Laser Hardning of Cutting Tools - DiVA portal1022680/FULLTEXT01.pdf · are hardened by flame hardening or induction hardening process. Indeed, VCBC has developed their own equipment

MASTER’S THESIS

2003:210 CIV

MARC MIRALLES

Laser Hardningof Cutting Tools

MASTER OF SCIENCE PROGRAMME(EEIGM)

Department of Applied Physics and Mechanical EngineeringDivision of Engineering Materials

2003:210 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 03/210 - - SE

Spring 2003

MASTER OF SCIENCE PROGRAMME

École Européenne d’Ingénieurs en Génie des Matériaux EEIGM

Department of Applied Physics and Mechanical Engineering Division of Engineering Materials

MASTER THESIS REPORT

Laser Hardening of cutting tools

MIRALLES Marc

Master Thesis Report Laser hardening of cutting tools

Student: Marc MIRALLES January-June 2003

TTAABBLLEE OOFF CCOONNTTEENNTTSS I. INTRODUCTION.........................................................................................................................1

II. LASER GENERALITIES ............................................................................................................2

II.1 THEORETICAL PRINCIPLE: ENERGY TRANSITION .........................................................................3 II.1.1 Spontaneous Emission ...........................................................................................................3 II.1.2 Stimulated Emission...............................................................................................................3 II.1.3 Stimulated Absorption............................................................................................................3 II.1.4 Population Inversion..............................................................................................................4 II.1.5 Gain & Loss...........................................................................................................................4 II.1.6 Three and Four Level Laser Schemes....................................................................................5

II.2 PROPERTIES OF LASER BEAMS ....................................................................................................6 II.2.1 Monochromaticity :................................................................................................................6 II.2.2 Coherence ..............................................................................................................................6 II.2.3 Divergence and Directionality...............................................................................................6 II.2.4 Brightness ..............................................................................................................................7

II.3 LASER TECHNOLOGY ..................................................................................................................8 II.3.1 Laser constituents ..................................................................................................................8 II.3.2 Common industrial lasers......................................................................................................9

II.3.2.1 Gas Lasers...................................................................................................................................9 II.3.2.2 Solid State Laser .......................................................................................................................10 II.3.2.3 Liquid Lasers ............................................................................................................................10 II.3.2.4 Semiconductors lasers...............................................................................................................10

II.4 SEMICONDUCTOR LASERS: DIODE LASER ..................................................................................11 II.4.1 Theory about semiconductors lasers....................................................................................11 II.4.2 Laser diode technology : p-n junction .................................................................................12 II.4.3 Diode laser advantages for hardening applications............................................................12

II.4.3.1 Absorption of radiation.............................................................................................................12 II.4.3.2 Influence of beam shape on geometry of the hardened tracks...................................................13 II.4.3.3 Temperature measurement and process control used at Duroc .................................................14

II.5 LASER & SURFACE TREATMENT ...............................................................................................15 II.5.1 Laser Surface Hardening.....................................................................................................16 II.5.2 Laser Alloying......................................................................................................................16 II.5.3 Laser Glazing.......................................................................................................................16 II.5.4 Laser Cladding ....................................................................................................................16 II.5.5 Laser Shock Hardening .......................................................................................................16

III. SELECTIVE HARDENING PROCESSES ..............................................................................18

III.1 PHASE TRANSFORMATION THEORY .......................................................................................19 III.1.1 Austenite formation ............................................................................................................19

III.1.1.1 From a ferrite-cementite structure .......................................................................................19 III.1.1.2 Particular example of the pearlitic structure ........................................................................20 III.1.1.3 Important parameters for austenite formation......................................................................21

III.1.2 Martensite formation ..........................................................................................................21 III.1.2.1 Cooling rate .........................................................................................................................21 III.1.2.2 Effect on the temperature of martensite formation ..............................................................22

III.2 SELECTIVE HARDENING PROCESSES OVERVIEW ....................................................................24 III.2.1 Flame Hardening ...............................................................................................................24 III.2.2 Induction Hardening ..........................................................................................................24 III.2.3 Laser Hardening.................................................................................................................25

III.3 LASER HARDENING SPECIFICITIES.........................................................................................27 III.3.1 Processing parameters .......................................................................................................27 III.3.2 Properties of transformed steels.........................................................................................27

III.3.2.1 Hardness ..............................................................................................................................27 III.3.2.2 Wear resistance....................................................................................................................27 III.3.2.3 Abrasion ..............................................................................................................................28 III.3.2.4 Adhesion..............................................................................................................................28 III.3.2.5 Fatigue.................................................................................................................................28

Master Thesis Report Laser hardening of cutting tools


III.3.2.6 Distorsion ............................................................................................................................28 III.3.3 Laser hardening specificities..............................................................................................28 III.3.4 Comparison of these processes...........................................................................................29

IV. LABORATORY INVESTIGATION.........................................................................................31

IV.1 THE REASONS AND THE GOAL ...............................................................................................32 IV.2 THE HARDENED ZONE REQUIREMENTS..................................................................................34 IV.3 FIRST INVESTIGATION ABOUT LASER ANGLE & LENS............................................................35

IV.3.1 Protocol ..............................................................................................................................35 IV.3.1.1 Duroc´s equipment ..............................................................................................................35 IV.3.1.2 Test description ...................................................................................................................36 IV.3.1.3 Material used .......................................................................................................................37 IV.3.1.4 Laboratory investigation description ...................................................................................37

IV.3.2 Results and Discussion .......................................................................................................40 IV.3.2.1 Fermo results discussion......................................................................................................40 IV.3.2.2 Carmo results discussion .....................................................................................................42

IV.4 SECOND INVESTIGATION (SPEED & TEMPERATURE).............................................................44 IV.4.1 Parameters rectification for Carmo tool ............................................................................44

IV.4.1.1 Test description ...................................................................................................................44 IV.4.1.2 Results and Discussion ........................................................................................................44

IV.4.2 Sleipner tests.......................................................................................................................46 IV.4.2.1 Test description ...................................................................................................................46 IV.4.2.2 Results & Discussion...........................................................................................................46

IV.5 FINAL PARAMETERS OVERVIEW............................................................................................49 IV.6 CRITICAL PLACES OF LASER HARDENING OF CUTTING TOOLS ...............................................51

IV.6.1 Inside / Corner problem......................................................................................................51 IV.6.2 Overlapping problem..........................................................................................................53

IV.6.2.1 Solid angle problem.............................................................................................................54 IV.6.2.2 General annealing problem..................................................................................................57

IV.7 DIMENSIONAL STABILITY .....................................................................................................58

V. LIFE CYCLE STUDY ................................................................................................................60

V.1 THE FIRST STEP: CHECKING PHASE ............................................................................................61 V.1.1 Dimensional stability ...........................................................................................................62 V.1.2 Tool preparation and microcracks check.............................................................................64 V.1.3 Test Protocol........................................................................................................................66

V.1.3.1 First loop...................................................................................................................................66 V.1.3.2 Second loop ..............................................................................................................................67

V.1.4 Burrs investigation...............................................................................................................67 V.1.5 Results and discussion .........................................................................................................68

V.1.5.1 Results of the first loop.............................................................................................................68 V.1.5.2 Results of the second loop ........................................................................................................69

V.2 THE SECOND STEP: LONG WEAR TESTS ......................................................................................71 V.2.1 Test description....................................................................................................................71 V.2.2 Tool investigation at TOPONOVA.......................................................................................73

V.2.2.1 Toponova’ s equipment ............................................................................................................73 V.2.2.2 Investigation Protocol ...............................................................................................................74

V.2.3 Cut parts investigation .........................................................................................................76 V.2.4 Tool preparation ..................................................................................................................77 V.2.5 Test protocol ........................................................................................................................77 V.2.6 Results and discussion .........................................................................................................77

V.2.6.1 Fermo results ............................................................................................................................77 V.2.6.2 Carmo results ............................................................................................................................79 V.2.6.3 Sleipner results..........................................................................................................................80

V.2.7 Results Synthesis ..................................................................................................................81

VI. CONCLUSION............................................................................................................................85

VII. THANKS TO ..........................................................................................................................86

VIII. REFERENCE .........................................................................................................................87

Master Thesis Report Laser Hardening of cutting tools


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III... IIINNNTTTRRROOODDDUUUCCCTTTIIIOOONNN Nowadays, most of the tools of Volvo Cars Body Components (VCBC) in Olofström are hardened by flame hardening or induction hardening process. Indeed, VCBC has developed their own equipment for flame and induction hardening: However, Volvo has detected that 74% of the total cost of one year maintenance was caused by trim dies and the main reason is actually trim edge problems which represent 26 % of year maintenance. This is why the R&D Forming & Material department is investigating some ways to reduce those excessive costs caused by cutting tools. As a consequence, in this project, we will study the laser hardening of cutting tools. The aims of this project were to investigate:

The laser hardening process The limits of different tool materials with this laser process

This master thesis should give a beginning of answer to the following question:

This work has been conducted at Volvo Cars Body Components in the R&D Forming & Material department. This report is divided in four main parts. The two first parts deal with theories about laser and selective hardening processes. The two last ones are the report of my experimental investigation that I run during the thesis at VCBC in Olofström.

Is the laser hardening process a suitable alternative to induction hardening process for trim dies at VCBC ?



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IIIIII... LLLaaassseeerrr GGGeeennneeerrraaallliiitttiiieeesss Laser is the acronym of Light Amplification by Stimulated Emission of Radiation. Laser is light of special properties, light is electromagnetic (EM) wave in visible range. It is a long history for human beings to realise that light is both wavelike and particle like. In 1704, Newton characterised light as a stream of particles. The Young’s interference experiment in 1803 and the discovery of the polarity of light convinced scientists of that time that light is wave. Maxwell’s electromagnetic theory explained light as rapid vibrations of electromagnetic field due to the oscillation of charged particles. At the turn of the 20th century, the black body radiation phenomena challenged the wavelike light theory. According to Maxwell’s electromagnetic theory, the energy intensity of electromagnetic emissions with frequency f is proportional to the square of this frequency, integrate the intensity from zero to infinity frequencies over the limited black body volume will result in infinite energy, which is of course impossible! It was until Plank introduced the "quantum" concept in 1900 when this was explained. Thus energy is not continuous, it is discrete and can only be the multiples of a small unit. Einstein proposed the concept of "photon", we can say light is composed of individual particles called photons which posses a discrete amount of energy or quanta. Einstein also predicted in 1917 that when there exist the population inversion between the upper and lower energy levels among the atom systems, it was possible to realise amplified stimulated radiation, i.e., laser light. Quantum Mechanics was developed to explain these new phenomena since 1920. Now we think that light is composed of "particles" whose motion probability is determined by its wavelike behaviour.



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IIIIII...111 TTThhheeeooorrreeetttiiicccaaalll ppprrriiinnnccciiipppllleee::: EEEnnneeerrrgggyyy tttrrraaannnsssiiitttiiiooonnn

In the following section, we will try to understand the laser principles more in depth. We will detail the theoretical principles which are at the base of the operation of a laser. The general concept is always identical and by consequent the following fundamental principles are valid for any type of laser used. To explain how laser light is generated, we need first to investigate the energy transition phenomena in atoms or molecules. These phenomena include: spontaneous emission, stimulated emission/absorption and nonradiative decay.

IIII..11..11 SSppoonnttaanneeoouuss EEmmiissssiioonn According to Plank fundamental hypothesis (quantum mechanics), the electrons of atoms can take different energy states, say E1, E2, E3, ... with E1<E2<E3<…. Lower energy level is more stable than higher energy levels, so electrons at high energy levels tend to decay to low energy levels, the energy difference between the two levels can be given out as electromagnetic radiation. This process is called Spontaneous Emission. The relationship is:

E2 - E1 = hν0

Where E2 is the upper energy level, E1 is the lower energy level, h is Plank’s constant, ν0 is frequency of the radiated electromagnetic wave.

IIII..11..22 SSttiimmuullaatteedd EEmmiissssiioonn The easiest laser model to understand is the two level system described on the figure I.1. In a two level system, the particles have only two available energy levels, separated by some energy difference which is typically referred to in terms of the photon energy, hv0. These two levels are generally referred to as the upper and lower laser states. When a particle in the upper state interacts with a photon matching the energy separation of the levels, the particle may decay, emitting another photon with the same phase and frequency as the incident photon. Thus we have gotten two photons for the price of one. This process is known as Stimulated Emission. It is important to notice that in the case of spontaneous emission, the radiation is in all directions and in random phases, while in stimulated radiation, the emitted waves of any atoms are in the same direction and in the same phase with the incident wave.

IIII..11..33 SSttiimmuullaatteedd AAbbssoorrppttiioonn If the atom is initially at level E1, if this is the ground level, the atom will remain in this level unless got excited. When an electromagnetic wave of frequency ν0 is incident on the material, there is a finite probability that the atom will absorb the

Figure I.1



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incident energy and jump to energy level E2. This process is called Stimulated Absorption. Normally the number of atoms at lower energy levels is larger than atoms at higher levels. Stimulated radiation/absorption, spontaneous emission and nonradiative decay are going on in the same time. Even if we ignore the decay factors, stimulated absorption still dominates over stimulated radiation, the incident electromagnetic wave can not be amplified in this case. Amplification of incident wave is possible only when the number of upper level atoms is greater than that of lower level atoms.

IIII..11..44 PPooppuullaattiioonn IInnvveerrssiioonn A fundamental concept in lasers is the idea of a "population inversion". A normal thermal population in any material will have most of the particles in the ground state. However, we would prefer to have most of the particles in the excited state so we can get free photons through stimulated emission. Thus in a laser we strive to create a "population inversion" where most or all of the particles are in the excited state. This is achieved by adding energy to the laser medium (usually from an electrical discharge or an optical source such as another laser or a flashlamp); this process is called pumping and described on picture I.2.

IIII..11..55 GGaaiinn && LLoossss Another fundamental concept in lasers is the idea of gain, which is basically a short way of referring to the "free" photons described earlier. Suppose we have just pumped our laser medium so that all of the particles are in their excited state. One of those particles now spontaneously decays back down to its ground state, emitting a photon (hv0). This photon is of the right frequency to stimulate emission from another excited state particle, which emits another photon which can stimulate another excited state particle, and so on. (see figure I.3 ). In addition to stimulated emission processes there are also stimulated absorption processes in which a ground state particle absorbs a photon matching the energy gap and jumps to the excited state. (represented by the grey arrow in the above figure).

Figure I.2

Figure I.3



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Thus we lose one photon to each stimulated absorption process. Since the probabilities for stimulated absorption and emission processes are equal (relative to population of the ground and excited states), it is clearly detrimental to the laser to have any particles in the ground state. For this reason, two level lasers are not practical. Indeed, it is not in general possible to pump more than half of the molecules into the excited state.

IIII..11..66 TThhrreeee aanndd FFoouurr LLeevveell LLaasseerr SScchheemmeess We have said that the two level schemes is not generally feasible for laser action. There are two main reasons for this. The first reason is that the energy being used to pump the particles into the upper laser state has an equal probability of stimulating them back down by stimulated emission. Thus it is not possible to perform an efficient pumping and get more than half of the particles into the excited state. The three level scheme gets around this problem by first exciting the particles to an excited state higher in energy than the upper laser state (See figure I.4). The particles then quickly decay down into the upper laser state. It is important for the pumped state to have a short lifetime for spontaneous emission compared to the upper laser state. The upper laser state should have a lifetime as long as possible, so that the particles live long enough to be stimulated and thus contribute to the gain. With this three level scheme, we clearly separate the energy used for pumping and the energy created by stimulated emission between the two laser state which is improving the population inversion and consequently the laser functionnement itself.

Figure I.4

Figure I.5 The four level laser scheme goes one step further and also depopulates the lower laser level by a fast decay process (see figure I.5). This greatly decreases the loss of laser photons (v2) by stimulated absorption processes since the particles in the lower laser level have a short life-time for spontaneous emission.



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IIIIII...222 PPPrrrooopppeeerrrtttiiieeesss ooofff LLLaaassseeerrr BBBeeeaaammmsss

IIII..22..11 MMoonnoocchhrroommaattiicciittyy :: This property is due to the following two factors:

First, only an electromagnetic wave of frequency ν0 = (E2-E1)/h can be amplified, ν0 has a certain range which is called linewidth, this linewidth is decided by homogeneous broadening factors and inhomogeneous broadening factors, the result linewidth is very small compared with normal lights.

Second, the laser cavity forms a resonant system (see Laser constituents), oscillation can occur only at the resonance frequencies of this cavity. This leads to the further narrowing of the laser linewidth, the narrowing can be as large as 10 orders of magnitude! So laser light is usually very pure in wavelength, we say it has the property of monochromaticity.

IIII..22..22 CCoohheerreennccee For any electromagnetic wave, there are two kinds of coherence, namely spatial and temporal coherence.

Let’s consider two points that, at time t=0, lie on the same wave front of some given electromagnetic wave, the phase difference of electromagnetic wave at the two points at time t=0 is k0. If for any time t>0 the phase difference of electromagnetic wave at the two points remains k0, we say the electromagnetic wave has perfect coherence between the two points. If this is true for any two points of the wave front, we say the wave has perfect spatial coherence. In practical the spatial coherence occurs only in a limited area, we say it is partial spatial coherence.

Now consider a fixed point on the electromagnetic wave front. If at any time the phase difference between time t and time t+dt remains the same, where "dt" is the time delay period, we say that the electromagnetic wave has temporal coherence over a time dt. If dt can be any value, we say the electromagnetic wave has perfect temporal coherence. If this happens only in a range 0<dt<t0, we say it has partial temporal coherence, with a coherence time equal to t0. We emphasise here that spatial and temporal coherence are independent. A partial temporal coherent wave can be perfect spatial coherent. Laser light is highly coherent, and this property has been widely used in measurement, holography, etc.

IIII..22..33 DDiivveerrggeennccee aanndd DDiirreeccttiioonnaalliittyy Laser beam is highly directional, which implies laser light is of very small divergence. This is a direct consequence of the fact that laser beam comes from the resonant cavity, and only waves propagating along the optical axis can be sustained in the cavity. The directionality is described by the light beam divergence angle.



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( )24

θπDPB =

IIII..22..44 BBrriigghhttnneessss The brightness of a light source is defined as the power emitted per unit surface area per unit solid angle. A laser beam of power P, with a circular beam cross section of diameter D and a divergence angle θ and the result emission solid angle is πθ2, then the brightness of laser beam is:



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IIIIII...333 LLLaaassseeerrr TTTeeeccchhhnnnooolllooogggyyy

IIII..33..11 LLaasseerr ccoonnssttiittuueennttss A laser device is consisted of: Laser medium like atoms, molecules, ions or semiconductor crystals; Pumping process to excite these atoms (molecules, etc.) into higher quantum-

mechanical energy levels; suitable optical feedback elements that allow the beam of radiation to either pass

once through the laser medium (as in laser amplifier) or bounce back and forth repeatedly through the laser medium (as in a laser oscillator).

A basic laser system (see figure I.6) has two mirrors that are placed parallel to each other to form an optical oscillator so that light can be transmitted back and forth between the mirrors along the optical axis. Between the mirrors is the active medium that can amplify the light by stimulated emission. Under certain pumping mechanism, the active medium can be excited from lower energy state to upper energy state, population inversion happens. Stimulated emissions give out photons in all directions at first. But the photons are transmitted through the system, only photons whose directions are along the optical axis can last, photons in other directions will either be scattered or absorbed. Soon almost all the photons in the system are oscillating in the optical axis direction. One incident photon can become two photons after every stimulated emission, so under proper conditions, light density is amplified. Usually one of the mirrors is nearly totally reflecting, another is partially reflecting (typically ~5% of the laser energy is transmitted), the partially reflecting mirror will output laser light either in continuous wave or in pulse. Lasing will occur in the cavity when the gain due to stimulated emission rises above some threshhold value determined by the loss mechanisms active in the cavity (some examples of loss mechanisms are: stimulated absorption, scattering from defects in the laser medium, and loss due to transmission). Working laser may give out a lot of heat, this heat can affect the operation quality seriously. So lasers usually have cooling systems.

Figure I.6



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IIII..33..22 CCoommmmoonn iinndduussttrriiaall llaasseerrss Lasers can be divided into gas lasers, solid state lasers, liquid lasers and semiconductors lasers according to the active medium used. All the previous theory is valid for those different type of lasers. The only difference between these various lasers is the technology implemented for the active medium.

IIII..33..22..11 GGaass LLaasseerrss Gas Lasers can be further divided into neutral atom, ion and molecular lasers, whose lasing mediums are neutral atoms, ions or gas molecules respectively.

Helium-neon laser (He-Ne) is a kind of neutral atom gas laser, the common wavelength of a He-Ne laser is 632.8 nm, it is tuneable from infrared to various visible light frequencies. He and Ne are mixed according to certain percentage, pumping is by DC electrical discharge in the low pressure discharge tube. First He atom is excited. Because Ne atom has an energy level very near to an energy level of He, through kinetic interaction, energy is readily transferred from He to Ne, and Ne atom emit the desired laser light. The typical power of He-Ne laser is below 50 mW, it is widely used in holography, scanning, measurement, optical fibre communication, etc. It is the most popular visible light laser.

Carbon dioxide laser is a typical molecular gas laser, it emits laser light at a wavelength of 10.6 µm, its beam power ranges from several watts to 25 kW or even to 100 kW, so CO2 laser is widely used in laser machining, welding and surface treating. The active medium of CO2 laser is a mixture of CO2, helium (He) and nitrogen gases (N2). Pumping is realised by AC or DC electrical discharge. First most of the electrical discharge energy is absorbed by nitrogen gas, only a small part of the energy is absorbed by CO2 molecules directly which raise them from ground state (000) to upper state (001). Large amounts of CO2 molecules collide with the nitrogen molecules and gain the excitation energy. Once excitation is achieved, the CO2 molecules at (001) state will give out energy and jump to lower energy state (100) or (020), thus giving out laser light at frequency 10.6m m or 9.6 µm respectively. The remaining decay from state (100) to (010), (020) to (010) or (010) to ground state (000) will dissipate energy in the form heat instead of light.

Figure I.7

Figure I.8

(001)

(100)

(010)

(000)



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IIII..33..22..22 SSoolliidd SSttaattee LLaasseerr In solid state lasers, ions are suspended in crystalline matrix to generate laser light. The ions emit electrons when excited, the crystalline matrix spread the energy among the ions. The first solid state laser is ruby laser, but it is no longer used because of its low efficiency. Two common solid state lasers are Nd:YAG lasers and Nd:glass lasers, there structures are very similar. Both use krypton or xenon flash lamps for optical pumping. For Nd:glass lasers, the glass rod has the advantage of growing into larger size than YAG crystals, but the low thermal conductivity of glass limits the pulse repetition rate of Nd:glass laser. So Nd:glass lasers are used in applications which require high pulse energies and low pulse repetition rates. It is suitable for hole piercing and deep keyhole welding operations. YAG crystal has a higher thermal conductivity than glass, so the thermal dissipation in Nd:YAG laser cavity can be improved, operation power can be up to several hundred watts in continuous mode, and high pulse rates (50kHz) can be reached. YAG is a complex crystal of Yttrium-Aluminium-Garnet with chemical composition of Y3Al5O12, it is transparent and colourless. About 1% Nd3+ ions are doped into the YAG crystal, the crystal colour then changed to a light blue colour. The wavelength of Nd:YAG laser is 1.06 µm.

IIII..33..22..33 LLiiqquuiidd LLaasseerrss Liquid Lasers use large organic dye molecules as the active lasing medium. These lasers can lase in a wide frequency range, i.e. they are frequency tuneable. The spectral range of dyes covers infrared, visible and ultraviolet light. Pumping is by another pulsed/continuous laser, or by pulsed lamp. These lasers are used in spectroscopic investigation and photochemical experiments.

IIII..33..22..44 SSeemmiiccoonndduuccttoorrss llaasseerrss Semiconductors lasers are said to be "the laser of the future". The reasons are: they are compact, they have the potential of mass production, they can be easily integrated, their properties are in rapid improvement, they are becoming more and more powerful and efficient.



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gfvfc EhEE ≥=− ν

IIIIII...444 SSSeeemmmiiicccooonnnddduuuccctttooorrr lllaaassseeerrrsss::: DDDiiiooodddeee lllaaassseeerrr

IIII..44..11 TThheeoorryy aabboouutt sseemmiiccoonndduuccttoorrss llaasseerrss The majority of semiconductor materials are based on a combination of elements in the third group of the Periodic Table (such as Al, Ga, In) and the fifth group (such as N, P, As, Sb) hence referred to as the III-V compounds. Examples include GaAs, AlGaAs, InGaAs and InGaAsP alloys. The laser emission wavelengths are normally within 630~1600 nm. The semiconductor lasers that can generate blue-green light uses materials which are the combination of elements of the second group (such as Cd and Zn) and the sixth group (S, Se). The principle of semiconductor laser is very different from CO2 and Nd:YAG lasers. It is based on "Recombination Radiation". We can explain this principle by referring to the following figure I.9. The semiconductor materials have valence band V and conduction band C, the energy level of conduction band is Eg (Eg>0) higher than that of valence band. To make things simple, we start our analysis supposing the temperature to be 0 K. It can be proved that the conclusions we draw under 0 K applies to normal temperatures. Under this assumption for non-degenerate semiconductor, initially the conduction band is completely empty and the valence band is completely filled. Now we excite some electrons from valence band to conduction band, after about 1 ps, electrons in the conduction band drop to the lowest unoccupied levels of this band, we name the upper boundary of the electron energy levels in the conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the valence band and electrons near the top of the valence band drop to the lowest energy levels of the unoccupied valence energy levels, leave on the top of the valence band an empty part. We call the new upper boundary energy level of the valence band quasi-Fermi level Efv. When electrons in the conduction band run into the valence band, they will combine with the holes, in the same time they emit photons. This is the recombination radiation. Our task is to make this recombination radiation to lase. Then, one condition must be met. Indeed, for the radiation to be amplified, the light energy hν must satisfy: From this relation we have Efc - Efv ≥ Eg. This decides the critical condition. The value of Efc and Efv is influenced by the pumping process, by the intensity (N) of the electrons being raised to the conduction band. When N is increased, Efc increases and Efv decreases.

Figure I.9



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The N satisfies Efc - Efv = Eg is named Nth. We inject carriers (called dopants) into the semiconductor material to make the free electron intensity to be larger than Nth, then the semiconductor exhibits a net gain. We put this active medium in a suitable cavity, laser action occurs when this net gain overcome losses. The pumping of semiconductor lasers can be realised by the beam of another laser, or by an electron beam, but the most convenient way is by using electrical current that flows through the semiconductor junctions. This uses the semiconductor laser in the form of diode.

IIII..44..22 LLaasseerr ddiiooddee tteecchhnnoollooggyy :: pp--nn jjuunnccttiioonn In a laser diode, the semiconductor crystal is fashioned into a shape somewhat

like a piece of A4 paper (very thin in one direction and rectangular in the other two). The top of the crystal is n-doped, and the bottom is p-doped, resulting in a large, flat p-n junction. In that way, we create a region where simultaneously free electrons and holes exist naturally without any current. This means that many more of the electron-hole pairs can contribute to amplification The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction will be reflected several times from each end face before they are emitted. Each time they pass through the cavity, the light is amplified by stimulated emission. Hence, if there is more amplification than loss, the diode begins to "lase".

The dimension of diode laser is very small, a typical value is 100µm * 200µm * 50 µm. The power conversion efficiency is a few percent for the low power units and can reach 30% for laser arrays. A problem with the semiconductor laser is its relatively large divergence angle (typical value 1~30 degrees), but its defects are being improved quickly. Lower power diode laser systems, of a few mW, are used in CD players, optical storage systems, laser printers and communications. Diode lasers with Power 0.5W/diode are available, when they are packed into arrays, they can generate power of several kW.

IIII..44..33 DDiiooddee llaasseerr aaddvvaannttaaggeess ffoorr hhaarrddeenniinngg aapppplliiccaattiioonnss

IIII..44..33..11 AAbbssoorrppttiioonn ooff rraaddiiaattiioonn The used diode laser is emitting dichromatic light at wavelengths of 808 nm and 940 nm. The emitted light is absorbed by the radiation-exposed material and turned into heat. Figure I.10 shows the absorption A of different metals as function of wavelength. Iron and steel posses an improved absorption characteristic for the radiation of diode lasers compared to Nd:YAG and CO2 lasers with their wavelengths of 1064 nm and 10.6 µm. This results in a faster heat up of the surface, when the work piece is exposed to the radiation of the diode laser. For hardening processes for a given hardness depth and output power the processing speed can be increased.



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IIII..44..33..22 IInnfflluueennccee ooff bbeeaamm sshhaappee oonn ggeeoommeettrryy ooff tthhee hhaarrddeenneedd ttrraacckkss The caustic of the laser beam and the resulting intensity distribution in the focal point have great influence on the geometry of the resulting hardened track on the work piece. Figure I.11 shows the geometry, hardness penetration depth and specific energy of hardened tracks for different beam shapes.

A round focal point with a Gaussian intensity distribution, as it is typical for Nd:YAG and CO2 laser, leads to unfavourable semicircular hardening lenses. With a diode laser hardened tracks with steep boundaries can be realised due to improved intensity distributions. At a given output energy a cylindrical shape gives the maximum track width. If smaller track width are required the beam can be shaped with a uniform top head, which enables higher feed rates and thus faster processing speeds. With a diode laser an intensity profile can be achieved which lies in-between a cylindrical shape and a top hat distribution.

Figure I.10

Figure I.11



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IIII..44..33..33 TTeemmppeerraattuurree mmeeaassuurreemmeenntt aanndd pprroocceessss ccoonnttrrooll uusseedd aatt DDuurroocc For best results of the quality of the hardened tracks a temperature control is essential. It enables to work at high heating rates with their austeniting temperatures without melting the workpiece. For temperature control during processing a two-color infrared pyrometer and a micro controller for the laser output power are used. The pyrometer is adapted to the lens system of the laser with an optical fibre and works in-axis to the laser beam. A dichroitic mirror transmits the laser radiation and reflects the backscattered thermal radiation of the workpiece to the internal optics of the pyrometer, which give a spot diameter of about 22 mm at the working distance of the 15*15 mm2 optic. The micro controller keeps the working temperature in a range of 650°C to 1300°C with an accuracy of 10 K by adjustment of the laser output power. Figure I.12 shows the principle of the in-axis adaptation of the two-color-pyrometer.

Figure I.12



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IIIIII...555 LLLaaassseeerrr &&& SSSuuurrrfffaaaccceee TTTrrreeeaaatttmmmeeennnttt

The laser has some unique properties for surface heating. The electromagnetic radiation of a laser beam is absorbed within the two first few atomic layers for opaque materials, such as metals, and there are no associated hot gas jets, eddy currents or even radiation spillage outside the optically defined beam area. In fact, the applied energy can be placed precisely on the surface only where it is needed. As a consequence, high power lasers provide a source of energy, and hence a method of heating materials, which can be accurately controlled. The development and industrial application of lasers in materials processing such as drilling, cutting and welding have progressed rapidly over the last twenty years. With the continuous laser of high output, surface treatment technology is rapidly growing with the identification of new and improved processing methods. If laser material processing (drilling, cutting etc…) is considered to be the first generation of laser application, then laser surface treatment, referred to as surface treatment hardening, allowing, glazing, cladding etc., may be considered to be the second generation of laser utilisation in industry. Nowadays, surface treatment is a subject of considerable interest because it seems to offer the chance to save strategic materials or to allow improved components with idealised surfaces and bulk properties. For most engineering applications, the laser, in simple terms, can be regarded as a device for producing a finely controllable energy beam, which, in contact with a material, generates considerable heat. The basic physics of laser surface treatment is simply heat generation by laser interaction with the surface of an absorbing material and subsequent cooling either by heat conduction into the interior, or by thermal re-radiation at high temperatures from the surface of the material. It belongs to the group of short time hardening processes which are characterised by extremely rapid heating and cooling.

LLAASSEERR SSUURRFFAACCEE TTRREEAATTMMEENNTT

HEATING Solid state processing

Shock

Glazing

Alloying

Hardfacing

Remelting

Transformation hardening

Ageing

VAPORISING

MELTING Liquid state processing

EN

ER

GY

Figure I.13



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The figure I.13 illustrates various laser surface treatment methods that are currently available.

IIII..55..11 LLaasseerr SSuurrffaaccee HHaarrddeenniinngg It has been found that the same metallurgical principles underlie both conventional and laser hardening techniques. In the latter case, however, much smaller time scales are involved, and very large rates of heating and cooling are obtained here. If a transformed phase heated by a laser beam is cooled rapidely, it will not have time to transform back to its previous low temperature stable phase, and a metastable phase will be obtained. The metastable phases are highly stressed internally, and are usually hard. A common example of such a phase martensite in ferrous materials. As in other heat treatment processes, the hardenability of the steel is determined by its carbon content. The cooling rates necessary for production of martensite from steel heated above its austenitization temperature can be predicted with the help of a Continuous Cooling Transformation (CCT) diagram. A line describing a typical laser heat treatment cooling rate (104 °C/sec) would be well to the left of the nose of the ferrite/pearlite/bainite c-curves, indicating direct transformation to martensite.

IIII..55..22 LLaasseerr AAllllooyyiinngg In this treatment, a shallow layer at the surface of the material is melted by the laser beam which becomes efficiently coupled to the surface, while alloying elements are added simultaneously to give a local composition having the desired surface properties on solidification.

IIII..55..33 LLaasseerr GGllaazziinngg Here, a suitable laser beam is used to melt and re-solidify the surface on the material. A thin layer of material with an extremely fine-grained structure is then obtained. This material possesses improved wear and corrosion metallic resistance properties.

IIII..55..44 LLaasseerr CCllaaddddiinngg The laser beam energy melts a pre-positioned material, causing it to flow over the surface of an alloy, freeze, and then form a protective or wear-resistant layer.

IIII..55..55 LLaasseerr SShhoocckk HHaarrddeenniinngg A laser beam with extremely high power densities and short dwell times can vaporise a thin layer adjacent to the surface of a material, very rapidly. With rapid removal of surface atoms, momentum impulse occurs and a shock or stress wave is generated in the material. A shock wave then propagates and reflects within the material. This causes significant work hardening in the material. As mentioned previously, the surface of a material treated by a laser beam undergoes a heating process, followed by a cooling process, that is, an athermal cycle. The type of thermal cycle obtained is principally determined by the laser power, beam size,



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beam velocity relative to the workpiece and also by the absorption coefficient, thermal diffusitivity and thermal conductivity of the material that is to be treated. Based on laser power density and interaction time, that is, the dwell time (defined as the diameter of laser beam spot divided by beam velocity), a laser beam can heat, melt or vaporise a thin surface layer of a material.



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IIIIIIIII... SSSEEELLLEEECCCTTTIIIVVVEEE HHHAAARRRDDDEEENNNIIINNNGGG PPPRRROOOCCCEEESSSSSSEEESSS

These processes, which do not change surface composition, are also called localised heat treatment processes because the surface is selectively heat-treated (i.e. the surface is austentized and then quenched very rapidly to produce martensite). The basic requirement for these processes is that the steel must have sufficient carbon and hardenability to achieve the required hardness at the surface. Medium carbon steels are usually suited for these processes. The processes are classified according to the heating source and these are flame hardening, induction hardening, laser hardening, and electron-beam heat-treating. In addition to increased wear resistance, the surface hardening (due to the martensite formation) also induces residual compressive stresses that result in improved bending and torsional strength as well as fatigue properties. We shall discuss only the first three processes. When is selective hardening necessary? Selective hardening is applied because of one or more of the following reasons:

(1) Parts to be heat-treated are so large as to make conventional furnace heating and quenching impractical and uneconomical - examples are large gears, large rolls, and dies;

(2) only a small segment, section, or area of the part needs to be heat-treated-typical examples are ends of valve stems and push rods, and the wearing surfaces of cams and levers;

(3) better dimensional accuracy of a heat-treated part; and (4) overall cost savings by using inexpensive steels to have the wear

properties of alloyed steels.



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IIIIIIIII...111 PPPhhhaaassseee tttrrraaannnsssfffooorrrmmmaaatttiiiooonnn ttthhheeeooorrryyy

IIIIII..11..11 AAuusstteenniittee ffoorrmmaattiioonn

IIIIII..11..11..11 FFrroomm aa ffeerrrriittee--cceemmeennttiittee ssttrruuccttuurree The response of ferrous material to rapid cool-down from the austenite region has been studied in great detail for many decades and is well understood. The same cannot be said for the heating period of the process. Basically, the problems associated with these high heating rates are that the formation of austenite as well as the redistribution of carbon, necessary to form a homogeneous γFe-C solid solution, are processes that require small but finite time intervals. It is known that austenite formation from aggregates of cementite and ferrite occurs by nucleation and subsequent growth. In general terms, the transformation process from the pearlite-cementite (or ferrite) aggregate to austenite (at temperature above A1) occcurs in the following four stages: Stage 1: The initial microstructure remains untransformed. During this period,

embryos for austenite nuclei are formed. Stage 2: Austenitic nuclei occur at the interface between ferrite and cementite,

simultaneously growing into both ferrite and carbide. Stage 3: Residual carbides dissolve during this period. Stage 4: Carbon atoms diffuse further in the austenite, until a homogeneous

equilibrium carbon content is achieved. It is clear that austenitic nuclei must have an approximately eutectoid composition when they are formed just above the A1 temperature. They must also form at a cementite/ferrite interface. Here, austenitic embryos readily acquire carbon atoms, to become austenite nuclei. The question of the location of these nuclei with respect to the initial structure is really probabilistic, since the number of available sites is exceedingly large. However, it can be stated that austenite nucleation is a structure-sensitive process. After austenitic nuclei form at the ferrite/cementite interface, two new interfaces (austenite/ferrite and austenite/cementite) occur. The process of austenitic growth is such that the austenite-ferrite and austenite-cementite interfaces move respectively into the ferrite and the cementite. The rate of austenite growth will depend upon the rate of solution of carbide and ferrite, and also the rate of diffusion of the carbon atoms in the austenite. The difference of the carbon concentration in austenite and ferrite at their interface is much less than at the austenite/cementite interface. The rate of austenite growth towards ferrite greatly exceeds the rate of growth towards cementite. Hence, ferrite always disappears before cementite, for austenizartion of the eutectoid steel. At the instant when the ferrite disappears, the residual carbide still exits, and this continues



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to dissolve further in austenite. Furthermore, when the residual carbide has dissolved completely, the distribution of carbon in austenite is not homogeneous. The carbon concentration at cementite sites exceeds the carbon concentration at the ferrite sites. Subsequently, the distribution of carbon atoms in the austenite becomes increasingly homogeneous, by the diffusion of carbon atoms.

IIIIII..11..11..22 PPaarrttiiccuullaarr eexxaammppllee ooff tthhee ppeeaarrlliittiicc ssttrruuccttuurree The pearlite is a lamellar constituent of steel consisting of alternate layers of ferrite (alpha-iron) and cementite (iron Carbide Fe3C) and is formed on cooling austenite at 723°C. This produces a tough structure and is responsible for the mechanical properties of unhardened steel. The transformation of the pearlite is thought to proceed by diffusion from the cementite plates into the ferrite plates, possibly starting from one end of a pearlite colony. This time dependent process does not take long but is sufficient to necessitate some superheat above the austenitising temperature to allow it to proceed to any extent during laser treatment. The superheat, and therefore the extent of the diffusion process, is thus slightly affected by the prior size of the pearlite colonies. These colonies, on transformation, become austenite having 0,8% carbon. Carbon diffuses down the concentration gradient into the ferrite regions where there is virtually no carbon. The ferrite regions may also have transformed to the fcc (face centred cubic) structure of austenite. The extent of homogeneity of the resultant martensite will be depend upon the size of the prior ferrite regions and the processing conditions in particular the interaction (beam diameter / traverse speed).

Figure II.1



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IIIIII..11..11..33 IImmppoorrttaanntt ppaarraammeetteerrss ffoorr aauusstteenniittee ffoorrmmaattiioonn The factors which affect austenite transformation are as follows:

Temperature : Both nucleation and growth rates increase with increasing temperature. Larger temperatures lead to an earlier disappearance of ferrite, and also to a greater concentration of residual carbides and a lower average concentration of carbon in austenite at the instant when ferrite disappears.

Carbon content in steels : An increase in carbon content in steels leads to an

increase in the rate of austenite growth. When the carbon content is higher, the time needed for the ferrite to disappear becomes smaller and the time needed for dissolution of carbide becomes larger.

Initial metallographical structure : Austenization is strongly dependent on

the shape and distribution of carbide in the initial structure. Fine carbides produce ferrite/carbide interfaces with large surface area. This increases the opportunity for nucleus formation. Hence, lamellar and needle-shape carbides yield more rapid austenizarion than is obtained with nodular carbides.

IIIIII..11..22 MMaarrtteennssiittee ffoorrmmaattiioonn Martensite is a non-equilibrium phase that forms when the cooling rate is fast enough to avoid the formation of pearlite and bainite. The mechanism is a shear or diffusionless transformation that does not require motion of atoms by diffusion, but only a slight rearrangement of the immediate neighbours. Typically, a lath or plate of the new arrangement begins at an imperfection such as an austenite grain boundary or the surface of another lath, and then propagates across the grain at the speed of sound. Since the volume occupied by the atoms in their new structure is larger than in the FCC arrangement, the laths are highly stressed and so is the matrix. This is why martensite is extremely hard and brittle.

IIIIII..11..22..11 CCoooolliinngg rraattee An alloy has a critical cooling rate. If cooled slower than this rate, the austenite will revert to a soft ferrite and iron

carbide structure. If forced to cool at a rate faster than the critical than the critical rate, austenite will

transform to a hard martensite structure. The critical cooling rate is determined by the alloying constituents of the steel. Metal additions such as Nickel, Chrome and Molybdenum act to restrict the rate of transformation, thus lowering the critical cooling rate. An alloy that has a low critical cooling rate is said to have high hardenability. The hardness of martensite is almost exclusively related to its carbon content. Thus, a 0,5% carbon steel which has no alloying elements, will form martensite with a Rockwell C hardness of 61. The 0,5% carbon steel which contains Cr, Ni and Mo will reach exactly the same hardness. What is conferred by the alloying elements is a property called hardenability, which is a measure of the cooling rate required to produce martensite.



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To completely convert the plain 0,5% carbon steel to martensite it must be cooled from 900°C to 500°C in less than 1 second. For the alloyed steel with 0,5% C this time is prolonged to 10 seconds. In laser hardening generally the cooling rates obtained from heat conduction into the substrate are high enough for martensitic transformation even in cheaper carbon steels. The more rapid heating and quenching of the laser process does result in variations in the type of martensite, particularly its fineness, amount of retained austenite and carbide precipitation as well as the homogeneity of the hardened zone. The transformed zone is also more highly restrained resulting in higher compressive stresses opposing the approximately 4% volume increase associated with martensitic phase changes.

IIIIII..11..22..22 EEffffeecctt oonn tthhee tteemmppeerraattuurree ooff mmaarrtteennssiittee ffoorrmmaattiioonn All alloying elements with the possible exception of Co, lower Ms the temperature of the start of the martensite formation, as well as Mf, the finish of the martensite formation, at 100% martensite. For the majority of steels containing more than 0,50% C, Mf lies below room temperature. This implies that after hardening these steels practically always contain some residual austenite. Ms may be calculated from the equation given below, by inserting the percentage concentration of each alloying elements in the appropriate term. The equation is valid only if all the alloying elements are completely dissolved in the austenite. Ms = 561 – 474 %C – 33 %Mn – 17 %Ni – 17 %Cr – 21 %Mo For high-alloy and medium-alloy steels, Stuhlmann has suggested the following equation:

Ms(°C) = 550 – 350 %C – 40 %Mn – 20 %Cr – 10 %Mo – 17 %Ni – 8 %W – 35 %V – 10 %Cu + 15 % Co + 30 %Al

Figure II.2



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In order to sum up, all alloying elements with the possible exception of Co and Al, lower temperature of the start of the martensite formation as well as the finish of the martensite formation which has a negative effect if we try to get a complete martensitic transformation. In the same time, this same alloying elements delay the formation of ferrite and cementite and also lower the critical cooling rate which are two good points for the martensitic transformation. As a consequence, the problem is complex and it is very difficult to formulate any general rules regarding the influence exerted by the various alloying elements.



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IIIIIIIII...222 SSSeeellleeeccctttiiivvveee hhhaaarrrdddeeennniiinnnggg ppprrroooccceeesssssseeesss ooovvveeerrrvvviiieeewww

IIIIII..22..11 FFllaammee HHaarrddeenniinngg The principle of flame hardening a steel component is to quickly heat its surface to the austenitic temperature range and then rapidly quench the component to produce a martensitic structure on the surface layer. In flame hardening the surface layer of steel is quickly austenitized by the direct impingement of a high-temperature flame (or a high-velocity combustion-product gas). Flame hardening can be applied by a variety of methods of which the principal ones are spot or stationary, progressive, spinning, or a combination of progressive and spinning. The quenching action after heating is accomplished by a combination of heat extraction by the cold metal beneath the case and by an external quenching medium. Steels for flame hardening usually contain 0.4 to 0.75% carbon, and since there is no change in composition, the steel to be flame-hardened is selected for both case and core properties. Since the core structure is not affected by the surface treatment, the core properties must be developed by proper heat treatment before the surface treatment. In general, a hard surface layer of martensite is produced and a softer inner core that has a ferrite-pearlite structure.

IIIIII..22..22 IInndduuccttiioonn HHaarrddeenniinngg The principle of induction hardening of a steel surface is to rapidly heat the surface of a steel component into the austenitic condition and then quickly quench the part so that its surface is transformed into a hard martensitic case. Since no change in composition is involved in the component, the steel must be selected for case and core properties. Thus, for surface induction hardening. steels usually contain 0.4 to 0.75% carbon, and the core properties must be produced by heat treatment before surface induction hardening.

Figure II.3



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The basic components of an induction heating system are: (1) an induction coil, (2) an alternating-current (AC) power supply, and (3) the workpiece itself. The coil is commonly a copper tubing through which cooling water passes and takes a variety of shapes to suit the part to be heated. The AC current flows through the coil, generates an electromagnetic field that cuts through the workpiece, and which induces the eddy currents to heat the workpiece. Induction heating of a steel part is accomplished by placing the part in a magnetic field generated by high-frequency alternating current passing through a water-cooled copper induction coil. The rapidly alternating magnetic field produced within the coil induces current flow within the steel surface. The induced current within the steel then produces heat according to the relationship heat = i2R, where R is the electrical resistance of the steel. Different types of heating patterns can be produced by various types of induction coils. The depth of current penetration and hence depth of heating the metal surface depends mainly on the frequency of the alternating current. The higher the frequency, the lower the penetration. When shallow heating or a thin case is desired, high-frequency current is used. Intermediate and low frequencies are used for applications requiring deeper case depths and even through hardening.

IIIIII..22..33 LLaasseerr HHaarrddeenniinngg Surface of Fe-C alloys can be hardened through a martensite structure by a moving laser beam, provided the energy input, beam radius, and beam velocity are controlled within certain limits. During laser hardening the surface of the work piece is exposed to the laser beam. The absorbed radiation heats up the work piece locally. Movement of the laser across the surface of the working piece produces hardened tracks. The phase transformation induced by laser hardening for steels take the following stages: 1. Formation of austenite from pearlite-cementite (hypereutectoid steels) or pearlite-

ferrite (hypoeutectoid steels) aggregate structure 2. Martensite transformation from austenite

Figure II.4 Figure II.5



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The high cooling rate (104 °C/s) in laser hardening treatment, exceeds the critical cooling rate of martensite formation. The metallographic microstructure after laser hardening , which determines the results of hardening, depends mainly on austenite formation in the thermal cycle, and hence we must give particular attention to the kinetic of its formation in a thermal cycle. Both heating and quenching stages of laser treatment can be described using classical Time-Temperature-Austenitisation and Time-Temperature-Transformation diagrams given in Figure II.6:

TTA diagram C60 Steel TTT diagram C60

Figure II.6



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IIIIIIIII...333 LLLaaassseeerrr hhhaaarrrdddeeennniiinnnggg ssspppeeeccciiifffiiiccciiitttiiieeesss

IIIIII..33..11 PPrroocceessssiinngg ppaarraammeetteerrss The results of laser surface treatment is determined principally by the factors of laser parameters (power, interaction time and traverse speed), beam properties (power density and its distribution in terms of time and space), conditions of material (composition, structure and pre-treatment), surface conditions and other parameters as e.g absorption layers, purging gas.

The three most important criteria for heat treatment by laser heating are:

The temperature for the zone being hardened must reach well into austenitising zone.

Between heating and cooling cycles, the substrate should be maintained at the austenitising temperature long enough for carbon diffusion.

There should be enough mass so that the cooling rate by self quenching is such that it could satisfy the critical quenching rate requirement.

IIIIII..33..22 PPrrooppeerrttiieess ooff ttrraannssffoorrmmeedd sstteeeellss

IIIIII..33..22..11 HHaarrddnneessss Laser transformation hardening produces thin surface zones which are heated and cooled very rapidely resulting in very fine martensitic microstructures even in steels with relatively low hardenability. This depends upon the carbon content. The next figure II.7 shows the hardness in vickers according the carbon content and also for three different martensite amounts. It has been found that the hardness value may be slightly higher than that found for induction hardening. This difference is probably due to the shallower zone in the laser process allowing a faster quench and therefore greater restraint and hence higher residual compressive stress.

IIIIII..33..22..22 WWeeaarr rreessiissttaannccee Laser surface hardening increases not only the local hardness but also the wear resistance. Indeed, wear resistance has also been found to improve with laser treatment compared to oil or water quench. There are many physical wear mechanisms, the four basic types are abrasion, adhesion, fatigue and oxidation.

Figure II.7



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IIIIII..33..22..33 AAbbrraassiioonn Abrasive wear occurs when a hard protuberance (asperity) or particles plastically deforms or cuts a surface as a result of motion. Generally, abrasive wear is determined by the hardness. It is desired to produce the surface which is much harder than the hardness of the abrasive materials.

IIIIII..33..22..44 AAddhheessiioonn Adhesive wear occurs when one surface bonds to another, and with subsequent motion, rupture occurs in one of the materials. This type of wear is determined by the coefficient of friction. Therefore in order to reduce frictional forces the low friction surfaces are needed.

IIIIII..33..22..55 FFaattiigguuee In steels and cast irons there is a residual compressive stress on transformation hardening due to the volume expansion on the formation of martensite (approximately 4% for 0,3 wt %C steel). This effect is particularly pronounced in the shallower hardened zones formed with the laser due to the greater restraint for such treatment. Fatigue cracks are generally initiated at the surface by tensile stresses; thus the fatigue load must be sufficient to overcome this residual compressive stress before a crack can propagate. Improved fatigue life compared to induction hardening has been reported with laser heat treatment.

IIIIII..33..22..66 DDiissttoorrssiioonn Due to the reduced thermal load and penetration possible with laser treatment there is less distorsion compared to flames or induction hardening. This is often the reason for using the laser.

IIIIII..33..33 LLaasseerr hhaarrddeenniinngg ssppeecciiffiicciittiieess In the field of metal heat treatment, the laser beam has a number od specific properties which make it superior to other surface treatment methods (induction, flame and electron beam). These are as follows:

Controlled thermal profile and therefore shape and location of heat affected region : The laser beam is easily, quickly and precisely shaped without the use of coils. Therefore, heat treatment can be performed selectively, and the process can be localised to the area required. The heat zone is narrow and there is no ”heat spillage”, as is sometimes encountered with induction hardening.

Controlled thermal penetration and therefore distorsion : The energy is pure, monochromatic, electromagnetic radiation. It is thus absorbed by most metals and many non-metals in a layer of two or three atomic diameters thickness. This very small interaction depth means it is a true surface heater, with an effectively zero depth penetration in comparison with the induction hardening. Therefore, the treatment is restricted to the component skin and the core properties remain unaffected.



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After-machining : The heat energy input to the workpiece is low, so that mechanical deformation is minimised. Thus, straightening operations and rejects are avoided. Because the technique is self-quenching and external quenching is not necessary, no surface damage can result. So, we get less after-machining, if any, is required.

Chemical cleanliness : The energy is chemically ”clean”, unlike flame heaters. No dust, swarf and x-rays can be generated, so that the process can be carried out in the atmosphere. Special environmental chambers, such as vacuum chambers used in electron-beam technique are therefore not required.

Flexibility and automation facilities : The laser process has a large focal depth. This feature of the laser beam enables it to treat complex and awkward shapes of components. The heating pattern can be modified rapidly to suit various applications. The process can be applied to all materials which can be heat treated by conventional methods. Cast iron, plain steel, alloy steel and even low carbon steel can be laser-treated. Metallurgical considerations for laser heating are no different from any other conventional heat-treating process. The process does, however, differ from conventional processes in the following respects:

The extremely rapid heating and cooling rates inherent in laser heating make possible hardening of low cost carbon steels. However, if the carbon content in surface layers is higher than 0,7 the Ms temperature decreases and a significant amount of austenite can remain in the layer.

Higher hardness as compared to that from conventional processes is often obtained. During the laser heat treatment, martensite is formed under unusually high restraint due to localised and rapid heating and cooling rates.

Differences in hardenability between carbon steel and alloy steels are not as prominent as for conventional processes since the cooling rate in laser heat is high enough compared to the critical cooling rate.

IIIIII..33..44 CCoommppaarriissoonn ooff tthheessee pprroocceesssseess The properties enumerated above, emphasise the industrial importance of laser surface treatment techniques. The development and utilisation of these techniques have therefore progressed rapidly. It is not surprising, therefore, that there is now considerable research interest in the area of laser surface treatment. As laser hardening is in competition with accepted classical hardening techniques, it is necessary to consider whether laser hardening is the correct tool. The evaluation depends on the size of the workpiece, its geometry and the number of pieces that have to be treated. A general comparison of advantages and disadvantages of various treatments are indicated in the two following tables:



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Advantages Disadvantages Laser Mimimum part distortion

Selective hardening No quenchant required Thin case capability Case depth controllable Eliminates post processing Improves fatigue life

⌧ High equipment cost ⌧ Coverage area restricted ⌧ Absorbent coatings necessary ⌧ Multiple passes give local tempering

Induction

Fast process rates Deep case obtainable Lower capital cost than

Laser Coverage area

⌧ Downtime for coil change ⌧ Quenchant required ⌧ Part distorsion ⌧ Coil placement critical ⌧ Large thermal penetration ⌧ Electro-magnetic forces may spoil

surface ⌧ Fabrication of complex coils for

specific processes Flame Cheap

Flexible Mobile process

⌧ Poor reproducibility ⌧ Lacks rapid quench ⌧ Component distorsion likely ⌧ Environmental problems

Arc (TIG) Relatively cheap Flexible process

⌧ Section thickness limited ⌧ Large thermal penetration ⌧ Stirring takes place ⌧ Poor control to avoid melting

Electron Beam Minimal distorsion Slective hardening No quenchant required

⌧ High equipment cost ⌧ Requires vacuum ⌧ Low production rate ⌧ High processing costs

Surface Hardening Process

Flame Induction Laser E-beam Spatial resolution Accessibility 0 0 0 Intensity modulation 0 0 Low technical effort 0 Low investment cost Flexibility 0 0 Low distorsion Self quenching Quality of result 0 Surface oxidation 0 0 Treatment of large components 0

Single pieces 0 Complex geometry 0 0 Small pieces 0 Large production Industrial value 0



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IIIVVV... LLLaaabbbooorrraaatttooorrryyy IIInnnvvveeessstttiiigggaaatttiiiooonnn



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IIIVVV...111 TTThhheee rrreeeaaasssooonnnsss aaannnddd ttthhheee gggoooaaalll

The first step of my work was to investigate the laser hardening process with the collaboration of Duroc Applications facilities. The final goal was to find the optimal parameters for laser hardening in the particular case of a cutting edge for each material tested. We have discussed about the laser hardening process and the parameters which were used at this time. This discussion made it possible to list the various parameters. Furthermore, I received the nominal values used in the normal process for each parameter but also the range in which it was possible and coherent to vary them. The different parameters were the following ones:

The power distribution : With laser diode, we have a good control of the power distribution. In the case of laser hardening, the uniform power distribution is more suitable than the conventional Gaussian distribution. This distribution has been chosen and fixed for all the laboratory investigation.

The beam size and the lens: Then, we have the lens choice. The lens allows to focus or defocus the laser beam in order to scan more or less large area. At Duroc Application facilities, they got two different lenses: the 8*8 mm lens and the 15*15 mm lens.

The laser speed: The laser speed means the speed to which the robot moves the laser along the cutting edge. The nominal value before this project was always 5 mm/s.

The laser angle: The laser angle this corresponds to the angle formed between the laser beam and the vertical as the following figure III.2 shows it. Figure III.2

LLLaaassseeerrr BBBeeeaaammm AAAnnngggllleee θθθ

LLLaaassseeerrr SSSpppeeeeeeddd

BBBaaarrr

AAAnnngggllleee (((999000°°°---θθθ)))

Figure III.1



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The temperature and the laser power

For Duroc´s equipment, the temperature control is based on pyrometer measurements. According to the pyrometer measurements, the power of the laser is automatically adjusted to reach the programmed temperature. These two parameters vary one according to the other, they are not independent.

Coating Finally, we have the coating question. An absorbing coating can be applied to the metal surface to avoid unnecessary power loss by reflection. Surface absorbs more heat what increases the process of hardening. Practically, it was a black painting applied on the surface just before that we use the laser.



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Before our first tests, I had some discussion with people of Tool & Die in order to have their opinions about how the hardened zone should look like. During this discussion, I get very rapidly the answer to this specific question. The laser hardened zone absolutely have to check two different requirements :

The first and the most important one is the condition required in the cutting direction (direction X). On the figure III.3, you can see what mean the terms “cutting direction” and “perpendicular to the cutting direction”.

In the cutting direction, we need the longest hardness depth as possible because this side of the tool is passing through the whole thickness of the sheet material as a consequence this side is subjected to a severe phenomenon of abrasion. In this direction, we must thus at least harden the surface of the tool which is in contact with the sheet in order to minimise this wear.

In the same time, we have also to guarantee a certain hardness depth in the direction which is perpendicular to the cutting direction. Indeed, we also need to harden a little bite in this direction because of the tool regrinding. When the tool need some maintenance, people from Tool & Die take away a layer from the cutting edge in the X direction (typically between 0,2 and 0,5 mm). As a consequence, we have to guarantee a minimal hardness depth in that direction in order to assure few regrinding processes for tool maintenance.

Figure III.3



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IIVV..33..11..11 DDuurroocc´́ss eeqquuiippmmeenntt Due to an increasing demand for surface hardening of tools, which are subjected to high loads or wear like cutting or bending tools for metal working or deep drawing dies for sheet metal forming, Duroc developed and set up this laser system. As heating source, a high power diode laser of 4 kW is used, which is adapted to a jointed-arm-robot ( 6 axis robot KUKA ) as guiding machine. Best results of the quality of the hardened surfaces can be achieved with temperature controlled transformation hardening. Therefore, the output power of the laser is controlled during the production process thanks to a pyrometer which measures several times per second the surface temperature heated by the laser. Duroc´s pyrometer measures 5% of the warmest area. In order to have a reliable measurement, the warmest area should be at least bigger than 5% of the total area. In the particular case of a cutting tool, the warmest area which is obviously the cutting edge does not represent 5% of the warmest area. As a consequence, the temperature which is indicated by the pyrometer is not exactly the right temperature obtained on the surface on the tool. Anyway thanks to their process experience, Duroc knows which temperature can be programmed to allow the maximum heat as possible in the tool without melting the edge. However, this temperature value should not be considered as absolute value. Indeed, this temperature value is only valid for the particular case of Duroc´s equipment.

Picture III.4

4 kW Laser Diode

Control Panel 6 Axis Robot



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The picture III.4 gives an overview of this equipment which provides all laser hardening for this project.

IIVV..33..11..22 TTeesstt ddeessccrriippttiioonn We started with a first loop of 12 tests. The purpose was to investigate the laser angle effects and the lens effects. During this first loop, we tested this two parameters separately in order to determine exactly the effects of each one of them. As a consequence, we tried during this tests : the two different lens ( the 8*8 mm lens and the 15*15 mm lens) three different angles which are 45°, 50°,60°

The other parameters like the laser speed and the temperature were fixed. We adopted the nominal values that Duroc was using at this time in their process i.e. 5 mm/s for the laser speed and 1060°C for the laser temperature. Figure III.5 shows the procedure for this first loop, it shows the different parameters combinations we tested. Figure III.5 shows how the tests has been practically performed. Indeed, we decided to laser harden different tracks on the same bar to save money and time. Each laser track was set with different parameters combinations and they measured 50 mm long which was the dimension required to cut properly a sample for the laboratory analysis. Furthermore, we separated the tracks from 10 mm in order to avoid any parasitic effects to occur between two tracks as annealing for instance.

Figure III.5

Angle 145º

Angle 2 50º

Angle 3 60º

Angle 1 45º

Angle 2 50º

Angle 360º

Temp 1060°C Speed 5 mm/s

Optic 1 8mm*8mm

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Optic 2 15mm*15mm



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444000000 mmmmmm

111000000 mmmmmm 40 mm

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50 mm 63 mm

IIVV..33..11..33 MMaatteerriiaall uusseedd For those tests, we used two different tool materials:

The first tool steel was Fermo. We used one Fermo rolled bar and performed the tests described before as picture III.6 and picture III.7 show it.

The dimensions of the Fermo bar was 100mm*40mm*400mm.

We also used Carmo as second tool steel for those tests. The next pictures show the Carmo rolled bar after laser hardening process.

The dimension of the Carmo bar was 63mm*50mm*400mm.

IIVV..33..11..44 LLaabboorraattoorryy iinnvveessttiiggaattiioonn ddeessccrriippttiioonn Then, we have to proceed to the laboratory investigation to collect the information that we need. We want to study two main parameters : The first parameter is the hardness profile of the cutting edge, we want to check if

we obtained correct hardness with the laser hardening process. The second parameter is the geometry of the hardened zone obtained which

should respect the requirements described before. All these laboratory analysis were done in the Volvo Cars Body Components laboratory (Quality & Environment Department 30 111).

Picture III.6 Picture III.7




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IIVV..33..11..44..11 SSaammppllee pprreeppaarraattiioonn

The first operation is to prepare the sample for the laboratory analysis. So, we cut a sample exactly in the middle of each track performed at Duroc.

Then, we had the mounting operation. Picture III.10 shows the device which allows the samples to be embedded in Bakelit to facilitate their handling and their preparation.

Then, we had the regrinding operation. This operation was necessary to get a clean and plane surface. The device that you see on picture III.11 removed a layer of 1,2 mm from the sample surfaces.

Then, we had the polishing operation. The picture III.12 shows the polishing machine which consists of two discs. We always begin by the 9 µm disc. It means that after this first step, we will obtain a surface roughness equal to Ra = 9µm. Finally, we washed the sample with alcohol and we used the second disk to increase surface roughness up to Ra = 3µm. Now, the samples are ready for the different analysis.

Picture III.11

Picture III.12

Picture III.10



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IIVV..33..11..44..22 MMiiccrroohhaarrddnneessss

Finally, when the samples are ready, we can proceed to the microhardness investigation. We would like to establish some kind of hardness profile for each sample. As a consequence, we decided to perform two series of measurements for each samples as it is described in the following sketch:

Figure III.13

During all this laboratory analysis, the measurements A will always be the vertical serie and the measurements B will always be the horizontal serie. Series A just like Serie B can be each one in their turn in “the cutting direction” or “perpendicular to cutting direction”. It depends for each sample which serie has the better hardness depth. Indeed, the measurement serie which has the better hardness depth corresponds to the “cutting direction” and of course the other measurement serie is perpendicular to the cutting direction. That was not possible to predict which side was corresponding to which direction because the etching has to be performed after the microhardness investigation. As a consequence, we could not see the heat affected zone as it is possible after the etching. This protocol had the big advantage to give us hardness profile information in the two directions which are the most interesting for us. Picture III.14 show the equipment which was used for this microhardness investigation.

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Picture III.14



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Once this microhardness investigation finished, we studied the geometry of the heat affected zone. For that, we proceeded to an etching with Nital ( chemical composition 96% Alcohol, 4% Nitric Acid ). After this etching, we can see optically the difference phase in presence in our sample and so we can easily differentiate the heat affected zone from the base material. In that way, we are now able to study the geometry of this heat affected zone and also measure to it.

IIVV..33..22 RReessuullttss aanndd DDiissccuussssiioonn On the next page, you can see the results synthesis for Fermo material and Carmo material for both direction (i.e. “the cutting direction” and in “the perpendicular to the cutting direction”). You can also see the whole laboratory report in APPENDIX 1.

IIVV..33..22..11 FFeerrmmoo rreessuullttss ddiissccuussssiioonn According to these two documents, we can very rapidly see that the lens

8mm*8mm was not suitable for the laser hardening of tool. Indeed, the hardness depths obtained with this 8*8 lens are significantly less than the hardness depths obtained with the 15*15 lens. People of Tool & Die immediately rejected the 8*8 results because the hardness depth was not sufficient. As a consequence, we definitely gave up this lens and decided to use exclusively the 15mm*15mm for the following tests.

If we look carefully on the heat hardened zone obtained on the laboratory report, we can notice that the angle 60° seems to be the most suitable angle in the laser hardening case. Indeed, we obtain with the 60° angle and the 15*15 lens a hardness higher than 600 Vickers for the first 3,5 mm in the cutting direction. This 600 Vickers is the hardness limit given by the Volvo Standard for a cutting tool. With the same 15*15 lens but with the 45° angle, we only get this hardness limit for the first 2,75 mm. Between this two angles, we get 0,75 mm difference which is significant. In the perpendicular direction, we get a hardness higher than 600 Vickers for the first 2,25 mm with the 60° angle and the 15*15 lens whereas we obtained this hardness limit up to the first 2,5 mm with the 45°angle and the 15*15 lens. In that direction, the difference between the two angles is smaller and in the same time it is not so critical to have a such important hardness depth as it is in the cutting direction. Indeed, according to Tool & Die People, 2 mm in Y direction is enough : to assure a good cutting edge support to allow some maintenance like regrinding operation

As a consequence, both of the two angles get a sufficient hardness depth in that direction.

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To sum up, we find out that the combination of the lens 15*15 mm plus the angle 60° were the best solution for Fermo material. Indeed, the two profiles obtained in both directions were satisfying. According to these acceptable results, it was useless to perform more investigation for the temperature and the laser speed parameters.

As a consequence, we decided to retain the following parameters as final parameters for Fermo material :

Laser Temperature 1060°C Laser Speed 5 mm/s Laser Angle 60° Lens 15 mm * 15 mm

IIVV..33..22..22 CCaarrmmoo rreessuullttss ddiissccuussssiioonn As you can see with the graphs on the previous page, the results for Carmo material

are not so good than Fermo results. Indeed, we do not obtain any “hardness plateau”. The hardness falls directly starting from the first millimetre. Even with lens 15*15 and the angle 60° (which was the best parameter combination for Fermo material), we only get 2,5 mm up to 600 Vickers for Carmo in the cutting direction instead of 3,5 mm for Fermo with the same parameters. If we still continue to compare Fermo and Carmo results with the previous parameters, we can see that the hardness falls down under 600 Vickers after 1,5 mm in the perpendicular direction for Carmo instead of 2,25 mm for Fermo.

As a consequence, we can notice that Carmo results are significantly under Fermo results but the question is why such big difference between the two materials ? In order to answer to this question, we should compare the chemical composition of these two tool steels. The following tables are the chemical compositions of Fermo and Carmo which are given in the Volvo Standard: Fermo

C Si Mn P S Cr Cu Al Mo Ni min % 0,46 0,30 0,80 - - 1,4 - 0,010 - - max % 0,50 0,50 1,00 0,025 0,035 1,6 0,15 0,030 0,15 0,25

Carmo

C Si Mn P S Cr Mo V Cu min % 0,57 0,20 0,70 - - 4,30 0,40 0,15 - max % 0,61 0,50 0,90 0,025 0,005 4,70 0,60 0,25 0,25

You can see that the main difference between these two steels is the chromium content. For Carmo, the maximum chromium content is 4,70% instead of only 1,6% for Fermo which is nearly three times more.



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As we explained it in theoretical part (chapter III.1), it is very difficult to formulate any general rules regarding the influence exerted by the alloying elements. Indeed, it depends on each alloying element and each situation. But, in our particular case, the chromium alloy seems to show very negative aspects for many different reasons : First, during the heating period, Chromium increases the austenitisation temperature. Indeed, elements like Chromium, Tungsten, Molybdenum, Vanadium and Silicon tend to stabilise ferrite. These elements are more soluble in α-iron than in γ-iron. They diminish the amount of carbon soluble in the austenite and thus tend to increase the volume of free carbides in the steel for a given carbon content. Thus, with above, a certain amount of each of these elements the austenite phase disappears and ferrite exists from the melting point down to room temperature which prevents normal heat treatment from occurring. Secondly, during quenching, the temperature at which martensite begins to form (Ms) is progressively lowered as the chromium content of the steel increases as the empirical formula (Steven and Haynes) shows it :

It means that the martensite begins to form later as a consequence the amount of martensite obtained after the room temperature quenching will decrease automatically. The only positive effect of Chromium for the hardening process is that Cr decrease significantly the “critical cooling velocity” which is the minimum cooling speed which will produce martensite from austenite. But, one of the specificity of laser hardening is its high cooling rate which can reach 104 °C/s. So, the only positive aspect of Chromium is completely useless for this particular situation. To sum up, we find out that Chromium prevents austenite from forming but it also delays the start temperature of martensite. These problems relative to the Chromium content can explain the difference between the Fermo results and the Carmo results.

As a consequence, we decided to proceed to new tests in order to get better results for Carmo and we decided to investigate the effects of the laser speed and laser temperature parameters. By reducing the laser speed, more time is given for the carbon diffusion process which can improve the results and we tried to increase the laser temperature which can have also positive effects. However, we decided to retain the 60° angle and the 15*15 mm lens as final parameters for Carmo but the two other parameters are still missing :

Laser Temperature ? Laser Speed ? Laser Angle 60° Lens 15 mm * 15 mm

Ms (°C)= 561 – 474 (%C) – 33 (%Mn) – 17 (%Ni) – 17 (%Cr) – 21 (%Mo)



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IIVV..44..11..11 TTeesstt ddeessccrriippttiioonn The first step was to make two extra tests with Carmo material with the following parameters:

Lens 15*15; Laser angle 60°; Laser Speed 4 mm/s; Laser temperature 1060°C Lens 15*15; Laser angle 60°; Laser Speed 4 mm/s; Laser temperature 1070°C

For this tests, we used a Carmo rolled bar with the following dimensions 32,5 mm* 63mm * 80 mm.

IIVV..44..11..22 RReessuullttss aanndd DDiissccuussssiioonn The following graphs show the results for the “cutting direction” and the perpendicular

with this direction. I also add on the same graph the result obtained with the first parameter i.e. 15*15; 60°; 5 mm/s; 1060°C to compare it with the new harness profiles. As you can see on the following graph, the hardness profiles obtained with the new parameters are much better than the old one. First, you can easily notice that the new profiles reach 800 Vickers instead of 750 Vickers for the old one. Secondly, we can see for the new profiles that we obtain a hardness higher than 600 Vickers for the first 4 mm instead of only 2,5 mm for the old one Furthermore, we can also notice that we get some kind of “hardness plateau”. Indeed, the hardness is still higher than 500 Vickers after nearly 8 millimetres.

Carmo materialHardness vs Depth

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The following graph shows the hardness profile in the perpendicular direction. In this direction, the difference is less important but we still notice that the two new profiles are a little bit better than the first one.

Now, if we compare the two new profiles, we can see that they are very similar for

both directions. Moreover, we can not program a temperature higher than 1070°C otherwise we will burn the cutting edge which must be absolutely avoid. If the results are not improved with this 1070°C, we should not use this temperature because we put more energy in the tool that it is necessary which is bad for the dimensional stability. As a consequence, we can affirm that it is no use to change the temperature and we should keep 1060°C which is nominal value for the process.

After this extra tests with Carmo, we decided to adopt the following parameters as final parameters :

Laser Temperature 1060°C Laser Speed 5 mm/s Laser Angle 60° Lens 15 mm * 15 mm

Carmo materialHardness vs Depth

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IIVV..44..22..11 TTeesstt ddeessccrriippttiioonn According to the previous results, we planed to test Sleipner material with the following parameters: Lens 15*15 and 60° angle (this combination gave the best heat affected zone geometry for

both Fermo and Carmo). Laser temperature: 1060°C Laser speed tested: 4mm/s and 5mm/s. Indeed, we have seen with Carmo that this

parameter had the major effects on the hardness profiles. We used a Sleipner rolled bar which was prehardened by Uddeholm Tooling. Indeed, Sleipner was the material which had the weakest hardness base material:

Material designation Hardness (HB) Fermo (V-2247) 250-290 Carmo (V-2249) 240-270 Sleipner (V-2263) 235

As a consequence, we decided to preharden this material in order to get better base material support. Actually, Uddeholm Tooling does not deliver Sleipner with any prehardening. If a market is created for this material, Uddeholm is ready to satisfy the request and to deliver a prehardened Sleipner. The dimension of this bar was: 50 mm * 50 mm * 300mm

IIVV..44..22..22 RReessuullttss && DDiissccuussssiioonn You can see on the following graphs the results of these tests in the two directions. We can notice that those results are not the best ones. Indeed, we didn’t get any hardness plateau, the hardness is directly falling down even with the laser speed of 4 mm/s.

Picture III.15



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As the above graph shows it, we can see that the laser speed doesn’t have as positive effect as for Carmo material. In the cutting direction, we are up to 600 Vickers for the first 2,75 mm which is acceptable but it is not the best results. In the other hand, we are still up to 500 Vickers for the first 6 mm in the same direction.

We exactly notice the same results in the perpendicular direction. We got a hardness up to 600 Vickers for the two first millimetres.

Sleipner materialHardness vs Depth


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Why this results ?

It is exactly the same reasons than for Carmo material. If we look carefully to the chemical composition, you can see that Sleipner has between 7,6% and 8% of Chromium which is 5 times more than Fermo and 1,5 times more than Carmo.

C Si Mn Cr Mo V min % 0,82 0,7 0,38 7,6 2,4 0,35 max % 0,97 1,1 0,62 8,0 2,6 0,55

As a consequence, the problems related with the Cr are amplified because of this significant Chromium content. This explanation seems to be verified. Indeed, we noticed that the hardness profile was severely affected each time that we tried to laser harden a steel which contained a big amount of Chromium.

We can also explain this weak results by the fact that Sleipner has much more carbon in its chemical composition than Fermo and Carmo. As we have already explained it, the Ms temperature is affected by the alloy content and the carbon content :

This rules shows that a majority of alloying elements lower Ms temperature of the start of the martensite formation as well as Mf, the finish of the martensite formation i.e. at 100% martensite (this temperature is generally about 215°C below the Ms). It can be noted that carbon has the strongest influence on the Ms temperature this is why for the majority of steels containing more than 0,50% C, Mf lies below room temperature. It means that it is not theoretically possible to get 100% of martensite with normal quenching (room temperature quenching). It can also be a reason which can explain those results. It is so that Sleipner seems to have the weaker results with laser hardening, however it does not want to say that Sleipner is not a suitable material for cutting tool.

We have thought to decrease one more time the laser speed from 4mm/s to 3mm/s but people from Duroc thought that a such parameter would create problems with the dimension stability. As a consequence, we decided to keep the following parameters as final parameter for Sleipner material:

Laser Angle 60° Laser Speed 4 mm/s Lens 15*15 Laser Temperature 1060°

We always tried to find out the parameters which were the most effective regarding the heat affected zone but we also tried in the same to minimise the energy during the laser hardening process to keep a good dimension stability.

Ms (°C)= 561 – 474 (%C) – 33 (%Mn) – 17 (%Ni) – 17 (%Cr) – 21 (%Mo)



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On the following page, you can see the three different hardness profiles in both directions.

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Final parameters for the three materialsHardness vs Depth


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Final parameters for the three materialsHardness vs Depth


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We also investigated some laser process critical places for the particular case of a cutting edge. These two examples are typically two cases where hardening is a little more delicate as a consequence we should pay attention to them. Those two cases are:

Is there any difference of hardness profiles between the middle and the corner of a track? Which are the problems when we overlap two different tracks?

In this chapter, we will discuss about those problems and try to find a solution to prevent them from occurring. All the following tests have been performed on Sleipner material. We used the same prehardened rolled bar as before.

IIVV..66..11 IInnssiiddee // CCoorrnneerr pprroobblleemm For this test, we used those parameters:

Laser Temperature 1060°C Laser Angle 60° Laser Speed 5 mm/s Lens 15 mm * 15 mm

We hardened a track along the bar and we decided to take one sample in the middle of the track and another one in the corner of the bar at the end of this same track as the following sketch shows it: As the following graphs show it, we can not see any significant difference between these two measurements in both directions. This is not a surprising result because we know perfectly that the laser process has the particularity to have a good temperature control. This is why we should not have any difference between those two measurements.

1st Measure: Corner

2nd Measure: Inside

Sleipner Bar

Figure III.16

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Sleipner materialHardness vs Depth (perpendicular to the cutting direction)

Comparison between two different measure points

0

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0 1 2 3 4 5 6 7 8Depth (mm)

Har

dnes

s (H

v)

CORNER 60 degrees 15*15 1060 C 5mm/s INSIDE 60 degrees 15*15 1060 C 5mm/sHardness limit

Bar

1st Measure : Corner

2nd Measure : Inside

In both directions, the difference is so small that we could not know in advance which curve corresponds to which profile.

Sleipner materialHardness vs Depth in the cutting direction

Comparison between two differents measure points

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CORNER 60 degrees 15*15 1060 C 5mm/s INSIDE 60 degrees 15*15 1060 C 5mm/sLimit hardness

Bar

1st Measure : Corner

2nd Measure : Inside



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IIVV..66..22 OOvveerrllaappppiinngg pprroobblleemm This second problem is encountered when closed forms are hardened. Indeed, when the laser arrives on its starting place, we form an overlapping zone. In this area, we often get some annealing. The annealing phenomenon occurs when the heat provided by the second pass of the laser approaches the hardened track. The heat gives annealing area which means that the hardness decreases or falls down to base material hardness. Of course, these effects are unwanted and we try as much as possible to avoid it. One way is for example to place one insert along the way traversed by the laser as shown on the next sketch III.17:

The idea is to remove the insert before the laser hardening. Then, we reassemble the insert which was hardened beforehand. In that way, we prevent overlapping zone from occurring and of course we avoid any weak area caused by annealing phenomenon. For these tests, we used the following parameters on the same prehardened Sleipner material than before.

Laser Temperature 1060°C Laser Angle 60° Laser Speed 4 mm/s Lens 15 mm * 15 mm

We made one laser hardened track with the previous parameters and then we created an overlapping area by performing a second identical laser track as it is described below on the sketch III.18:

TRACK WHICH HAS TO BE HARDENED

REMOVABLE INSERT

Figure III.17

DIRECTION 2nd TRACK

DIRECTION 1st TRACK

OVERLAPPING

NORMAL

OVERLAPPING LENGTH 15mm

Figure III.18



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IIVV..66..22..11 SSoolliidd aannggllee pprroobblleemm When we look to the two profiles below, we can notice two different zones for both directions. In the cutting direction:

For the first 2,75 mm, we can notice that the two hardness profiles are exactly the same in the normal hardened zone and in the overlapping area. The two curves are matching perfectly. After the 2,75 mm, we can see that the hardness profile of the overlapping area is falling down directly to the base material hardness whereas the hardness of the “normal” profile is still up to 500 Vickers for the first 6 mm.

In the perpendicular direction, we got the same kind of results:

For the first millimetre, the two curves of the two different profiles are exactly the same, no difference at all. After this first millimetre, we can see the same phenomenon. The hardness of the overlapping area is falling down directly to the base material hardness whereas the hardness of the normal hardened track is still up to 500 Vickers after 3 mm.

Now, we can ask the following questions: Why this difference between the two profiles ? Why do we obtain this difference only in one special zone of the profile?

Figure III.19 The energy distribution of Duroc´s Diode laser was uniform. As a consequence, the temperature is uniform when the laser heats a plane surface but it is not the case for the geometry of cutting edge. Indeed, for a cutting edge, we have of course an angle which makes a big difference with the laser hardening of a plane surface. Because of this angle, the corner will not receive the laser light in the same way everywhere on the cutting edge as the previous sketch shows it. The surface absorption is depending on different parameters. One of them is the solid angle. The solid angle translates mathematically the fact that a surface does not absorb in the same way according to the angle between the laser beam and the workpiece surface. You can easily understand that the temperature on the cutting edge will not be the same everywhere.

AAnnnneeaalliinngg zzoonnee

TTooooll aallrreeaaddyy LLaasseerr

hhaarrddeenneedd

BBeetttteerr aabbssoorrppttiioonn aarreeaa HHiigghheerr tteemmppeerraattuurree

LLeessss aabbssoorrppttiioonn LLoowweerr tteemmppeerraattuurree

Laser beam Even energy distribution



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In the middle of the laser beam, close to the cutting edge where the laser heats nearly perpendicularly the surface, we will get the warmest area. At the edges of the beam, where the laser is oblique compared to the surface, we get less absorption and a lower temperature. As a consequence, in the first area, the laser will reharden the cutting edge and there will not be any hardness difference during the overlapping track. But, in the second area, the temperature is not as high as the first one. This is why in this second zone we will get some annealing phenomenon which can explain the sudden fall of hardness starting from a certain depth. As you can understand, this problem is caused by the fact that the temperature is not uniform all around this cutting edge because of this angle. But, is there any solution to that problem? Can we obtain better temperature uniformity on the cutting edge? The answer to this question is YES. To increase this temperature uniformity, we just have to modify the energy distribution. Indeed, we have to put more energy on surfaces which absorb less (at the edges of the laser beam) and we should decrease the laser intensity on surfaces which absorb more (in the centre of the laser beam).

To sum up, we can say that the uniform distribution is maybe the most suitable energy distribution to harden plane track but obviously, it is not the optimal energy distribution to harden a cutting edge because of the problem explained before. More investigation should be done in that way to check this theory and to find out if we can get better results by using such kind distribution.

Figure III.20

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0

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60 degrees 15*15 1060 4mm/s Annealed 60 degrees 15*15 1060 4mm/s Lower Limit

DIRECTION 2nd TRACK

DIRECTION 1st TRACK

OVERLAPPING

NORMAL




0

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0 1 2 3 4 5 6 7 8Depth (mm)

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60 degrees 15*15 1060 4mm/s Annealed 60 degrees 15*15 1060 4mm/s

DIRECTION 2nd TRACK

DIRECTION 1st TRACK

OVERLAPPING

NORMAL




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IIVV..66..22..22 GGeenneerraall aannnneeaalliinngg pprroobblleemm The annealing problem discussed before is specific to laser hardening of cutting tools. Indeed, this problem does not concern induction hardening because in this process, the heat is not provided via light beam. So, the solid angle problem does not occur with induction hardening process. However, there is always one area where we get annealing problem for every selective hardening processes as the following sketch shows it:

As you can see on figure III.21, we obtain a weak zone caused by annealing phenomenon just after the end of the second track. Indeed, the heat generated by the second track will diffuse a little bit after the end of this track. As a consequence, we inevitably get one annealing area in front of the second track. This problem is not specific to laser hardening, it occurs for all the selective hardening processes. This problem is very often avoided by using inserts as we seen before.

More research can be done on this specific area because I did not investigate this problem which is a recurring problem for all heat treatment processes. In order to sum up, we can say that we must be very attentive with these overlapping problems. Indeed the laser hardening problem seems to be even more sensitive than other processes to this annealing phenomena because of solid angle problems for example.

Figure III.21



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IIIVVV...777 DDDiiimmmeeennnsssiiiooonnnaaalll SSStttaaabbbiiillliiitttyyy

We have also tested the dimensional stability problem which is a crucial point. Indeed, the most interesting thing with laser hardening is that we should not make any post machining after this heat treatment process. But, in order to do so, we have to keep a good dimensional stability during this process otherwise it is not possible to use directly those hardened tools. Indeed, between the upper tool and the lower tool, there is a small clearance which is an essential parameter in order to get a good cut quality (this clearance will be discussed later in chapter IV.1.2). The tolerance related to the dimension changes should not of course exceeds this clearance. This is why, it was so important to choose the parameters which were the most optimal for the hardness and the dimension stability. As a consequence, we performed a test on a Carmo rolled bar which had representative dimensions. We made a laser harden track over the entire length of the bar with the parameters which has been chosen for Carmo material.

As you can see on the picture, we made a lot of measurement points in the three directions in order to detect the dimension changes. We used a measuring machine which is able to measure with a margin of 0,005 mm. We get those results:

Before Laser Hardening

(mm)

After Laser Hardening

(mm) 1 63,405 63,420 2 63,405 63,425 3 63,405 63,425 4 63,405 63,425 5 63,405 63,425 6 63,410 63,425


1 3 2

4 5 6

7 8 9

10 11 12

250 mm

63 mm

100 mm



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Before Laser

Hardening (mm)


(mm)

Before Laser Hardening

(mm)


(mm) 7 100,065 100,100 13 248,255 248,215 8 100,065 100,090 14 248,255 248,255 9 100,065 100,090 15 248,245 248,260 10 100,065 100,100 16 248,255 248,255 11 100,070 100,090 17 248,245 248,255 12 100,070 100,090 The values in red are the values for which we obtain the greatest difference in each direction. The biggest difference is obtained for the measurement 13 which is totally normal because this measure was in the longest direction of the bar. We have found out that those results were acceptable but it was the upper limit for this material. It means that we should not decrease the laser speed one more time for Carmo otherwise we will put too much energy into the tool and get some problem with dimensional stability.

63 mm

100 mm

15 14 13

16 17



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VVV... LLLiiifffeee CCCyyycccllleee SSStttuuudddyyy After the laboratory investigation, we proceed to press tests in order to check the ductility and the wear properties of the laser hardened tools with the parameters chosen in the laboratory investigation.



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VVV...111 TTThhheee fffiiirrrsssttt sssttteeeppp::: ccchhheeeccckkkiiinnnggg ppphhhaaassseee

Before we begun the long wear tests, we have:

To check the laser hardened tools. We have to verify that the parameters adopted for each material are correct to allow those tools to cut the material sheets planned for the long wear tests.

To plan the investigation for this life cycle study. The wear is not something very easy to measure as a consequence we have to find some ways to evaluate this wear. This is why we performed some press tests with the three different tools that we built especially for this checking phase. Those tests take place in IUC and we used a conventional mechanical press which you can see on the following pictures: We laser hardened the three different tools that we built especially for this checking phase. We used the parameters chosen in the laboratory investigation to laser harden this Fermo, Carmo and Sleipner tool.

Figure IV.1

PPRREESSSS PPAARRAAMMEETTEERRSS PPrreessss SSppeeeedd == 5555 ssttrrookkeess//mmiinn

CCuuttttiinngg ddeepptthh == 33 ttiimmeess tthhee tthhiicckknneessss ooff tthhee sshheeeett mmaatteerriiaall CCuuttttiinngg cclleeaarraannccee == 77%% ooff tthhee tthhiicckknneessss ooff tthhee sshheeeett mmaatteerriiaall



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VV..11..11 DDiimmeennssiioonnaall ssttaabbiilliittyy After the laser hardening, we checked the dimensional stability. We used the same measuring machine as for the laboratory investigation. We performed measurements before and after the laser process with the protocol described on the sketch:

UUPPPPEERR PPAARRTT FFEERRMMOO LLOOWWEERR PPAARRTT FFEERRMMOO Nr Before After Nr Before After 1 78,225 78,235 1 38,380 38,405 2 78,230 78,230 2 38,375 38,370 3 78,235 78,245 3 38,385 38,410 4 98,250 98,280 4 70,035 70,070 5 98,255 98,265 5 70,030 70,045 6 98,260 98,280 6 70,035 70,075 7 200,205 200,160 7 200,530 200,460 8 200,310 200,270 8 200,555 200,555

1

2

3

4

5

6

8

7 UUPPPPEERR

9988 mmmm

220000 mmmm

7788 mmmm

1

2

3

4

6

5

78

7700 mmmm

220000 mmmm

3388 mmmm

LLOOWWEERR

Measurements Protocol for FERMO and CARMO tools

Picture IV.2



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UUPPPPEERR PPAARRTT CCAARRMMOO LLOOWWEERR PPAARRTT CCAARRMMOO Nr Before After Nr Before After 1 78,230 78,220 1 38,420 38,440 2 78,230 78,220 2 38,405 38,400 3 78,235 78,235 3 38,410 38,430 4 98,255 98,270 4 70,035 70,060 5 98,250 98,255 5 70,030 70,025 6 98,250 98,270 6 70,030 70,045 7 200,270 200,210 7 200,610 200,530 8 200,210 200,210 8 200,600 200,600

The Sleipner tools were a little bite smaller in one direction than Fermo and Carmo tools as you can see on the following sketch:

UUPPPPEERR PPAARRTT SSLLEEIIPPNNEERR LLOOWWEERR PPAARRTT SSLLEEIIPPNNEERR

Nr Before After Nr Before After 1 78,230 78,235 1 38,410 38,440 2 78,230 78,225 2 38,400 38,385 3 78,230 78,235 3 38,385 38,385 4 98,260 98,285 4 70,025 70,055 5 98,255 98,260 5 70,025 70,035 6 98,255 98,270 6 70,030 70,065 7 181,905 181,820 7 171,545 171,375 8 181,850 181,810 8 171,535 171,530

1

2

3

4

5

6

8

7 UUPPPPEERR

9988 mmmm

118811 mmmm

7788 mmmm

1

2

3

4

6

5

78

7700 mmmm

117711 mmmm

3388 mmmm

LLOOWWEERR

Measurements Protocol for SLEIPNER tools



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VV..11..22 TTooooll pprreeppaarraattiioonn aanndd mmiiccrrooccrraacckkss cchheecckk Then, we brought back the tools to IUC in order to prepare them for the tests. We

prepared the tools as close as possible to what would be the reality if the laser process is used one day at VCBC. Indeed, the main advantage of the laser process should be the suppression of the post machining. As a consequence, we decided to only polish the tools to respect this consideration. So, we only used this stone called BRYNE to polish manually the two edges of the cutting tools. It was the only preparation of the tools.

Then, we have also checked the possibility of microcracks existence after the laser process. First, we have clean and degreased the tools as show on picture IV.3. Secondly, we have recovered the tools with a special red painting as shown on picture IV.4. Finally, we have sprayed a second painting: this one was white. After this step, the eventual microcracks should appear clearly in red in this white background (picture IV.5). But, as you can see for example on the picture IV.6, nothing at all appeared during this test. So, it means that the tools were perfect without any damage on the surface.

Picture IV.3 Picture IV.4




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Finally, we set the tools for the tests. This operation consists to set the cutting

clearance between the upper tool and the lower tool. The cutting clearance is the perpendicular distance between the shearing blades as shown in figure IV.7. Cutting clearance is the most important parameters in obtaining satisfactory results from the shearing process. Optimum cutting clearance depends on plate thickness and material strength. This parameter is determined on the basis of wear, burr height, dimensional tolerances, cut edge quality or as a compromise between these factors. A general rule is that cutting clearance increases with material strength. The following values of cutting clearance as a percentage of plate thickness should be regarded as rough estimate of cutting clearance in relation to the ultimate tensile strength (UTS) of the workpiece material.

If the clearance is too large or too small, the cracks will fail to meet, producing an uneven cut edge and increased burring as shown on figure IV.8.

(a) optimal cutting clearance (b) too small cutting clearance (c) too large cutting clearance

UTS Rm (MPa)

Cutting Clearance

< 450 6-8 % of plate thickness

> 450 9-12 % of plate thickness

Figure IV.7

Figure IV.8



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If the cutting clearance is too small, tool wear increases, which in turn increases both tooling costs and cutting force. The cutting clearance also has an effect on the dimensional tolerances which can be attained in shearing. If the cutting clearance is too large, we get a bad result on the geometry of the cut edge, with increased taper and larger rollover. As we have just said, a too small cutting clearance increases the tool wear and the cutting force. But, the aim of this checking phase is precisely to verify the tools ductility this is why we voluntarily adopted a too small cutting clearance of 7%. In order to set this clearance, the operators from IUC have mounted the tool on a special press which is only used for tool setting. This press is shown on picture IV.9.

VV..11..33 TTeesstt PPrroottooccooll

VV..11..33..11 FFiirrsstt lloooopp

Fermo tool We run 10 000 strokes with Docol 600 DP 1,2 mm thickness. The ten thousand strokes have been performed with the coil Nr 1 (see APPENDIX 5 for the tensile stress and the oil measurement).

Carmo tool We have run 5 000 strokes with Docol 600 DP 1,2 mm thickness. The five thousand strokes have been performed with the coil Nr 1 (see APPENDIX 5 for the tensile stress and the oil measurement).

Sleipner tool We have run 5 000 strokes with Docol 600 DP 1,2 mm thickness. The five thousand strokes have been performed with the coil Nr 1 (see APPENDIX 5 for the tensile stress and the oil measurement).

Picture IV.9



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VV..11..33..22 SSeeccoonndd lloooopp

Fermo tool We run 5 000 strokes with Docol 600 DL 1,95 mm thickness. The five thousand strokes have been performed with the coil Nr 1 (see APPENDIX 6 for the tensile stress and the oil measurement).

Sleipner tool We have run 5 000 strokes with Docol 600 DL 1,95 mm thickness. The five thousand strokes have been performed with the coil Nr 1 (see ANNEXE 6 for the tensile stress and the oil measurement).

Carmo tool We have run 5 000 strokes with Docol 600 DL 1,95 mm thickness.

From 0 to 600 strokes Coil Nr 1 was used From 600 to 5000 strokes Coil Nr 2 was used

See APPENDIX 6 for the tensile stress and the oil measurement.

VV..11..44 BBuurrrrss iinnvveessttiiggaattiioonn As we have seen, cutting clearance is an important factor in cut edge quality. Normal shearing methods produce a cut edge which consists of a bright zone and a fracture zone. There is also an indentation at the top edge known as rollover, and a ragged edge or burr at the bottom as shown on next figure IV.10: The material deforms plastically in a very small region around a line connecting the edges of the upper tool and die. This plastic deformation is greatest in the areas nearest to the upper and lower cutting edges. Added to the elastic bending which takes place in the initial stage, this plastic deformation causes a residual deformation, or rollover, of blanked components. The plastic deformation experienced by the material before it breaks produces a notable increase in hardness within the cut surface, so that residual stresses occur. The bright zone is where the upper tool penetrates the material before the formation of the cracks.

Figure IV.10



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The eventual fracture produces a rough and somewhat irregular surface known as the fracture zone. This area extends into the burr which is formed as the material is draw out from the original cut edge. This occurs during the final phase of shearing. Burr height depends on cutting clearance, workpiece strength and tool condition. It should be kept as low as possible. As the following sketch explained it, we decided to investigate the cut edge.

As a consequence, we picked up regularly some cut parts during the press tests in order to make this investigation. We chose one point measurement which was always at 20 mm from the same edge of the cut part. At this point, we cut the part and look at the two cut edges. We analysed and measured the bright zone, the fracture zone and the burrs.

VV..11..55 RReessuullttss aanndd ddiissccuussssiioonn

VV..11..55..11 RReessuullttss ooff tthhee ffiirrsstt lloooopp

The Fermo tool run 10 000 strokes with Docol 600 DP 1,2 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Fermo.

The Carmo tool run 5 000 strokes with Docol 600 DP 1,2 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Carmo.

The Sleipner tool run 5 000 strokes with Docol 600 DP 1,2 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Sleipner. The following graph is showing the burr heights according to the number of strokes for the different tool materials with Docol 600 DP 1,2 mm thickness sheet material.

150 mm

8 mm

20 mm

CUT PARTS

CUT EDGE ANALYSIS

CUT EDGE ANALYSIS



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We can see on this graph that the results for Fermo and Carmo are very close for the first five thousand strokes and we can notice that Sleipner burrs are smaller than the burrs of Fermo and Carmo. We can already see with only 5 000 strokes that Sleipner seems to produce less burrs than Fermo and Carmo.

See the LABORATORY REPORT in APPENDIX 2.

VV..11..55..22 RReessuullttss ooff tthhee sseeccoonndd lloooopp

The Fermo tool run 5 000 strokes with Docol 600 DL 1,95 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Fermo.

The Carmo tool run 5 000 strokes with Docol 600 DL 1,95 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Carmo.

The Sleipner tool run 5 000 strokes with Docol 600 DL 1,95 mm thickness without any problem. Nothing has been detected on the tool. The test is successful for Sleipner. The following graph is showing the burr heights according to the number of strokes for the different tool materials with Docol 600 DL 1,95 mm thickness sheet material.

Burrs height on Docol 600 DP 1,2 mm vs

Strokes number

05

101520253035404550

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Stokes number

Bur

rs h

eigh

t (µm

)

Fermo tool Carmo tool Sleipner tool

We have checked that Fermo, Carmo and Sleipner are able to cut Docol 600 DP 1,2 mm thickness sheet material.



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We can see on this graph that the results for Fermo, Carmo and Sleipner are very close with this Docol 600 DL 1,95 mm thickness. Sleipner still seems to be a little bit better than the two other tool materials. We can also notice that the burrs are generally higher than the last loop with Docol 600 DP 1,2 mm thickness sheet material.

See the LABORATORY REPORT in APPENDIX 3.

Burrs height on Docol 600 DL 1,95 mmvs

Strokes number

0

10

20

30

40

50

60

70

0 1000 2000 3000 4000 5000 6000

Strokes number

Bur

rs h

eigh

t

Fermo Tool Sleipner Tool Carmo Tool

We have checked that Fermo, Carmo and Sleipner are able to cut Docol 600 DL 1,95 mm thickness sheet material.



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VVV...222 TTThhheee ssseeecccooonnnddd sssttteeeppp::: lllooonnnggg wwweeeaaarrr ttteeessstttsss

Now, we know that the tools did not have any major problem so we could perform long wear tests in order to study the life cycle study of those tools. But, this time, each test of each tool material has been performed simultaneously with induction hardened inserts and laser hardened inserts. As a consequence, the aims of those long wear tests were:

To perform semi industrial speeded tests

To compare results of different tool materials

To compare results of two different processes ( Laser & Induction)

VV..22..11 TTeesstt ddeessccrriippttiioonn We have continued to use the same mechanical press test at IUC but this time the tests were much longer in order to study the wear behaviour of the different tools. For those long wear tests, we used exclusively the hard sheet material tested in the checking phase:

Docol 600 DP 1,2 mm thickness Docol 600 DL 1,95 mm thickness

The width of these two sheet materials was 150 mm and the feed rate was set to 8 mm as it is described on the figure IV.11.

The press speed (55 stokes/min), the cutting depth (3 times the sheet thickness) and the cutting clearance (7% of the sheet thickness) remain unchanged.

Cut Pieces Cutting tool

8 mm

1,2 mm and 1,95 mm

150 mm

Docol 600

Figure IV.11



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In order to test simultaneously the laser process and the induction process, we decide to mount on the same tool two different inserts: one was laser hardened and the second one was induction hardened. As you can see on the figure IV.12, we mounted on the upper part of the tool the two inserts. On the other hand, the lower part was mounted with one unique insert which was laser hardened.

Picture IV.13 shows the two inserts mounted on the upper tool. The picture IV.14 is showing the Fermo laser hardened insert in the foreground whereas we can see the Fermo induction hardened insert in the background.


TOOL

INSERTS

UUPPPPEERR PPAARRTT LLOOWWEERR PPAARRTT

Laser hardened

Induction hardenedFigure IV.12



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VV..22..22 TTooooll iinnvveessttiiggaattiioonn aatt TTOOPPOONNOOVVAA

VV..22..22..11 TTooppoonnoovvaa’’ ss eeqquuiippmmeenntt After each test, we have analysed the cutting edge of the tool in order to evaluate the wear which also means the material loss. This tool investigation has been performed at TOPONOVA which is a company specialised in surface engineering in Halmstad (SWEDEN). We used the profilometer that you can see below which has a resolution better than 6 nanometers.

The stylus that you can see above is the organ of the profilometer which is in contact with the surface to be analysed. For our investigation, we used the following stylus: Stylus Ruby ball tip R = 0,5mm as you can see on the figure IV.15:

RR == 00,,55 mmmm

Figure IV.15

TThhee ssttyylluuss HHiigghh RReessoolluuttiioonn PPrrooffiilloommeetteerr



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VV..22..22..22 IInnvveessttiiggaattiioonn PPrroottooccooll The aims of this tool investigation were:

Make profiles on the 3 different tools before the wear test Make profiles on the 3 different tools after the 50.000 strokes Finally, compare those profiles & estimate the material loss

But with this protocol, we get a major problem. Indeed, we absolutely need a surface reference which was indispensable to:

To match correctly two profiles To estimate as close as possible the material loss

This is why we decided to adopt the non working surfaces as reference surfaces as it is explained on the following figure IV.16: In the non working area, there is not possibility of wear. It means that those surfaces will remain unchanged during the press test. As a consequence, those surfaces are excellent reference surfaces because of this last consideration. Thanks to this surface reference, we were able to match the initial profile with the profile after the wear in good conditions for each different tool. We used the software Autocad 2000 in order to match the profiles and also to calculate the difference area between them at the cutting edge. This difference area between the two profiles was calculated in square of 0,1 mm side centred exactly on the cutting edge angle as it is described on the figure IV.17.

figure IV.16



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In order to scan the cutting edge, we decided to perform three different measurement tracks in order to get three different profiles for each tool.

The above picture is showing you the profilometer tracks protocol. As you can see on the figure IV.18, we performed three different tracks which were all three in the working area. The name of these tracks means the distance which separates the tracks from the non working side. As you can see, the 99 mm track is corresponding to a measure which exactly performed at the working corner whereas the 40 mm and 90 mm tracks are corresponding to two other different measures of the cutting edge in the working area.

IINNIITTIIAALL PPRROOFFIILLEE

PPRROOFFIILLEE AAFFTTEERR RRHHEE TTEESSTT

MMAATTEERRIIAALL LLOOSSSS

LLiimmiitt ooff tthhee ssqquuaarree ((00,,11mmmm ssiiddee)) FFoorr tthhee ccaallccuullaattiioonn ooff ssuurrffaaccee aarreeaa bbeettwweeeenn tthhee ttwwoo pprrooffiilleess

Figure IV.17

LASER INDUCTION

Sheet width 150 mm

40 mm90 mm

99 mm

40 mm90 mm

99 mm Figure IV.18



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As the Sleipner tool was a little bit smaller, the 90 mm and 99 mm tracks were replaced by 80 mm and 89 mm tracks.

On the previous pictures, you can see the stylus of the profilometer which is measuring the cutting edge profile.

VV..22..33 CCuutt ppaarrttss iinnvveessttiiggaattiioonn We also performed investigation on the cut parts. Indeed, we wanted to investigate both the tools themselves but also the cut parts as we did in the checking phase. As we explained before, the sheet material was cut by two different inserts (laser & induction) in the same time. As a consequence, the cut part had a side which was cut by the laser hardened insert and the second side was cut by the induction hardened insert as you can see on the following sketch:

We analysis the appearance of the cut edge for both laser and induction side as you can see on the previous sketch. As a consequence, we cut each part at the two measurement points to be able thanks to an optical microscope to measure the bright zone, the fracture zone and the burrs.


Induction side Laser side

150 mm

8 mm

20 mm 20 mm

Measurement of the burr heights

Measurement of the burr heights

Figure IV.21



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Of course, this time we only analysed the cut edge of the part which was formed due to the upper tool. The other cut edge of the part caused by the lower tool was not interesting so we did not investigate it.

VV..22..44 TTooooll pprreeppaarraattiioonn We adopted the same tool preparation as for the checking phase. We only used the BRYNE stone to polish manually the two edges of the cutting tools in order to be as close as possible of the laser hardened tool preparation which would be adopted in the production. We performed this preparation for both laser and induction hardened tools. Indeed, it was not possible to prepare one tool in one way and the second tool in an other way.

VV..22..55 TTeesstt pprroottooccooll See APPENDIX 5 & APPENDIX 6

Fermo tool

We have run 50 000 strokes with Docol 600 DP 1,2 mm thickness.

From 0 to 28 500 strokes Coil Nr 1 was used From 28 500 to 50 000 strokes Coil Nr 2 was used

Fermo tool

We have run an other 50 000 strokes with Docol 600 DP 1,2 mm thickness.

From 50 000 to 71 000 strokes Coil Nr 2 was used From 71 000 to 100 000 strokes Coil Nr 3 was used

Carmo tool

We have run 50 000 strokes with Docol 600 DL 1,95 mm thickness.

From 0 to 18 000 strokes Coil Nr 2 was used From 18 000 to 50 000 strokes Coil Nr 3 was used

Sleipner tool

We have run 50 000 strokes with Docol 600 DL 1,95 mm thickness.

From 0 to 50 000 strokes Coil Nr 3 was used

VV..22..66 RReessuullttss aanndd ddiissccuussssiioonn

VV..22..66..11 FFeerrmmoo rreessuullttss We run 100 000 strokes with Fermo tools and Docol 600 DP 1,2 mm in two steps:

The first 50 000 strokes The last 50 000 strokes



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The first diagram is the results of the cut parts investigation. It gives the burrs height vs. the strokes number. The burrs height is one mean to evaluate the wear of the upper tool. We can notice that the burrs height difference between the laser and the induction tool after 100 000 strokes is only 8 µm which is nothing (See APPENDIX 4).

The second diagram is the results of the tool investigation. It gives the difference area between the profiles before and after the test in square millimetre for each track measurement.

Burrs height vs Strokes number

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000Strokes number

Hei

ght (

µm)

Laser Induction

Material loss

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0,0035

0,004

40 90 99Track position

Area

(mm2 )

Laser Induction



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We can notice that after 100 000 strokes, we detected for the three measurements less material loss for Fermo laser tool than for Fermo induction tool.

VV..22..66..22 CCaarrmmoo rreessuullttss For Carmo tools, we run 50 000 strokes with Docol 600 DL 1,95 mm. After 50 000 strokes with Docol 600 DL 1,95 mm, we can see that the burrs height difference between the cut parts of the laser and induction tools is not more than 10 µm (See APPENDIX 4). One more time, it seems that there is not significant difference between the two inserts after this test.

According to the tool investigation, the next diagram shows that Carmo laser results are worse than Carmo induction ones for the 40 mm track but the results are quite the same for the 99 mm track and even better for the 90 mm track.


0102030405060708090

100

0 10000 20000 30000 40000 50000Strokes number

Hei

ght (

µm)

Laser Induction

After 100.000 strokes with 600 DP 1,2 mm thickness, Fermo tools and the cut quality are still perfect



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0

10

20

30

40

50

60

70

0 10000 20000 30000 40000 50000Strokes number

Hei

ght (

µm)

Laser Induction

VV..22..66..33 SSlleeiippnneerr rreessuullttss For Sleipner tools, we run 50 000 strokes with Docol 600 DL 1,95 mm. You can notice that the results of the two inserts are still very close but we can also say that Sleipner burrs heights are lower than the Carmo and Fermo ones (See APPENDIX 4).

After 50.000 strokes with 600 DL 1,95 mm thickness, Carmo tools and the cut quality are still perfect

Material loss

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0,0035


Area

(mm

2 )

Laser Induction



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One more time, we can clearly see that the wear of the Sleipner tools seems to be much lower than the Carmo tools however the Sleipner and Carmo inserts run exactly the same test i.e. 50 000 strokes with Docol DL 1,95 mm. Furthermore, for the Sleipner tool investigation, we find out that the laser results were all significantly better than the induction ones.

VV..22..77 RReessuullttss SSyynntthheessiiss First, we have to say that it was very difficult to differentiate those close results because the tools and the cut quality were still very good for the three tool materials after their respective tests. It was necessary to perform two different investigations in order to determine which of the two processes is the most suitable for hardening of cutting tools.

Anyway, we have to keep in mind that all those results were already very promising. However, we can highlight the following points:

It is perfectly possible to cut Docol 600 material with laser hardened tools. Those results were not expected at all. Indeed, people of Tool & Die thought that laser hardened tools were able to cut only material up to 1,2 mm thickness. They thought also that laser hardened tools were not suitable to cut Docol 600 material. We have proved that this thinking is not right because we run tests with this Docol 600 for the three tool materials without any problem.

Material loss

0

0,0005

0,001

0,0015

0,002

0,0025


Area

(mm

2 )

Laser Induction

After 50.000 strokes with 600 DL 1,95 mm thickness, Sleipner tools and the cut quality are still perfect



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We have compared the results of the Fermo tools after 50 000 strokes and after 100 000 strokes. Indeed, as we performed the 100 000 strokes in two steps, we had the possibility to investigate the tools after each step. In that way, it gives us an idea about the evolution of the wear during the test: Do the wear of the tools increase progressively during the tests? Do the wear of the tools increase rapidly to a plateau and then stabilise? The results show that the wear occurs mainly during the first 50 000 strokes then it seems to stabilise as the two following diagrams show it:

AAfftteerr 5500..000000 ssttrrookkeess DDooccooll 660000 11,,22 mmmm AAfftteerr 110000..000000 ssttrrookkeess DDooccooll 660000 11,,22 mmmm

Laser hardening generally results in lower material loss than induction hardening.

Indeed, we found out that we get more often better results for laser hardened tools than for induction tools. We can also say that the results of the cut parts investigation do not show any significant difference between the burrs of the laser and induction hardened tools.

We also highlighted during those tests the excellent properties of Sleipner material. If we look at the tool investigation results, we can clearly see that Sleipner material loss is much lower than Carmo material loss, however Carmo and Sleipner have run exactly the same test. Moreover, if we look at the cut parts investigation results, we can notice that the burrs heights of Sleipner cut parts are also lower than the burrs heights of Carmo cut parts. As a consequence, both investigations seem to show that Sleipner material is the most efficient material for cutting tool.

Material loss

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0,0035

0,004


Are

a (m

m2)

Laser Induction

Material loss

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0,0035

0,004


Area

(mm

2)

Laser Induction

Mas

ter T

hesi

s Rep

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Lase

r Har

deni

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Stud

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Bur

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Stro

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020

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4000

060

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8000

010

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Stro

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Height (µm)

Lase

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Burr

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Stro

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num

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2000

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Height (µm)

Lase

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Burr

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VVVIII... CCCOOONNNCCCLLLUUUSSSIIIOOONNN After those unexpected and very good results, laser hardening seems to be a real alternative to induction hardening. We have proved thanks to our laboratory scale tests that laser results were as good as induction results and even better. We find out that the laser can be a real opportunity to save money and reduce the maintenance cost of trim dies at VCBC but of course more investigation remain to be done before the definitive adoption of the laser hardening process at VCBC. As a consequence, extra investigation should be carried out in order to answer to the following questions: What will be the results with laser and induction hardened casted steels on the scale of

production?

Is it possible to improve the laser process with a different energy distribution ?

More investigation is required about overlapping & annealing zone which are the weak point of the laser process. More investigation is required about Sleipner material which has given high quality

results during the tests? As this material tool has shown a very good dimensional stability, can we put more energy into this material in order to get even larger hardness depth ?



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VVVIIIIII... TTTHHHAAANNNKKKSSS TTTOOO Nader ASNAFI, VCBC (34423)

Thanks to Mr ASNAFI, I was under the best working conditions during this 5 months in this R&D Forming & Materials department at VCBC.

Tuve JOHANSSON, VCBC (34423)

Mr JOHANSSON was my supervisor during the training, I would like to thank him very much for his precious help which allowed me to conduct and complete successfully the thesis.

Istvan SARADY, LULEÅ UNIVERSITY OF TECHNOLOGY Mr SARADY accepted to be my examinator for Luleå University and provided me documentation about laser processing which was very useful. Jan KVIST, DUROC Mr. KVIST helped very much during the laser process investigation by sharing his knowledge. Roger STIGSSON, VCBC (33462) Mr STIGSSON helped me a lot to discuss the results and took important decisions. Muamer LAPOVSKI, Stefan SVENSSON, Jerzy SZADURSKI, VCBC (30111) They helped me a lot with their support during my laboratory investigation. Bengt OHLSSON, Jukka RAJALAMPI, IUC They helped me a lot to prepare the tools and perform the press tests during all the project. Ronny LUNDBERG, VCBC (33465) Mr LUNDBERG helped me very much with the Autocad support.



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VVVIIIIIIIII... RRREEEFFFEEERRREEENNNCCCEEE Publications

[1] “Sheet Steel Forming Handbook”, SSAB TUNNPLÅT, Edition 1 [2] “Laser Surface Hardening”, Swedish Institute for Metals Research [3] D.I. Pantelis, E. Bouyiouri, N.Kouloumbi, P Vassiliou, A. Koutsomichalis, “Wear and corrosion resistance of laser surface hardened structural steel”, Surface and Coatings Technology 298 (2002) 125-134 [4] Sung-Joom Kim, Chang Gil Lee, Tae-Ho Lee, Chang-Seok Oh, “Effect of Cu, Cr and Ni on mechanical properties of 0,15 wt.% C TRIP-aided cold rolled steels”, Scripta Materialia 48 (2003) 539-544 [5] A. Roy, I. Manna, “Laser surface engineering to improve wear resistance of austempered ductile iron”, Materials Science and Engineering A297 (2001) 85-93 [6] Y. Totik, R. Saleder, H. Altum, M. Gavgali, “The effects of induction hardening on wear properties of AISI 4140 Steel in dry sliding conditions”, Materials and Design 24 (2003) 25-30

Internet web site:

[1] http://www.key-to-steel.com/ [2] http://www.unc.edu/~dtmoore/laser_intro.html [3] http://www.columbia.edu/cu/mechanical/mrl/ntm/ch2index.html

Documents

Laser Hardning of Cutting Tools - DiVA portal1022680/FULLTEXT01.pdf · are hardened by flame hardening or induction hardening process. Indeed, VCBC has developed their own equipment