The electrodes generally are made of a low resistance alloy, usually copper, and are designed in many different shapes and sizes depending on the application needed.

The two materials being welded together are known as the workpieces and must conduct electricity. The width of the workpieces is limited by the throat length of the welding apparatus and ranges typically from 5 to 50 inches. Workpiece thickness can range from 0.008in. to 1.25in.

After the current is removed from the workpiece, it is cooled via the coolant holes in the center of the electrodes. Both water and a brine solution may be used as coolants in spot welding mechanisms.

2-2-2 Seam Welding

Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material.

A transformer supplies energy to the weld joint in the form of low voltage, high current AC power. The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed together by the welding pressure that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.

A common use of seam welding is during the manufacture of round or rectangular steel tubing. Seam welding has been used to manufacture steel beverage cans but is no longer used for this as modern beverage cans are seamless aluminum.

Fig.(2-31): seam welding

2-2-3 Flash Welding

Flash welding is a type of resistance welding that does not use any filler metal. The pieces of metal to be welded are set apart at a predetermined distance based on material thickness, material composition, and desired properties of the finished weld. Current is applied to the metal, and the gap between the two pieces creates resistance and produces the arc required to melt the metal. Once the pieces of metal reach the proper temperature, they are pressed together, effectively forging them together.

Fig.(2-32): flash welding

Applications

According to the Journal of Materials Processing, the railroad industry is using flash welding to join sections of mainline rail together. This mainline rail is also known as continuously welded rail (CWR) and is much smoother than mechanically joined rail because there are no gaps between the sections of rail. This smoother rail reduces the wear on the rails themselves, effectively reducing the frequency of inspections and maintenance. [2] In other countries, continuously welded rail is used on high speed rail lines because of its smoothness. A study published in Materials Science and Design proved that flash welding is also beneficial in the railroad industry because it allows dissimilar metals and non-ferrous metals to be joined. This allows crossings, which are generally composed of high manganese steel, to be effectively welded to the carbon

steel rail with the use of a stainless steel insert, while keeping the desired mechanical properties of both the rail and the crossing intact. The ability of this single process to weld many different metals with simple parameter adjustments makes it very versatile. Materials and Design discusses the use of flash welding in the metal building industry to increase the length of the angle iron used to fabricate joists.

2-3 Welding Defects 2-3-1 Introduction

Common weld defects include:

i. Lack of fusion ii. Lack of penetration or excess penetration iii. Porosity iv. Inclusions v. Cracking vi. Undercut vii. Lamellar tearing

Any of these defects are potentially disastrous as they can all give rise to high stress intensities which may result in sudden unexpected failure below the design load or in the case of cyclic loading, failure after fewer load cycles than predicted.

2-3-2 Types of Defects

i and ii. Lack of fusion and Lack of penetration

To achieve a good quality join it is essential that the fusion zone extends the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a join. However thicker material will normally need edges cut at a V angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends the full thickness of the metal. Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or

electric).

Excess penetration arises from too high a heat input and / or too slow transverse of the welding torch (gas or electric). Excess penetration - burning through - is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal.

Fig.(2-33): Lack of fusion

iii. Porosity - This occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal or, from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and work is carefully cleaned and degreased prior to welding.

iv. Inclusions - These can occur when several runs are made along a V join when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run.

Fig.(2-34): Inclusions

v. Cracking: This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.

Solidification Cracking

This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels.

Solidification cracks are normally distinguishable from other types of cracks by the following features:

they occur only in the weld metal - although the parent metal is almost always the source of the low melting point contaminants associated with the cracking.

they normally appear in straight lines along the centreline of the weld bead, but may occasionally appear as transverse cracking.

solidification cracks in the final crater may have a branching appearance. as the cracks are 'open' they are visible to the naked eye.

On breaking, the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries and are characteristically inter dendritic. There may be evidence of segregation associated with the solidification boundary. The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

insufficient weld bead size or inappropriate shape. welding under excessive restraint. material properties - such as a high impurity content or a relatively large

shrinkage on solidification.

Joint design can have an influence on the level of residual stresses. Large gaps between components will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking.

In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese and sulphur can help to reduce the risk. To minimize the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03% might be needed.

Weld metal composition is dominated by the filler and as this is usually cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Parent metal composition becomes more important with autogenous welding techniques, such as TIG with no filler.

Avoiding Solidification Cracking

Apart from choice of material and filler, the main techniques for avoiding solidification cracking are:

control the joint fit up to reduce the gaps. clean off all contaminants before welding. ensure that the welding sequence will not lead to a buildup of thermally induced

stresses. choose welding parameters to produce a weld bead with adequate depth to

width ratio or with sufficient throat thickness (fillet weld) to ensure the bead has sufficient resistance to solidificatiuon stresses. Recommended minimum depth to width ratio is 0.5:1.

avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains. As a rule, weld beads with a depth to width ratio exceeds 2:1 will be prone to solidification cracking.

avoid high welding speeds (at high current levels) which increase segregation and stress levels across the weld bead.

at the run stop, ensure adequate filling of the crater to avoid an un-favorable concave shape.

Hydrogen induced cracking (HIC)

also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself.

Four requirements for HIC to occur are:

a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses into the HAZ.

b) A HAZ microstructure susceptible to hydrogen cracking.

c) Tensile stresses act on the weld.

d) The assembly has cooled to close to ambient - less than 150oC.

HIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction.

Fig.(2-35): Cracks and porosity

vi- Undercutting: In this case the thickness of one (or both) of the sheets is reduced at the toe of the weld. This is due to incorrect settings / procedure. There is already a stress concentration at the toe of the weld and any undercut will reduce the strength of the join.

vii- Lamellar tearing: This is mainly a problem with low quality steels. It occurs in plate that has a low ductility in the through thickness direction, which is caused by non- metallic inclusions, such as sulphides and oxides that have been elongated during the rolling process. These inclusions mean that the plate cannot tolerate the contraction stresses in the short transverse direction.

Lamellar tearing can occur in both fillet and butt welds, but the most vulnerable joints are 'T' and corner joints, where the fusion boundary is parallel to the rolling

plane. These problem can be overcome by using better quality steel, 'buttering' the weld area with a ductile material and possibly by redesigning the joint.

Vii- Distortion: Welding methods that involve the melting of metal at the site of the joint necessarily are prone to shrinkage as the heated metal cools. Shrinkage then introduces residual stresses and distortion. Distortion can pose a major problem, since the final product is not the desired shape. To alleviate certain types of distortion the workpieces can be offset so that after welding the product is the correct shape. Figure (2-36) describe various types of welding distortion:

(a):Transverse shrinkage (b): Angular distortion

(c): Longitudinal shrinkage (d): Fillet distortion

(e): Neutral axis distortion

Fig.(2-36): distortion

2-4 Detection

2-4-1 Visual Inspection

Prior to any welding, the materials should be visually inspected to see that they are clean, aligned correctly, machine settings, filler selection checked, etc. As a first stage of inspection of all completed welds, visual inspected under good lighting should be carried out. A magnifying glass and straight edge may be used as a part of this process. Undercutting can be detected with the naked eye and (provided there is access to the reverse side) excess penetration can often be visually detected.

2-4-2 Liquid Penetrant Inspection

Serious cases of surface cracking can be detected by the naked eye but for most cases some type of aid is needed and the use of dye penetrant methods are quite efficient when used by a trained operator. This procedure is as follows:

Clean the surface of the weld and the weld vicinity Spray the surface with a liquid dye that has good penetrating properties

Carefully wipe all the die off the surface Spray the surface with a white powder Any cracks will have trapped some die which will weep out and discolour the

white coating and be clearly visible

2-4-3 X - Ray Inspection Sub-surface cracks and inclusions can be detected 'X' ray examination. This is expensive, but for safety critical joints - eg in submarines and nuclear power plants - 100% 'X' ray examination of welded joints will normally be carried out.

2-4-4 Ultrasonic Inspection

Surface and sub-surface defects can also be detected by ultrasonic inspection. This involves directing a high frequency sound beam through the base metal and weld on a predictable path. When the beam strikes a discontinuity some of it is reflected beck. This reflected beam is received and amplified and processed and from the time delay, the location of a flaw estimated. Porosity, however, in the form of numerous gas bubbles causes a lot of low amplitude reflections which are difficult to separate from the background noise. Results from any ultrasonic inspection require skilled interpretation.

2-4-5 Magnetic Particle Inspection

This process can be used to detect surface and slightly sub-surface cracks in ferro-magnetic materials (it cannot therefore be used with austenitic stainless steels). The process involves placing a probe on each side of the area to be inspected and passing a high current between them. This produces a magnetic flux at right angles to the flow of the current. When these lines of force meet a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more than elsewhere, indicating the location of any discontinuities. This process may be carried out wet or dry, the wet process is more sensitive as finer particles may be used which can detect very small defects. Fluorescent powders can also be used to enhance sensitivity when used in conjunction with ultra violet illumination.

Lecture No.15 Week No.15 No. of hours: 2 theoretical and 1 tutorial

METAL FORMING 3-1 OVERVIEW OF METAL FORMING 3-1-1 Definitions Plastic Deformation Processes are those operations that induce shape changes on the workpiece by plastic deformation under forces applied by various tools and dies.

Bulk Deformation Processes

These processes involve large amount of plastic deformation. The cross-section of workpiece changes without volume change. The ratio cross-section area/volume is small. For most operations, hot or warm working conditions are preferred although some operations are carried out at room temperature.

Sheet-Forming Processes

In sheet metalworking operations, the cross-section of workpiece does not change-the material is only subjected to shape changes. The ratio cross-section area/volume is very high. Sheet metalworking operations are performed on thin (less than 6 mm) sheets, strips or coils of metal by means of a set of tools called punch and die on machine tools called stamping presses. They are always performed as cold working operations.

3-1-2 Material considerations Material Behavior

In the plastic region, the metal behavior is expressed by the flow curve:

σ = Κεn

where K is the strength coefficient and n is the strain-hardening (or work-hardening) exponent. K and n are given in the tables of material properties or are calculated from the material testing curves.

Flow stress

For some metalworking calculations, the flow stress Yf of the work material (the instantaneous value of stress required to continue deforming the metal) must be known:

Yf = Κεn

Average (mean) flow stress

In some cases, analysis is based not on the instantaneous flow stress, but on an average value over the strain-stress curve from the beginning of strain to the final (maximum) value that occurs during deformation:

Fig.(3-1): Stress-strain curve indicating location of average flow stress Yf in relation to yield strength Y and final flow stress Yf

The mean flow stress is defined as:

here εf is the maximum strain value during deformation.

Work-hardening

It is an important material characteristic since it determines both the properties of the workpiece and process power. It could be removed by annealing.

3-1-3 Temperature in metal forming The flow curve is valid for an ambient work temperature. For any material, K and

n depend on temperature, and therefore material properties are changed with the work temperature:

Fig.(3-2): True stress-strain curve showing decrease in strength coefficient K and strain-hardening exponent n with work temperature.

There are three temperature ranges-cold, warm, and hot working:

Fig.(3-3): Temperature range for different metal forming operations. TA is the ambient (room) temperature, and Tm is the work metal melting temperature

Cold working is metal forming performed at room temperature.

Advantages

(i) Good surface finish of the product.

(ii) High dimensional accuracy.

(iii) Superior mechanical properties, e.g. hardness and strength increase due to strain hardening.

(iv) Strain hardening may eliminate the surface hardening heat treatment required in some components such as cold rolled gears.

(v) The material develops anisotropy which may be used to advantage in subsequent forming processes. For instance, the anisotropy developed in cold rolled sheet metal helps in getting deeper draws.

Disadvantages:

(i) High loads on the equipment require it to have high strength and rigidity. This increases the cost of machines.

(ii) With strain hardening the material becomes less ductile.

(iii) limitations to the amount of forming

(iv) additional annealing for some material is required

(v) some material are not capable of cold working

Warm working is metal forming at temperatures above the room temperature but below the recrystallization one. Warm Forming has come to be used in order to gain the advantages of hot as well as cold forming, though to a lesser extent. Warm forming is carried out at a temperature higher than room temperature but lower than the re-crystallization temperature. Since yield strength decreases with increase in temperature, the load on the equipment in warm forming is lower than in cold forming. Also the temperatures are not that high that the surface layer damage can occur. Therefore, the advantages of cold forming are achieved, that is, better surface quality, better dimensional accuracy and better mechanical properties than in hot forming.

Hot working involves deformation of preheated material at temperatures above the re-crystallization temperature.

Advantages:

(i) At high temperatures, the metals become soft, its yield strength decreases and hence low forces are required for forming. This reduces the cost of equipment needed for the process.

(ii) Metals are more ductile at higher temperatures and their formability in hot state is higher than in cold state. Therefore, large deformations may be given in hot working.

(iii) The casting defects in ingots like internal shrinkage cavities (not those in contact with atmosphere) and blow holes get welded during hot working. The structure becomes more homogeneous resulting in better mechanical properties.

(iv) Due to low flow stresses at high temperatures, very large components may be made by plastic deformation.

Disadvantages:

(i) The products have low surface quality due to oxidation of surface layer.

(ii) The components formed have low dimensional accuracy.

(iii) There is little improvement in mechanical properties.

(iv) The forming tools also get heated up due to contact with hot metal and wear of tools is rapid.

3-1-4 Friction effects Homogeneous Deformation

If a solid cylindrical workpiece is placed between two flat platens and an applied load P is increased until the stress reaches the flow stress of the material then its height will be reduced from initial value of ho to h1 . Under ideal homogeneous condition in absence of friction between platens and work, any height reduction causes a uniform in-crease in diameter and area from original area of Ao to final area Af .

Fig.(3-4): Homogeneous deformation

The load required, i.e. the press capacity, is defined by

P = YfAf

Inhomogeneous deformation

In practice, the friction between platens and workpiece cannot be avoided and the latter develops a “barrel” shape. This is called inhomogeneous deformation.

Fig.(3-5): Inhomogeneous deformation with barreling of the workpiece

3-2 BULK DEFORMATION PROCESSES Classification of Bulk Deformation Processes

Fig.(3-6): Basic bulk deformation processes: (a) rolling, (b) forging, (c) extrusion, (d) drawing

Rolling: Compressive deformation process in which the thickness of a plate is reduced by squeezing it through two rotating cylindrical rolls.

Forging: The workpiece is compressed between two opposing dies so that the die shapes are imparted to the work.

Extrusion: The work material is forced to flow through a die opening taking its shape

Drawing: The diameter of a wire or bar is reduced by pulling it through a die opening (bar drawing) or a series of die openings (wire drawing)

3-2-1 Rolling Definition

Rolling is a Bulk Deformation Process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls:

Fig.(3-7): the process of flat rolling

Rolling is one of the most important bulk deformation techniques. For example, it is used to reduce the cross-section of large ingots or plate for producing a wide variety of finished and semifinished components. These include structural steel sections, automotive body sheet, food/beverage container sheet, building siding etc. Rolling mills vary in size from hand operated units for light gauges of soft metals, to units requiring thousands of horsepower.

Salient points about rolling

1-Rolling is the most extensively used metal forming process and its share is roughly 90%.

2-The material to be rolled is drawn by means of friction into the two revolving roll gap.

3-The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional.

4-The geometry of the product depend on the contour of the roll gap.

5-Roll materials are cast iron, cast steel and forged steel because of high strength and wear resistance requirements.

6-Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish.

7-In rolling the crystals get elongated in the rolling direction. In cold rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone.

8-The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough.

9-In the deformation zone the thickness of the strip gets reduced and it elongates. This increases the linear speed of the at the exit.

10-Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverses.

11-When the angle of contact α exceeds the friction angle λ the rolls cannot draw fresh strip.

12-Roll torque, power etc. increase with increase in roll work contact length or roll radius.

Steps in rolling

The preheated at 1200 oC cast ingot (the process is known as soaking) is rolled into one of the three intermediate shapes called blooms, slabs, or billets.

• Bloom is the product of first breakdown of ingot. It has a square cross section of 150/150 mm or more (cross sectional area > 230 cm2 ).

• Slab (40/250 mm or more) is rolled from an ingot or a bloom (cross sectional area > 100 cm2 and with a width ≥2 x thickness).

• Billet (40/40 mm or more) is rolled from a bloom (cross sectional area > 40x40 mm2 ).

Further rolling steps are:

• Plate is the product with a thickness > 6 mm.

• Sheet is the product with a thickness < 6 mm and width > 600 mm.

• Strip is the product with a thickness < 6 mm and width < 600 mm.

The figure below shows the rolling steps with some intermediate shapes:

Fig.(3-8): Production steps in rolling

Lecture No.16 Week No.16 No. of hours: 2 theoretical and 1 tutorial

Types of Rolling

1-Based on workpiece geometry :

a– Flat rolling: used to reduce thickness of a rectangular cross section.

b – Shape rolling: square cross section is formed into a shape. Shape rolling is accomplished by passing work through rolls that have the reverse of desired shape.

These products include:

–Construction shapes such as I-beams, L-beams, and U-channels.

– Rails for railroad tracks.

– Round and square bars and rods.

2-Based on work temperature :

a-Hot Rolling: most common due to the large amount of deformation required. Hot rolling is a rolling operation carried out at a temperature just below the metal melting point, permitting large amount of deformation.

b- Cold rolling: produces finished sheet and plate stock and is carried out at room temperature. Cold rolling is commonly conducted after hot rolling when good surface quality and low thickness tolerance are needed. Cold rolling causes material strengthening.

Below are some steel products made in a rolling mill:

Fig.(3-9): steel products made in a rolling mill

Rolling mill configurations:

– Two-high: two opposing rolls

– Three-high: work passes through rolls in both directions

– Four-high: backing rolls support smaller work rolls

– Cluster mill: multiple backing rolls on smaller rolls

– Tandem rolling mill: sequence of two-high mills

Two-High Rolling Mill

This is the simplest arrangement which consist of upper and lower driven rolls between which the workpiece passes. This mill has the advantage of relatively low momentum and, therefore, can easily reverse direction so that the workpiece can pass back and forth through the mill stand. Reversing two-high breakdown mills are often used for reducing large ingots into long slender plates. In the reversing two-high mill configuration the upper and lower rolls are driven by separate motors, to provide faster reversing action and smaller individual motor sizes. However, it is also common to drive both rolls from a single motor via a gearbox.

Fig.(3-10): ( a )Two-high mill Rolling ( b ) Reversing two-high mill Rolling

Three-High Rolling Mill

Consist of upper and lower driven rolls and a middle roll, which rotates by friction.

Fig.(3-11): three-High Rolling Mill

Four-High Rolling Mill

The four-high mill consists of two driven work rolls, with large back-up rolls that provide increased stiffness. The back-up rolls prevent the work rolls from bowing due to the rolling pressure which, if not controlled, results in rolled products, thicker at the centre than at the edges. For this reason four-high mills are used when the sheet thickness must be controlled accurately.

Fig.(3-12): four-High Rolling Mill

Tandem Rolling

After the blooming mill has reduced the ingot into a plate, many more rolling passes may be required before the strip reaches the required thinness. For efficient production the strip is rolled on a continuous production line, passing from one mill station to another at high speed without stopping or reeling between stations. A standard tandem mill will contain about six individual mill stands. The mill plant will employ dozens of these six-stand tandem mills, and some strip may continuously pass through most of them on a path a mile long.

Between the tandem mills a reservoir of strip may be provided through several mechanisms. The most common mechanism is a bend and variable position roll. This small-capacity storage is required for contingencies and for temporary speed discrepancies.

At some points along the production line, larger reservoirs of strip may be needed to assure uninterrupted running of the plant equipment. Reservoirs for coils of 100 to 200 tons of 60-in-wide steel strip, called Sendzimir spiral loopers, have been developed by T. Sendzimir, Inc. In this design the excess strip is collected as a coil, turning on a table. The incoming strip collects onto the coil on the outside while the payoff is removed from the inside, or vice versa. Two coils may share the same axis of symmetry, one above the other. The top coil may collect layers on the outside and pay off on the inside into the inside of the bottom coil, which then pays off through its outside. When excess material is coming in, the coil gets larger and reserve is built up to be released when needed. The speeds of accumulation and release of strip are controlled independently according to conditions.

While the initial stages of rolling of the ingot are performed at elevated temperature, the rolling of thin strip is done at room temperature.

Speed and roll gap control between individual stations of a tandem mill are very critical. The volume rates of strip passing through all pairs of rolls must be identical. Thus, if the volume production rate is V , and the width of the strip is assumed constant, then:

V1 t1 = v1 t2 = = vn tn = V /w (a)

The exit speed vf from each station is uniquely determined by the emerging strip thickness from that station. The rolling circumferential velocity Ů is slightly lower than the exit velocity of the strip, vf . The mill gap and speed at each station is preset before rolling starts; thereafter, both have to be continuously controlled to accommodate normal fluctuations in temperature, thickness of the incoming strip, etc. The monitoring and control station, usually above and in full view of the mill, is manned by teams of operators, each assigned the limited task of controlling the speed or gap for each station. Observing the behavior of the strip between a pair of rolls, the operator judges the changes required to increase or decrease the tension between stands. The action of each operator strongly affects the occurrences on both sides of his station and (with diminishing strength) the occurrences farther along the line. For example, an increase in speed at an

intermediate station will increase the back tension and decrease the front tension of that station. In turn, the increased back tension is also an increase in the front tension of the preceding station, which causes thinning of the emerging strip from that station. That in turn affects the incoming speed to that station, and so on. The entire team must cooperate, reacting swiftly and in complete harmony. This harmony is attained by long periods of experience combined with training of new members one at a time. At speeds of 2000 meters a minute, when a slack is starting to show, precise response must be immediate. Otherwise, tension is eliminated, the excess length of strip between the two stations doubles up and folds, and a triple layer of strip enters the gap between the rolls downstream. The station cannot handle the separation force, and a costly ($40,000) break of a roll at its transition from bearing to full diameter results.

Today, fully automated tandem mills controlled by digital computers are a reality. To utilize the mill most efficiently, all stations must deliver power to their full capacity. A program to distribute the reductions properly so that no station will be running at less than full capacity. When each station is running at full power capacity and at the optimal conditions for that station, the mill is producing at its peak rate.

Fig.(3-13): schematic of tandem mill

Fig.(3-14): volume conserved in tandem mill

Cluster Rolling Mills

The thinner the strip to be rolled, the smaller is the required roll diameter before the limiting thickness is reached, unless hydrodynamic lubrication is established. If a four-high rolling mill is used with working rolls of very small diameter and too large backup rolls (say more than twice the diameter of the working rolls), the working rolls may start to deflect horizontally. To prevent the horizontal deflection the cluster, rolling mills were introduced, using working rolls of very small diameter with a train of supporting rolls of progressively increasing diameter. Each roll is supported by two larger-diameter rolls. Thus the working roll is supported by two rolls, while the two support rolls are supported in turn by three backup rolls, as Figure (3-15) shows. The mill of Figure (3-15) is called a 1-2-3-4 cluster rolling mill. Today, 1-2-3-4-5 cluster rolling mills are available. The designs by Sendzimir have introduced new features into the old technique, so that today’s nearest to perfectly uniform thin strip is produced worldwide by the Sendzimir cluster rolling mills. Some of these features are as follows:

1. The support rolls in the last line are supported by the mill housing through their entire length. An eccentric-cam arrangement, controlled by continuous thickness measurement through the width, can rectify thickness variations locally across the width.

2. The small-diameter (12-mm) working rolls can be made of carbide, which is twice as rigid as steel and by far more wear-resistant, and which can be polished much smoother.

3. The cast or welded housing is much more rigid than in previous designs.

4. Working rolls can be changed smoothly with little time and effort.

With the introduction of the Sendzimir cluster rolling mills, the rolling speed increased impressively over that of older cluster rolling mills. Today, Sendzimir cluster mills can reach speeds of up to 150 meters per minute. However, a four- high

rolling mill still runs at 10-fold higher speeds. Small-diameter rolls cannot develop very great circumferential speed.

Fig.(3-15): cluster Rolling Mill

Planetary Rolling Mills

In a planetary rolling mill many small-diameter working rolls are arranged around a single, much larger support roll on each side of the strip. The working rolls are in rolling motion both over the workpiece and over the support rolls, thus eliminating sliding friction. The small area of contact between each roll and the workpiece minimizes the roll separation force. While the workpiece is moved slowly forward by the feed rolls, the working rolls rotate rapidly, each working roll taking a small reduction. A large total reduction in one pass of the billet is accomplished by the many passes of the many rolls. When the operation of the planetary rolling mill is preceded by a continuous casting and followed by a cluster rolling mill, a very compact plant can cover the full operation, on a continuous basis, from the melt to thin-strip product.

Sendzimir Planetary Rolling Mill

The concept of planetary rolling was first introduced by Sendzimir. In his design the backup rolls are driven while the working rolls, flexibly held in the cage (Figure 3-16), roll over the workpiece and over the backup roll. Thus, each working roll, as it contacts the workpiece, rotates around its own axis of symmetry while advancing at the same time circumferentially around the axis of symmetry of the backup roll. The circumferential speed of the working roll is intermediate between the speed of the strip and that of the back-up roll. From the moment of contact with the

strip at the entrance to the moment each individual working roll leaves the strip at the exit, its rotational speed around its own axis and its circumferential speed around the axis of symmetry of the backup roll are constantly increasing. Thus, the distances between the working rolls that are in contact with the strip at any time undergo constant change. The positioning of the individual work rolls in the cage permit changes in distance between the rolls. While each working roll takes a small reduction on the strip, it also leaves a very shallow but visible feed mark on it. These feed marks are insignificant, but if desired, they can be removed by subsequent rolling, either conventional or with a cluster rolling mill.

Krupp Planetary Rolling Mill

The concept later introduced by the Krupp-Platzer planetary mill ( figure 3-17) eliminates the problem of feed marks on the strip. Here the backup rolls are stationary, while the work rolls are separated from the backup roll by a set of small-diameter intermediate rolls. The two layers of working and intermediate rolls are housed in a driven cage. The arrangement of the stationary backup rolls permits the provision of a flat region in the exit area of the strip where the working rolls on the opposite sides of the strip move for a short while in a linear motion which eliminates the feed marks. Another alternative design is provided by replacing one set of planetary rolls and their backup with a regular large-diameter roll in direct contact with the strip. The planetary arrangement is then retained on only one side of the strip.

Fig.(3-16): Sendzimir Planetary Rolling Mill Fig.(3-17): Krupp Planetary Rolling Mill

3-2-1-1 Types of rolling processes

There are different types of rolling processes as listed below;

• Continuous rolling

• Transverse rolling

• Shaped rolling or section rolling

• Ring rolling

• Powder rolling

• Continuous casting and hot rolling

• Thread rolling

Conventional hot or cold-rolling (continuous Rolling)

The objective is to decrease the thickness of the metal with an increase in length and with little increase in width.

The material in the centre of the sheet is constrained in the z direction (across the width of the sheet) and the constraints of undeformed shoulders of material on each side of the rolls prevent extension of the sheet in the width direction. This condition is known as plane strain. The material therefore gets longer and not wider, otherwise we would need the width of a football pitch to roll down a steel ingot to make tin plate!

Fig.(3-18): continuous rolling

transverse rolling

• Using circular wedge rolls, heated bar is cropped to length and fed in transversely between rolls which are revolved in one direction.

Fig.(3-19): transverse rolling

Shaped rolling or section rolling

A special type of cold rolling in which flat slap is progressively bent into complex shapes by passing it through a series of driven rolls. No appreciable change in the thickness of the metal during this process. This Process is suitable for producing moulded sections such as irregular shaped channels and trim.

A variety of sections can be produced by roll forming process using a series of forming rollers in a continuous method to roll the metal sheet to a specific shape.

Applications:

- construction materials,

- partition beam

- ceiling panel

- roofing panels.

- steel pipe

- automotive parts

- household appliances

- metal furniture,

- door and window frames

- other metal products.

Fig.(3-20): shaped rolling

Lecture No.17 Week No.17 No. of hours: 2 theoretical and 1 tutorial

Ring Rolling

It is a rolling process in which a thick-walled ring of smaller diameter is rolled into a thin-walled ring of larger diameter.

Applications:

ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery.

As thick-walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged Hot working process for large rings and cold working process for smaller rings Ring rolling used to reduce the wall thickness and increase the diameter of a ring:

Fig.(3-21): ring rolling (1) start and (2) completion of process.

Powder rolling

Metal powder is introduced between the rolls and compacted into a ‘green strip’, which is subsequently sintered and subjected to further hot-working and/or cold working and annealing cycles.

Advantages :

- Cut down the initial hot-ingot breakdown step (reduced capital investment).

- Economical - metal powder is cheaply produced during the extraction process.

- Minimize contamination in hot-rolling.

- Provide fine grain size with a minimum of preferred orientation.

Re-rolling Mill

Fig.(3-22): powder rolling

Continuous casting and hot rolling

In this process a metal is melted, cast and hot rolled continuously through a series of rolling mills within the same process. It is usually used for steel sheet production.

Fig.(3-23): continuous casting and hot rolling processes

Thread rolling

In this process dies are pressed against the surface of cylindrical blank. As the blank rolls against the in-feeding die faces, the material is displaced to form the roots of the thread, and the displaced material flows radially outward to form the thread's crest. Threads can also be produced by feeding a blank two grooved die plates.

Advantages over thread cutting (machining):

– Higher production rates because rolled threads are produced in a single pass at speeds far in excess of those used to cut threads.

– Better material utilization

– Stronger threads with higher strength and better fatigue resistance due to work hardening.

Fig.(3-24): thread rolling with cylindrical dies

Fig.(3-25): thread rolling with flat dies