Induction Surface Hardening

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TECHNOLOGY FORCES (Technol. forces): PAF-KIET Journal of Engineering and Sciences

Volume 01, Number 02, July-December 2007

Abstract

In this paper, major factors in design of high frequency induc-

tion-heating equipment for surface hardening purpose are dis-

cussed. After a brief theoretical review of the basic concepts

involved in surface hardening, the influence of ratio current

penetration depth to the workpiece diameter (d/) on the effi-

ciency and the power induced in the workpiece are discussed.

Different types of hardening and quenching techniques, which

are to be given due considerations while designing the equip-

ment, are also described in detail. Mathematical computationshave been intentionally avoided to ease matters for non-special-

ist readers. In conclusion, factors affecting the inductor design

and the design requirements for radio frequency power source

are outlined.

1. INTRODUCTION

Induction surface hardening is widely applied in transport, ma-

chine tool and metal industries, engaged in heat treatment of

machining elements. The advantages that led to the widespread

application of induction surface hardening include: rapid heat-

ing, low scaling, less machining, fast cycle time, precise control

of temperature, localized heating, no decarburization and no

large scale grain.

Induction surface hardening is a heat treatment process used to

increase the durability of machine tool elements subjected to

high stresses. By durability is meant the improvement in the

resistance to wear and greater torsion strength. Many types of

steel are surface hardened with heat to increase toughness and

resistance to wear. High quality alloy steel can be replaced by

cheaper carbon steel which has been surface hardened by induc-

tion method. The induction method of hardening offers the pos-

sibility of confining the heat to the outer layer subjected to

stresses without affecting the hardness of the core. A tough

original core with hardened surface layer offers considerable

mechanical and dynamic advantage.

2. INDUCTION HARDENING CONCEPTS

In induction surface hardening only the outer surface layer is

heated to a hardening temperature (7500C for steel) and then

cooled immediately by means of spray of water, air or oil after a

certain metallurgical allowable time. Not all the materials are

capable of being hardened. The hardenable material must contain

alloy or carbon content as it plays a vital role in the build-up of

the desired hardness. For adequate hardness, the carbon content

present in the material should not be less than 0.35%. Steel,

alloy steel and castings are some examples of the material ca-

pable of being hardened.

Fig: 1 shows the basic arrangement for induction surface harden-

ing comprising of an inductor made of hollow copper tubing

which surrounds the workpiece to be hardened. and a power

source supplying the inductor with AC frequency power. The

magnetic field established in the inductor induces a voltage in the

workpiece, which drives a current on the surface of the workpiece

and heats the steel temperature,

Increase in the work piece is both due to Joules heat and under

certain circumstances due to losses that occur when the mag-

netic field reverses (hysteresis). Since the magnetic field changes

with the frequency of the applied voltage, the current distribu-

tion over the cross-section of the work piece is not uniform.

Instead the current density is maximum and concentrated on the

upper surface decreases exponentially according to: I = I0 e-x/

.....(1) from the upper surface to the interior of the workpiece

[Fig: 2]. At high frequencies the effect of the current being con-

centrated at the upper surface of the workpiece (skin effect] is

Major Factors in the Design of Induction Heating Equipment

for Surface Hardening

Junaid A. Siddiqui* and Ghulam Rasool Mughal**

College of Engineering

Pakistan Air Force-Karachi Institute of Economics and Technology, Korangi Creek

Karachi-75190 (Pakistan)Received on October 3, 2007; Accepted on November 28, 2007

* Author for correspondence. E-mail:

** E mail:

72

Fig. 1. Basic arrangement for induction hardening.


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more pronounced so that the center of the core is practically free

of current. The thickness of the layer in which the current is

attenuated to 37% of its initial value at the surface is termed as

current penetration depth . The heat distribution over thesurface of the workpiece is dependent on the depth of penetra-

tion of electrical current. For practical purposes, equation

= 503 /......(2) holds good. Here, (mm) represents thecurrent penetration depth, ( m), the specific resistance, the relative permeability (mm2) and frequency, (Hz) of the

power source required for hardening. Equation (1) indicates that

for high frequencies and low resistivity of the material selected

for hardening, the current penetration depth will also be low.

Consequently, the heat distribution and hardening depth, which

is a function of current penetration, will also be shallower andconfined to the upper surface (skin effect). This property has

been made useable in induction surface hardening where low

penetration depths are desired and are obtained at high frequen-

cies and high power densities. The skin effect alone, however

does not determine the temperature distribution over the

workpiece cross-section. As a result of conduction and depend-

ing upon heating time, a part of the heat also flows to the interior

of the workpiece. The heating time selected, is therefore, very

short so that the amount of heat that travels to the interior of the

workpiece as a result of conduction is minimum. Given the

dimensions and the hardening temperature, the hardening depth

will depened on the steel quality, frequency, power density,

heating time and the geometry of the workpiece [1, 3].

Current penetration depth is an important parameter in the de-

sign of induction hardening equipment, because it finally deter-

mines the depth of the upper surface layer being heated. Since

the magnitude of the frequency of the power source and the

parameters of the workpiece material determine the current pen-

etration depth, they have a considerable influence in the design

of induction hardening equipment including the power source.

The material parameters and are strongly temperature de-

pendent. In surface hardening of ferromagnetic materials and

steel below the Curie point, r increases in direct proportion tothe increase in temperature and it can lie in the range between 1

and 103-104. For non-magnetic materials and steel above the

Curie point (7800C), r approaches unity, thereby resulting inrapid increase of. This rapid increase of beyond the Curie

point is especially useful, since it minimizes the danger of uppersurface getting overheated. Fig. 3 shows as a function of theoperating frequencies for different material. This rapid increase

is especially visible in case of iron between 15 840 0C [3].

73

Surface distance

Currentdensity

Fig. 2. Exponential curve showing distribution of current

density as a function of distance from the

upper surface of the workpiece [1].

Operating frequency

Pene

trationdepth

Al: Aluminium; Cu: Copper; Fe: Iron; Ni: Nickel; B: Brass

Fig. 3. Current penetration depth as a function of operating

frequencies.

3. CHOICE OF FREQUENCY

In the design of induction heating equipment, it is important to

know the frequency range within which the equipment must

operate. The suitable operating frequency depends on the work

piece dimension and the desired hardening depth. The usual

frequency range lies between 500Hz - 10 kHz medium frequency

and 100 kHz - 2MHz high frequency range. The maximum en-

ergy transfer in induction hardening between inductor and

workpiece occurs at low current penetration depth (i.e high

frequency). However, it is to be noted that the thermal effi-

ciency deteriorates with decreasing . According to Kretzmann[3], the frequency selected for hardening must therefore, be such

that the current penetration depth does not exceed 1/8 times the

workpiece diameter. The substitution of this size of the

workpiece in equation 1, gives an expression: Fmin

= 16106/d2 Hz.......eq. 3. This expression can be used to find out theminimum frequency value [3]. The minimum frequency value

sets a limit while designing the power source for induction hard-

ening equipment. This limit can be exceeded if reasonable effi-

ciency is to be obtained but it cannot be lowered. The effect of

ratio d/ is shown in the Table: 1. The plot of fmin

[3] against the

workpiece dimensions gives the minimum values of frequencies

for different types of materials and is shown in Fig: 4. For a

definite frequency, there is also a minimum workpiece dimen-


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sion at which the efficiency is the most reasonable. Table1 gives

approximate formulae for calculating the minimum frequency

corresponding to minimum workpiece diameter [1, 2].

Table 1. Influence of the ratio d/ on the efficiency.

Workpiece dia 8 6 4 2 1 0.4 0.1

Current Penetration

Depth (d/)

Efficiency(%) 95 85 65 30 10 4 1

(Energy conversion)

4. INDUCED POWER

Induced power is another factor, which influences the design of

induction hardening equipment. For various geometrical shapes

to be hardened the induced power is calculated by:

P = 1,987 x 10-9 x H2 x A x m x (KW) eq. (4)

where m= f (d/) is a factor depending on the actual shape, anddimensions of workpiece and the value of the current penetra-

tion depth.

The plot of m=f (d/) for different shapes in Fig: 5 showthat mexhibits maximum at a particular value of d/. [1]. For larger ratioof workpiece diameters to the current penetration depth d/, the

power absorbed approaches nearly 100% [Table 2].

Hence, in practice attempts are made to keep the ratio d/ equalto or greater than 4. This means a power absorption of 65%,

with the decreasing ratio, however, the power absorbed reduces

so much that it nearly equals the radiation losses [3].

5. HARDENING DEPTH

Hardening depth is always greater than penetration depth of

current because even after the heated layer is cooled, the heat

penetrates still further as a result of conduction. Hardening depth

is dependent on frequency, heating time and power densities.

The heating time is further dependent on quenching techniques

and quenching medium. Besides, hardening depth depends on

factors, which vary partially during the heating cycle. The exact

determination of hardening depth is, therefore, for the same

reason not possible. However, curves have been developed which

give practical results. Fig 6(a),and 6 (b) show curves for both

single shot (static) and scanning (progressive) method of hard-ening with the help of which hardening depth for different power

densities, heating time and feed rate can be found. It is obvious

74

Fe: Iron; M: Bronze; Cu: Copper

Fig. 4. Minimum frequency in relation to the work piece

diameter for different materials [2].

Fig. 5. Effect of the ratio d/ on the induced power fordifferent geometrical shapes [3].

Diameter

Frequency

Hz

Table 2. Minimum diameter of the workpiece for definite

frequency range.

d in mm

Geometry of 25 100 400 900

the workpiece

f(KHz) 50 100 400 -

Rectangular 2500 70 50 25 1.7

d2

Circular 15000 7.8 3.9 6.5 13

d2

d/ (Ratio)

Indu

cedpower


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from the fig: 7 that by changing the heating time and power

densities, hardening depths of 0.25m and 3m can be obtained. In

case (a) power density of 2.5KW/cm2 will be required for a

heating time of 0.30 s. In case (b), 0.4KW/cm2 are needed for a

heating time of 10s.[4,5]. The surface temperatures achieved in

these cases are 850 and 1100 0C, respectively.

A comparison of these figures shows that larger hardening depths

can be obtained with smaller high frequency power. Small power

densities result in longer heating time till the hardening tempera-

ture is reached. During this long heating time the heat penetrates

deeper into the workpiece, which gives a greater hardening depth

after it, is cooled. Smaller hardening depth therefore requires

larger high frequency energy source and results in high investition

costs. [6, 4, 5].

6. HARDENING TECHNIQUES

The optimum results for a definite hardening job depend on the

hardening techniques. While designing the equipment for induc-

tion hardening, care must be taken to select the techniques, which

incur minimum costs per hardened workpiece under given elec-

trical, thermal and metallurgical conditions. Basically, two meth-

ods of surface hardening are in use, the single shot (static) andscanning (progressive) method of hardening Fig: 8a, 8b. All other

methods are either a combination or a modification of either of

these two methods. In single shot (static) hardening the

workpiece is placed inside a suitable designed inductor, the power

is switched on for a predetermined time and the workpiece is

removed and then quenched.

The quench-head is often located below the inductor and sprays

water into the workpiece at the end of the heating cycle. Single

shot method is usually applied to harden smaller cross -sections

of oblong workpieces such as hardening of bearing surfaces,

collars, undercuts and fillets.

In scanning operation the workpiece inductor are moved rela-

tively close to each other. This method is particularly suited to

the hardening of the whole surfaces of long shafts and pipes.

With the inductor usually being fixed, while the workpiece moves,

the heat is continuous and so is the quenching. The scanning

speed is 2-60m/s. The scanning speed must be such that the time

required for the movement of the workpiece into the cooling ring

lies within the metallurgical limit. In general, the length of the

hardening zone in scanning operation is greater than the length of

the inductor and the cooling ring. Instatic-hardening the induc-

75

Fig. 6. (a)Variation of hardening depth with power

density [17].

Fig. 7. Variation of hardening depth with the surface power

density at a frequency of 1 MHz [5]. T= Heating time.

f = 500 KHz; S = 00.75 mm

Fig. 6. (b) Variation of hardening depth with density [13].

Hardening depth (mm)

Powerdensity

f =1MHz

0 2 4 6 8 10 12 14


Powerdensity

800C =0.75 mmkwcm2

2

00 0.5 1.0 1.5 2.0


Powerdensity

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

100

200

kwcm2


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are used as buffers. The machine cycle is mainly determined by

the hardening sequence. Stalling of the material flow i.e a lack of

components on the incoming side and an accumulation of

workpieces on the outgoing side trigger the cycle of the devices,

without influencing the hardening results and no action has to be

taken by the operator. Fully automatic hardening machines are

usually applied for a throughput of one type only for verysimilar type of the workpieces, whereas the semi-automatic

machines are mostly used for a medium throughput of a large

variety of component type [4, 10].

In practice, machines have been developed which can be used for

both static and scanning mode of operation for surface harden-

ing. Especially two basic designs of machines have been usually

adopted, namely the vertical type design and the rotary bank

type. In the vertical type machine, Fig: 9 the workpiece is brought

into the inductor and clamped on both sides. Heating of the

workpiece takes place with or without rotation in static or scan-

ning mode of operation. This design serves the purpose of hard-

ening the workpieces such as shafts and bolts etc. Another fea-

ture of this design is that zonewise heating of the workpiece as

required in the hardening of camshafts is possible. Here, the

workpiece is moved with scanning speed.

Rotary type machines,Fig: 10, have been designed for hardening

of faces and corners of parts such as axle shaft and wheel shafts.

Two parts can be processed at a time with a production rate of

280 parts per hour or even more. The machine has a turn-table

with as many as six stations: In loading, reloading, checking

stations to determine if parts are properly loaded, heating and

quenching. The quench-head is integrated into the inductor etc.

Machines are used for static hardening but can be equipped with

vertical type scanning machines [10, 7].

8. QUENCHING TECHNIQUES

Besides frequency, heating time and power, hardening depth is

also dependent on quenching techniques, medium of quenching

and speed of quenching. Quenching techniques consideration in

the design of induction hardening equipment are, therefore, as

important as hardening techniques. In general, quenching tech-

niques can be direct or indirect. Usually, direct method of pres-

sure spray quenching technique is chosen as it directly adapts to

the induction hardening set-ups. The quenching mechanism is

built into the inductor, Fig. 11, so as to heat and quench concur-

rently. By adjusting the distance between the quench ring and

the coil, the hardening depth can be influenced.

A quench ring is built around a multiturn coil. It sprays water

through the turns or it can be placed in the line. Inline construc-

tion (workpiece moves into the coil to be quenched) is preferred.

In a separate method, workpiece is quenched in a bath of oil or

water. Usually, bath is agitated, the degree depending on the

steel and shape of the workpiece. The quenching medium water,

oil, air or polymer based liquid is chosen to give the required

metallurgical properties.

Quenching equipment should be designed so as to produce the

desired results. For instance coupling clearance must be consid-

ered to ensure quench effectiveness. Generally in single shot or

static hardening, coupling is quite close (0.06 in). Clearance may

be increased greatly, if it is necessary to harden parts of more

than one diameter.

Since stream velocity of the quenchant drops rapidly as the

quenchant stream lengthens, quench equipment must include a

separate heat exchanger for the control of quenching tempera-

ture. Pressure control must also be provided to ensure optimum

heat removal over the entire surface. Cooling like the heating rate

and austenitising temperature in the surface zone must be uni-

form and at a rate consistent with the type of steel or geometry

of the workpiece [11].

9. INDUCTORS

The inductor is the heart of induction hardening equipment pro-

ducing a magnetic field, which is induced into the workpiece to

be hardened. In surface hardening, the workpiece is very rapidlybrought to a high temperature by means of high frequency cur-

rents, with current densities as much as 6000A/mm2. Such cur-

rent densities therefore require that the material selected for

inductors must possess high thermal conductance and low resis-

tivity. Nearly all the inductors are made of hollow, water-cooled

copper tubes of sufficient cross-section so as to give requisite

mechanical strength and also to carry currents without getting

overheated. The greater the current and power requirements, the

greater the losses. The impedance of the inductor and with that

the resistive losses are affected by various other factors such as

turns, coil diameter and frequency. This means that a coil with a

small diameter and low resistivity material (e.g. copper) and

77

Fig. 11. Inductors with built-in quenching mechanism.


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operating at low frequency will result in minimum losses. How-

ever, greater power inputs result from higher frequencies, so the

designer has to compromise between the most efficient type,

the shape and the power requirement of the material to be hard-

ened. In general high coil efficiencies (taking into account the

power loading into the metal) are achieved at higher frequencies

and large diameters. Coil design, therefore, must be such as togive the best heat pattern and highest degree of effciency.

While designing inductors for surface hardening, the impedance

of the power source must also be kept in mind. This is particu-

larly important if the maximum available power from the energy

source is to be utilized. Inductors are either conected directly to

the resonant circuit of the energy source or through a radio

frequency transformer (Fig. 12). The impedance appearing at

the terminals of the single turn and multiturn inductor lies be-

tween 5-200 m and has to be matched to the impedance of theenergy source.

The form and shape of the workpiece to be hardened alongwith

the contours of the hardening zones determine the design of theinductor. The coil or inductor must be designed to adapt exactly

to the form of the workpiece and the path of the hardening zone

so as to avoid undesireable heating zones and the heat flow

resulting due to mutual effects of the magnetic fields, in case the

inductor only approximately fits into the workpiece. Basically

two inductors are practically in use: the internal and the external

field inductor(Fig: 13), depending on whether the inductor is

surrounded by the workpiece or the workpiece encircles the

inductor. Complication arise when work pieces such as teethed

gearare to be hardened in such cases both internal and external

field configuration are incorporated in the design workpiece.

Coupling is another factor, which must be taken care of whiledesigning inductors. The current density distribution is depen-

dent on the coupling i.e. the distance between the inductor, and

the workpiece. Since magnetic fields are stronger near to the coil

than at any distance away from it, it is advantageous to place the

workpiece close to the inductor, so that the maximum of the heat

energy is transferred to it. The strength of the field varies in-

versely with the square of the distance between the workpiece

and the coil, which means that this consideration will have direct

relation to the amount of heat generated in a workpiece in a given

length of time. With multiturn coils closely coupled to the

workpiece, there is a tendency for the eddy currents to provide

a heat pattern corresponding to the helix of the coil. The wider

the pitch of the coil, the more pronounced will be this heatpattern. Therefore, with a closely wound coil the rotation of the

workpiece becomes essential. When the inductor is more loosely

coupled i. e., at a greater distance from the surface to be heated,

Fig: 14, the stream of eddy currents spreads over a wider area

and rotation of the workpiece may not be necessary..In surface hardening, the inductor currents are of higher frequen-

cies and therefore, the current densities in the inductors will not

be uniform due to skin effect. A further factor, which causes

Fig. 12. HF - matching transformer.

Fig. 13 (a,b). Field distribution pattern of internal and

external field inductors.

Fig. 13(c). Some designs of internal and external

field inductors.

departure from the uniformity, is the proximity effect, which

will cause the bulk of the inductor currents to flow in that part of

the conductor nearest to the workpiece. Although air insulation

between adjacent turns of the coil is adequate from the electrical

point of view, it does not always give the requisite mechanical

rigidity for a coil used in production situation and spacers of

suitable insulation material, which must be capable of with-standing the high temperatures, are used [7, 8, 3].

The production of coils for heating simple shapes to a uniform

depth is straightforward, but designs are very complex in many

instances such as heating of gears where the correct distribution

of magnetic field is important. In many cases, it may be neces-

78

(a) Internal (b) External


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The DC terminals of the inverter are tightly coupled to RF

bypass capacitor whose capacitance is sufficient to pass the AC

component of inverter input without substantially changing its

DC potential. The Ac terminals of the inverter drive the RF load

circuit which is essentially a high Q series resonant circuit formed

by tuning capacitor, C and inductive coil Lf.A radio frequency

transformer matches the load impedance to the VA capability of

the inverter, while coupling capacitor prevents any DC current

from flowing in the primary winding and saturating the core.

In operation, the MOSFET transistors are switched as diagonal

pairs, Q1and Q

2alternating each half cycle with Q

3and Q

4to

provide a square wave voltage output at the AC terminals of the

inverter. The waveform of the output current depends on the

inverter frequency, which is the switching rate of the MOSFET

transistors.

Driving the series resonant load off resonance i.e. at a MOSFET

switching frequency differing from the natural resonant fre-

quency of the C1and L

fresults in a low output current, while

driving it at resonance results in a maximum power to the load

coil. Infact the output current is controlled in a closed loop byvarying the driving frequency [14].

10. REFERENCES

[1] RWE-Essen: Inductive Erwaermung, Physikalische

Grurundlagen und technische anwendungen, 2.Band 1979

Energie-Verlag, GmbH, and Heidelberg.

[2] W.Barth: Die anwendbarkeit und die Grenzen der

induktives Haertung Teil:I Fertigungstechnik 7.Jg, Heft

6,june 1957

[3] Benkowsky Induktives Erwaermung 4.bearbeitete Auflage,

VEB-Verlag Berlin. , 1980

[4] G.W.Seulen: An up-to-date look on induction hardening

equipment.and F.H.Reinke Elektrowaerme International

31 (1973) B4

[5] AEG-Elotherm: Induktives Randsicht heartens von

Stahteilen Merkblatt 236

[6] Kurt Flicke: Hochfrequenz-Induktionshaerteanlage Draht-

Welt Ausgabe 1969.Nr.3, Seite 65-169.

[7] W.E. Mulane: Coil design for HF induction heating Metal

treating Aug-sept.1963.

[8] P.G.Simpson: Induction heating, coil and system design.

McGraw-Hill.

[9] F.W. Curtis High frequency Induction Heating McGraw-

Hill, Newyork

[10] Oliver S. ParkSingle shot hardening by induction heating

[11] George Lendl: Why quenching is important in inductionHeating? Metal progress Dec 1967

[12] S.N.Okele Application of Thyristor inverters in induction

heating and melting. Electronics and power March 1978.

[13] S.R.Pelly: Latest developments in static high frequency

power sources for induction Heating.

[14] Solid -state Generator for induction hardening print from

Conference record industry applications Society IEEE,

IAS, annual meeting, October 4-7, 1982 , San Fransisco.

[15] G.W.Seulen: Entwicklungsstand der Induktionhaerte-

technik fuer Kurbelwelle Klepzig Fachberichte 1/72

[16] J. A. Siddiqui: Leistungselektronische Speisegeraete fuer

die inductive erwaermung Unter besondere Beruecksichti-

gung der verschiedenen technologischen Verfahren unter

der Netzanschluessprobleme. [Unpublished Thesis]

[17] VDIArbeitsblatt Induktives Haerten

81

Professor of electronics at the College

of Engineering PAF-KIET, Korangi

Creek Karachi received his B.Sc. (Hons.)

in 1964, M. Sc (for Hons.) in 1965 from

the University of Sindh, Jamshoro Sindh

and PhD in 1974 from the University of

Southampton, England.

Prior joining the PAF-KIET, Dr. Mughal was professor of

Microelectronics and an associate Dean at the Institute of

Business and Technology, BIZTEK, Main Ibrahim Hydri

Road, Karachi.Before adopting full time teaching profession at PAF-KIET

and BIZTEK, Dr. Mughal was Chief Scientific Officer (BPS-

20), and served as a Head of the Research Division, Applied

Physics, Computers and Instrumentation Technology Re-

search Division, PCSIR Karachi Laboratories Complex,

Karachi-75280.

Dr. Mughal joined PCSIR Karachi Laboratories in June 1966

as Research Assistant and successively rose to the post of

Chief Scientific Officer. The main task was full time scien-

tific and Industrial Research.

During his stay at PCSIR, he was visiting professor part-

time, under the UGC (Higher Education Commission) teach-

ing programme in the Department of Physics, University of

Sindh, Jamshoro (1975-77) and taught of Microelectronics

to M. Sc (Final) students. Dr. Mughal also went to Iraq and

served as a visiting professor in the College of engineering

and the College of Sciences, University of Basrah, Basrah,

Iraq and taught Microelectronics and Physics to B.E. stu-

dents (1980-82).

Dr. Mughal assisted in establishing, Institute of Industrial

Electronics Engineering (IIEE-PCSIR) faculty of NED Uni-

versity of Engineering & Technology. He taught Solid-state

Devices Technology/Integrated Circuits & Physics to B. E

students (1989-94). Simultaneously Dr. Mughal was teach-

ing Industrial Electronics and Physics to the Post-diploma

students at PSTC-PCSIR (1989-94).

Dr. Mughal has 36 years R&D experience and 15 years teach-ing experience in Pakistan and abroad. He has also industrial

experience in respect of repair and calibration of electronic

gadgets.

Dr. Mughal has 30 publications of international repute to his

credit. He is member of a number of societies in Pakistan.

Research Concentration and Areas of Subject

Microelectronics, Physics & Technology of Solid-state De-

vices, Instrumentation and Education Delivery Systems in

Science and Engineering Faculty.

Professor Dr. Ghulam Rasool MughalCMILT

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Induction Surface Hardening