Canale L.C.F., Mesquita R.A., Totten G.E. Failure Analysis of Heat Treated Steel Components

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treating conditions based on the criterion of

minimum dimensional deviations, that is, mini-

mum workpiece distortion. Currently, measure-

ment of individual workpiece dimension is a

very practical, uncomplicated, and reliable

method of product quality assessment. Through

the appropriate selection of the grinding wheel

and grinding conditions and taking into account

the physical and mechanical properties of the

workpiece material, very favorable compressive

residual stresses in the hardened surface layer

can be retained.

How is it possible to assure a desirable surface

and surface layer quality after induction hard-

ening and ﬁne grinding? Finding an answer to

this question requires a very good knowledge of

the grinding process on the microlevel as well as

knowledge of mechanical and heat effects acting

on the layer of the workpiece, including the type

of grinding wheel and the grinding conditions.

An all-inclusive consideration of the numer-

ous inﬂuences of the tool type and condition on

the changes on the surface and in the surface

layer of the workpiece in the given machining

conditions is described by the term surface

integrity. This is a scientiﬁc discipline providing

an integral assessment of the surface and sub-

surface layer. It was deﬁned at the beginning of

the 1960s. For high-quality machine parts and

parts subjected to heavy thermomechanical

loads, different levels of description of the sur-

face integrity were deﬁned. A basic level of the

surface-integrity description includes measure-

ment of roughness and analysis of the micro-

structure and microhardness in the thin surface

layer resulting from the machining process

under given machining conditions. The second

level of the surface-integrity description in-

cludes studies of residual stresses in the surface

layer and of mechanical properties of the given

material. The third level of the surface-integrity

description includes tests making clear the be-

havior of the given part under the operating

conditions.

As essential advantage of induction surface

hardening is that it is possible to achieve a suf-

ﬁcient repeatability of the hardened layer thick-

ness on the workpiece as well as a desirable or

even prescribed hardened layer proﬁle, ensuring

sufﬁcient hardness and favorable distribution

of residual stresses in the hardened layer. A

variety of steels and a whole range of induction-

hardening methods provide the possibilities

for very accurate planning of residual stress size

and distribution. This is of growing importance,

since manufacturers are frequently required to

produce machine parts that, among other surface

properties, must have quite speciﬁc residual-

stress distribution along the depth of the hard-

ened layer. It has become a proven fact that high

compressive stresses ensure high fatigue

strength of machine components and reduce the

danger of crack occurrence and growth on the

surface of components.

In thermal hardening, the surfaces of suitable

materials, usually plain carbon, low-alloy steels,

or cast irons, are austenitized and then quenched

to produce a hard martensitic case that is usually

tempered in a subsequent operation. Case depths

are normally in the range of 0.5 to 5 mm. Case

hardness is typically approximately 700 HV on

hardening and 600 to 650 HV after tempering at

200



C (Ref 1).

Heating processes include electrical induction

and resistance (Ref 2–4), and direct impinge-

ment methods using ﬂames, (Ref 5), lasers

(Ref 6–11), and electron beams (Ref 12). Of

these, induction heating is the most widely used.

Laser and electron beam heating have recently

become established in a number of applications,

mainly where distortion was a problem with the

induction method.

Thermal treatments are mainly employed

when only local areas on machine parts require

hardening and wear resistance. Consequently,

such methods are energy efﬁcient compared

with thermochemical methods where bulk heat-

ing of batches of components is undertaken.

Normally, the surface-hardening process also

introduces compressive stresses into the surface

layers, leading to an improvement in fatigue

properties. For example, the drive shafts of

heavy lorries and buses are induction surface

hardened to improve their fatigue properties

(Ref 10, 11).

The skills and experience required for manual

ﬂame hardening have been largely superseded

by automated ﬂame techniques or by the in-

duction method. However, for one-off machine

parts, the simplicity of manual ﬂame hardening

and the extremely low capital investment ensure

that the method is still used on a regular basis

(Ref 5, 13).

From a heat treatment point of view, laser can

be considered as a versatile and ﬂexible high-

intensity heat source that can operate in air. It is

capable of undertaking a range of processes,

essentially simultaneously, since the laser beam

can be directed through air by metal mirrors and

switched and shared among a number of work

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stations. Manipulative techniques using mirrors

allow the beam to be directed to areas not

accessible by other techniques, for example, the

bores of tubes. Set against these considerations

are the high capital cost and low energy efﬁ-

ciency of the technique (Ref 1, 6, 10).

Steels for Surface Hardening

Heat treatable steels contain between 0.2 and

0.6% C and can be carbon steels or low-alloyed

steels (Table 1) (Ref 14, 15).

In practice, engineers choose these steels

because, in the soft condition (normalized or soft

annealed), they are more suitable for machining,

while their strength properties and hardness can

be subsequently reﬁned by additional heat

treatment. Soft steels have good machinability,

while additional heat treatment improves their

mechanical properties to a desirable level.

Figure 1 presents a schematic of the heat treat-

ment methods used to achieve the desired

mechanical properties (Ref 15, 16). Besides the

mentioned surface hardening, other thermo-

chemical methods can be used to create a de-

sirable wear resistance. The steels for surface

hardening are from the group of heat treatable

steels, except that they usually contain between

0.35 and 0.5% C. Heat treatable steels can be

Table 1 Suitable steels for induction surface hardening

Type of steel

Chemical composition %

C Si Mn P, max S, max Cr Mo Ni

1 0.33–0.39 0.15–0.40 0.50–0.80 0.035 0.035 ... ... ...

2 0.38–0.44 0.15–0.40 0.50–0.80 0.035 0.035 ... ... ...

3 0.43–0.49 0.15–0.40 0.50–0.80 0.035 0.035 ... ... ...

4 0.48–0.55 0.15–0.40 0.60–0.90 0.035 0.035 ... ... ...

5 0.50–0.57 0.15–0.40 0.40–0.70 0.035 0.035 ... ... ...

6 0.42–0.48 0.15–0.40 0.50–0.80 0.035 0.035 0.40–0.60 ... ...

7 0.34–0.40 0.15–0.40 0.60–0.90 0.035 0.035 0.90–1.20 ... ...

8 0.38–0.44 0.15–0.40 0.60–0.90 0.035 0.035 0.90–1.20 ... ...

9 0.38–0.44 0.15–0.40 0.50–0.80 0.035 0.035 0.90–1.20 0.15–0.30 ...

10 0.38–0.44 0.15–0.40 0.70–1.00 0.035 0.035 0.40–0.60 0.15–0.30 0.40–0.70

11 0.37–0.43 0.15–0.40 0.50–0.80 0.035 0.035 0.60–0.90 0.15–0.30 0.70–1.00

Source: Ref 14, 15

Fig. 1 Heat treatment methods for carbon and low-alloyed steels. Source: Ref 15

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supplied in soft or reﬁned conditions. Carbon

steels with less than 0.35% C are in the nor-

malized condition, and the products made from

them can be in the soft or reﬁned condition. For

improvement of the wear resistance of these

steels, additional surface hardening can be

applied (Ref 14, 17, 18).

Below 0.35% C, the treated surface is insuf-

ﬁciently hard, and above 0.6% C, there is a

danger of surface cracking after quenching. To

obtain a satisfactory hardening response in

induction surface hardening, it is necessary for

the carbides to dissolve when the steel is heated.

Thus, steels that have been quenched and tem-

pered and have small carbide particles respond

better than steels with large spheroidized car-

bides. Similarly, the rate of solution of carbides

in steels other than plain carbon and low-alloy

steel is too slow for them to be satisfactorily

induction hardened. Steels must also have suf-

ﬁcient hardenability to achieve the required

hardness at the speciﬁed depth (Ref 17).

The procedures are similar for fabrication of

machine parts made of carbon steels with more

than 0.35% C and alloyed steels, expect that

because of the higher machinability required,

they must be in the soft-annealed condition.

For induction surface hardening, it is recom-

mended to use steels with an appropriate carbon

content (0.35 to 0.45%) and a careful selection

of alloyed elements. Usually, these steels have

an increased silicon (0.15 to 0.40%) and man-

ganese (0.50 to 1.00%) content and other

alloying elements, such as chromium, moly-

bdenum, and nickel. More highly alloyed tool

steels (O1, D2, D3, A1, and S1) and some mar-

tensitic stainless steels (AISI 416, 420, and

440C) are also sometimes induction hardened.

Among alloyed steels, chromium, chromium-

molybdenum, and chromium-molybdenum-

nickel steels prevail. With given combinations of

alloying elements in induction-hardened mach-

ine parts, the internal stresses in heating as well

as in cooling can be controlled. The steels are

normally quenched in water. In certain cases, the

alloy steels can be cooled by means of an oil

emulsion. Steels can also be oil quenched. Hav-

ing selected the right shape of the product and the

right choice of technology, it is possible to

expect only minimal distortions of the workpiece

after completion of the heat treatment (Ref 14).

If the starting points are the hardness and

residual-stress proﬁles, then the induction-heat-

ing conditions should be adapted to the selected

steel. Induction heating of a machine-part

surface to the austenitic zone above T

is of

major importance, since not only the case depth

but also the right through-depth residual-stress

and microhardness proﬁles of the machine part

are to be provided. The through-depth micro-

hardness proﬁle of the surface layer depends,

with the given steel, on the induction-hardening

conditions. Induction heating may be controlled

by infrared thermometers. An infrared thermo-

meter is placed close to the induction coil, which

means that only the maximum surface temper-

ature may be measured. This technique is quite

simple and practical. It allows a user to determine

the required, that is, optimal, power density for

heating. When a multiturn coil is used, for

example, with longer workpieces, several infra-

red optical-ﬁber thermometers may be em-

ployed. Thus, with sufﬁcient spacing between

two adjoining turns, the temperature may be

measured at different locations on the machine

part. A difﬁculty may arise due to nonuniform

heating along the coil movement. By collecting

and processing the gathered measurement data,

the process quality may be assessed. In scientiﬁc

research, temperature measurement with

thermocouples may be applied too. In induction

surface heating, temperature measurement using

thermocouples is quite exacting, since they are

embedded in the machine part. The hardened

state of steel is to be obtained at an appropriate

depth. The thermocouples embedded at a parti-

cular location in the workpiece permit tem-

perature measurements at this location only. In

progressive hardening, however, the momentary

temperature is measured. Thus, the so-called

temperature cycles in heating, as well as in

cooling, are obtained. The temperature cycles

make it possible to predict the efﬁciency of sur-

face hardening. The choice of energy input, that

is, power density, in an appropriate nomogram,

that is, the choice of power and heating time with

reference to the shape of the induction coil and

case depth, applies only to stationary heating and

hardening of machine parts. This means that in

progressive hardening the data obtained on the

energy input are of informative character only.

Main Features of Induction Heating

Induction heat treatment is a segment of the

much larger technical ﬁeld of induction heating,

which combines many other industrial processes

using the phenomenon of heating by induction

(Ref 1, 16, 18–22).

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Induction heating is often one of the most

effective heat treatment processes available for a

variety of applications, including:

 Surface hardening

 Through hardening

 Tempering and stress relief

 Annealing and normalizing

 Grain reﬁnement

 Precipitation hardening or aging

 Sintering of powdered metals

In most of these applications, induction

heating is used to selectively heat only the por-

tion of the workpiece that requires treatment

(Ref 2). This usually means that the process can

be accomplished in a relatively short time and

with high efﬁciency, because energy is applied

to the workpiece only where it is needed.

One of the main features of induction heating

compared with conventional heating procedures

is that heat is generated in the workpiece itself.

In conventional heating procedures, the heat

input achieved is only 5 to 200 kJ/m

s energy,

whereas in induction heating this energy input

is 300 MJ/m

s. In induction heating, heat

penetrates into the workpiece by the aid of

high-frequency alternating current, the choice of

frequency depending on heating requirements.

Induction heating power supplies are frequency

changers that convert the available utility line

frequency power to the desired single-phase

power at the frequency required by the induction

heating process. They are often referred to as

converters, inverters, or oscillators, but they are

generally a combination of these. The converter

portion of the power supply converts the line

frequency alternating current input to direct

current, and the inverter or oscillator portion

changes the direct current to single-phase alter-

nating current of the required heating frequency.

Many different power supply types and

models are available to meet the heating require-

ments of a nearly endless variety of induction

heating applications (Ref 21, 22). The speciﬁc

application will dictate the frequency, power

level (Fig. 2), and other inductor parameters

such as coil voltage, current, and power factor

(cos Q)orQ factor (Ref 15, 19).

Advantages in Surface Hardening of

Machine Components. Induction hardening

is most often used for surface hardening of

machine components and has the following

Fig. 2 Typical power-frequency regions of induction heat treatment applications. Source: Ref 15, 19

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advantages over other procedures (Ref 2,

19–22):

 Relatively short heating times

 Heating procedure is not strictly governed

by hardening temperature. All that matters is

that the heating process does not end at too

low a temperature, because sometimes it is

necessary for the transformation into auste-

nite. The heating temperature is limited by

the solidus-line temperature, since the pro-

cess is carried out while the material is in the

solid condition. Due to a short heating time,

there is no danger that the austenite grains

would grow at higher austenitization tem-

peratures, which also means there is no

danger of coarse and brittle martensite for-

mation.

 The quenching procedure is easy to perform,

contributing to short surface-hardening

times. In progressive induction hardening, a

spray coil is located directly below the in-

ductor and quenches the heated surface. In

single-shot hardening, the inductor is de-

signed to perform the function of heating as

well as quenching. The coil around the work-

piece functions as an inductor in the heating

phase. After the austenitization temperature

has been reached, the current is interrupted,

and the coil functions as a spray for

quenching.

 Induction hardening is a short procedure that

does not require any additional protection

against oxidation. Thus, compared with

other similar procedures, such as cementa-

tion, it does not require much subsequent

machining.

 Due to the nature of the procedure, the

workpieces, especially if symmetrically

shaped, are less susceptible to undesirable

deformations after induction hardening. The

volume changes in the workpieces after hard-

ening the surface layer can be very accurately

predicted or estimated. The volume changes

after induction hardening of thin layers are so

small that quite often the function of the

machine component is not affected.

 Especially in induction hardening of thin

layers and workpieces with low mass, it is

possible to achieve the desired critical rate of

cooling by self-cooling in air alone, that is,

by heat conduction from the heated surface

layer into the remaining cold part of the

workpiece. With thicker layers and work-

pieces of greater mass, it is necessary to use

quenching agents that move the actual

cooling rates close to the critical cooling

rate. These requirements can be met with the

right selection of quenching oils or polymer

water solutions. Practical experience has

shown that polymer water solutions are very

suitable for quenching of induction-heated

surfaces, since optimal quenching can be

ensured with the right choice of concentra-

tion of the polymer water solution.

 The induction-hardening procedure enables

the engineer, by simply adapting the shape

of the induction coil, to ensure the desired

shape of the hardened proﬁle of the surface

layer. Likewise, the engineer can surface

harden only that part of the surface (local

hardening) on which a certain increased level

of hardness and wear resistance is desired.

One of the main advantages of induction

hardening is the possibility of hardening a

surface layer only in certain places, at a

deﬁned penetration depth and shape.

 These advantages make it possible for

induction hardening to be fully automated

and are especially suitable for a large series

of workpieces.

 Induction hardening always leaves com-

pressive residual stresses in the surface layer,

which makes machine components more

resistant to dynamic loads. Compressive

residual stresses in the surface layer after

induction hardening prevent the occurrence

of cracks in dynamically loaded components

and prevent the growth of existing cracks

on the workpiece surface, if these are present

due to hardening or grinding.

 Induction hardening is appropriate for small-

sized workpieces, since, by a well-chosen

technology of heating and cooling or quen-

ching, a hardened surface layer and a reﬁned

core can be ensured. Thus, the required wear

resistance of the machine component at a

certain location as well as the required load-

bearing capacity of the component can be

created, experiencing only a slight loss in

toughness of the core.

Induction Hardening of Machine Parts

Working Coil and Procedure during

Induction Hardening. Methods of induction

hardening of thin surface layers are always

adapted to the product size and shape and the

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requirements of location, size, and shape of the

hardened layers proﬁle.

Designing an induction coil, however, is very

speciﬁc to a given application and can be con-

sidered the most important aspect of the engi-

neering of the system. There are several major

functions that an induction coil must perform to

make a job successful:

 Induce current in the load so that the proper

heating pattern is obtained

 Accomplish the proper heating pattern with

as great an efﬁciency as possible

 Provide an impedance match to the gen-

erator so that adequate power can be trans-

ferred to the load

 Have a geometry that will accomplish the

aforementioned three major functions and

permit easy loading and unloading of the

part being heated

First, the outer and inner surface on machine

components must be distinguished. The size and

shape of the induction coil must be adapted to

the size and shape of the product, and during the

heat treatment process, the distance should be

adapted to the workpiece. It is important that the

design of the machine component be planned for

induction-hardening speciﬁcs. A uniform gap

size between the induction coil and the work-

piece surface is very important, since the energy

penetration depends on it. This is why single-

shot hardening, where the workpiece and the coil

are at a standstill, is rarely used. To ensure a

uniform gap, single-shot hardening is almost

always combined with a rotating motion of the

workpiece, or, on long workpieces, progressive

hardening is used.

Figure 3 shows different types of coils for

induction hardening designed for external sur-

faces (Fig. 3a), internal surfaces (Fig. 3b), and

front surfaces (Fig. 3c) of the workpiece. In all

three cases, the shape of the induction coil is

adapted to the size and shape of the work-

piece surface that is to be hardened (Ref 2, 15, 16,

19–22).

Workpiece sizes depend on the manufacturing

possibilities of high-frequency coils and their

types. This restricts the use of induction hard-

ening primarily to small-sized machine compo-

nents. Induction hardening of internal surfaces

requires a great deal of knowledge and experi-

ence in the design and manufacturing of induc-

tion coils of small diameters. The smallest

internal diameter of a machine component that is

to be induction hardened depends on the manu-

facturing possibilities of making small-diameter

coils, the capacity of the high-frequency gen-

erator (power, frequency), and the positioning

accuracy of the induction coil inside the

workpiece.

The induction coils are made of materials with

the highest possible electrical conductivity.

Consequently, copper or silver is usually chosen

to produce the induction coils. The induction

coils are adapted to the shape of the machine part

at the location where heating, that is, surface

hardening, is performed. The choice of the size

and shape of the induction coil depends on the

Fig. 3 Signiﬁcant types of coils for induction heating. Source: Ref 15, 16

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method of heating, that is, hardening. With the

single-shot heating method, the induction coil

may be adapted by choosing an adequate pipe

cross section and by adapting the number of

turns used so as to provide simultaneous heating

of the entire area at the machine part. Figure 4

shows the inﬂuence of the induction coil shape

on the heating proﬁle (Fig. 4a) and that of the

distance between the individual turns of the

induction coil and the gap on the heating proﬁle

(Fig. 4b).

The concentration of the heat generated in the

machine part heated by the induction coil is

symmetrically and uniformly distributed across

the cross section, so that no distortion of the part

may occur after quenching. The symmetry of the

hardened-pattern proﬁle thus reduces the risk of

distortion of the machine part. The induction

coils are made of pure copper, which can be

nicely shaped as required to produce coils. Be-

cause the electrical conductivity of the material

is reduced due to plastic deformation of the

material, it is necessary to adequately anneal the

coils to soften the material. Thus, the highest

possible electrical conductivity is ensured. It is

also necessary to prevent the formation of cop-

per oxide at the coil surface to preserve highest

possible electrical conductivity of the material.

This will provide a high efﬁciency of induction

heating.

In single-shot induction heating, that is,

induction surface hardening, it is important to

provide the same gap size all around the cylin-

drical machine part. Because such an arrange-

ment can be very exacting, the heating process is

usually performed with the rotation of the

machine part. Figure 5(a) shows single-shot

hardening performed with a different number of

inductor turns along the speciﬁed hardened

pattern. This means different divisions among

the individual coil turns. Under the same heating

conditions, considerable difference in the

hardened-pattern depth will be obtained with

the same heating time.

Figure 5(b) shows different examples of

shaping a multiple-turn induction coil around

the cylindrical machine part. The ﬁgure indi-

cates that the gap size along the cylindrical part

changes, which produces different hardened-

pattern proﬁles. Figure 5(b) shows the inﬂuence

Fig. 4 Inﬂuence of shape and arrangement of induction coil around machine part on heating, that is, hardened-pattern proﬁle

Fig. 5

(a) Inﬂuence of multiturn coil on hardened-pattern

proﬁle. (b) Inﬂuence of shape and gap size of induction

coil turns on hardened-pattern proﬁle. (c) Inﬂuence of number of

induction coil turns on hardened-pattern depth

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of a multiple-turn coil with too-large divisions of

the individual coil turns; therefore, the same heat-

ing depth, that is, hardened-pattern depth, along

the coil length is not achieved. Thus, individual

coil areas showing a smaller hardened-pattern

depth are obtained at the machine part.

Figure 5(c) shows the inﬂuence of the cross

section of a single-turn coil on the hardened-

pattern proﬁle.

For hardening of axles, shafts, and similar

lengthy machine parts, the scan-hardening pro-

cess with a single-turn coil of an adapted cross

section is chosen. Such a hardening process is

necessary, otherwise, high-energy densities and

very long multiple-turn induction coils would be

needed. Figure 6 shows a diagram of heating, that

is, through hardening, of a cylindrical rod of

different diameters (D, in centimeters) with dif-

ferent inductor powers (P, in kilowatts), and a

given frequency ( f, kilohertz) and inductor

movement (v, in cm/s). The diagram shows a

linear dependence between the connected power

at the coil and the depth of hardening, which

means that the choice of a higher power produces

hardening at a greater depth, the movement

velocity being the same. It can also be established

which diameters of the axles or shafts can be

through hardened if the same power is available

and different movement velocities are chosen.

It is possible to distinguish between:

 Single-shot hardening

 Scanning or progressive hardening

 Tooth-by-tooth hardening or gap-by-gap

hardening

Quenching techniques are an important

design feature of induction-hardening equip-

ment. The important questions to be answered

when determining quenching systems include

(Ref 2, 17, 23, 24):

 Workpiece size and geometry

 Hardenability of steel

 Type of austenitizing operation (surface or

through hardening)

 Type of heating method (single-shot or

scanning)

 Type of quenchant

The two most common types of systems

consist of spray quench rings and immersion

techniques. When quench rings are used for

round bars, their shape, like the coil, is generally

round (Ref 2, 23, 25). Figure 7 shows different

ways to single-shot harden the surface layer.

Common to all of them is that heating is

performed along the length of the layer, so that

the induction coil embraces the particular part of

the workpiece where heat treatment is to be

performed (Ref 15, 16). The ring may be located

concentric with the coil (Fig. 7c) or directly

underneath or alongside it (Fig. 7b), as in the

single-shot induction hardening setup.

In any of these ways, it is important to ensure a

constant gap size between the induction coil and

the workpiece surface. This is usually achieved

by a rotating motion of the workpiece. In the ﬁrst

case, shown in Fig. 7(a), the cooled induction

coil embraces the object at a certain height. After

heating to the austenitization temperature is

Fig. 6 Selection of heating conditions in scan hardening to provide through hardening of a cylindrical rod

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completed, the workpiece is quenched in a

quenching bath, or quenching is done by moving

the workpiece into the quenching spray located

beneath the induction coil (Fig. 7b). Figure 7(c)

shows a method of heating and cooling in the

induction coil, which is made so that it enables

heating of the workpiece surface for a certain

period, and then, on switching the current off, the

same coil is activated as a spray for quenching.

The induction coil is usually made of copper

tubing with additions of alloying elements

that do not affect the electrical conductivity of

copper but contribute to good machinability

of the material. Good machinability is required

because it is necessary to make the openings

for the quenching spray. Thus, during the entire

heat treatment, the workpiece is rotating, and in

this way, uniform heating and quenching are

ensured. The heating and quenching conditions

created in this way produce a uniform hardened

surface layer and ensure repeatability of thick-

ness and shape of the hardened proﬁle.

In induction scanning or progressive hard-

ening, workpieces move through the quench ring

and coil, with quenching occurring immediately

after heating (Fig. 8). For nonsymmetric work-

pieces, the quenching system, like the coil, is

generally the same shape as the workpiece (Ref

15, 16).

The second method of induction hardening

involves a group of procedures of progressive

or progressive-rotating hardening. This type of

Fig. 7 Single-shot induction surface hardening of a cylindrical workpiece. Source: Ref 15, 16

426 / Failure Analysis of Heat Treated Steel Components

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hardening is employed when the required length

of the hardened surface is longer than the length

(size) of the coil. In this case, the correct heating

mechanism can be ensured by the optimal in-

duction coil design (number of coil turns, length

of the coil) and suitable energy inputs for a given

size and proﬁle of the hardened layer (Ref 23,

25, 26). To ensure the uniformity and repeat-

ability of the hardened layer, progressive motion

of the induction coil must be provided, or the

workpiece is fed progressively through the coil

while rotation of the workpiece takes place at the

same time. This method of induction hardening

is presented in Fig. 8. It is possible to design the

induction coil separately from the spray (Fig. 8a)

or combined so that the induction coil has

downward-directed spray openings for quench-

ing in its bottom part (Fig. 8b).

Gear wheels belong to very demanding

machine elements, from the point of view of

mechanical machining as well as heat treatment.

In hardening smaller gears, that is, gears with

small modules or small diameters, the method of

single-shot hardening can be used to ensure a

varying thickness of the hardened layer (Ref 15,

16). The thickness is maximum on the tooth tip

and then decreases toward the tooth root.

Figure 9(a) shows single-shot hardening of tooth

tips, where the coil encircles the whole gear.

This method is very simple and suitable for high-

frequency heating of gears with a module up

to 3 mm and for smaller gear diameter. In this

case, it must be ensured that the tooth ﬂanks are

hardened at least along the length that is other-

wise activated in the mesh, since the thickness of

the hardened layer varies, being highest at the

tooth tip and gradually decreasing toward the

tooth root. This method of hardening is suitable

only for gears subjected to low loads, just to

increase the wear resistance of the gear (Ref 16,

23, 26, 27).

Figure 9(b) shows induction surface hard-

ening, where the whole tooth and a certain area

below the tooth root are hardened. This method

of induction surface hardening is appropriate for

gears with a module up to 5 mm. Heating is

achieved by two systems: ﬁrst, with a current of

medium frequency, and then in the ﬁnal phase

for a relatively short period by heating with a

high-frequency current. By quenching, a desir-

able hardness of the tooth ﬂank surface and

increased strength of the gear tooth can be

achieved. In the root of the tooth, due to a reﬁned

microstructure, an increased material fatigue

Fig. 8 Progressive induction hardening of a cylindrical workpiece. Source: Ref 16

Induction Hardening / 427

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