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

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strength can be noted. Gears heat treated in this

way are suitable for highly loaded machine parts

(Ref 28–30).

Figure 9(c) shows hardening of individual

gear teeth, where the coil encircles the tooth and

the individual teeth are heated in turn and

quenched directly after heating. In the case of

large-module gears, simultaneous heating of the

left and right ﬂank of the same gear tooth can be

performed (Fig. 9d).

Induction hardening of all gear teeth (Fig. 10)

can be done only for gears with a module smaller

than 3 mm and a gear diameter up to 100 mm.

This kind of hardening requires high-power

frequency generators, for example, 200 kW.

For gears exposed to higher loads, where

reﬁnement of the whole tooth is desirable, a

power of 30 to 40 kW and longer heating times

can be selected.

To ensure a desired penetration depth and

proﬁle of the induction-hardened layer, the fol-

lowing heat treatment parameters can be varied

in a single-shot or progressive induction hard-

ening by choosing a suitable power density and

feed rate:

 The size and shape of the induction coil

adapted to the workpiece

 Kind of steel and its thermal properties

 Size and mass of the workpiece on the

location where induction hardening is to be

performed

 Quenching agent and method of quenching

A number of graphs and nomographs are

available for this purpose, offering a selection of

heat treatment conditions for heating and cool-

ing. The most important data are the starting

point data on energy input and frequency of the

current, and the temperature to achieve in

induction hardening. From these data, the time

necessary for heating, in the case of single-shot

hardening, or the feed rate of the workpiece or

the rate at which the coil should move along the

workpiece can be deﬁned (Ref 2, 18–20, 23, 27,

31, 32).

Figure 11(a) illustrates induction surface

heating with a medium-and high-frequency

current. The procedure is known as double-

frequency heating (Ref 27). Here, the gear is ﬁrst

placed into the coil fed by the medium-

frequency current. Then, the gear is moved into

the high-frequency coil, where only the surface

layer of the gear tooth is reheated with a high-

frequency current. When both phases are com-

pleted, the gear is dropped or moved into a

quenching bath. In this method of heating with a

double frequency, the progressive motion of the

workpiece can be combined with additional

rotation.

Additional rotating motion ensures a uniform

reheating of the surface layer and results in

uniform microstructural changes on the left and

right side of the tooth. In this way, undesirable

and nonuniform dimensional deviations be-

tween the left and right tooth ﬂanks are pre-

vented. The same method of induction surface

Fig. 9 Different methods of induction surface hardening of gear wheels. Source: Ref 15, 16

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heating is shown in Fig. 11(b), where the same

coil is used for heating with a medium- and high-

frequency current (Ref 27).

The second method is hardening of individual

gear wheels or individual gear gaps. Figure 12

shows induction heating of a gear wheel tooth-

by-tooth and Fig. 13 shows induction heating

with the gap-by-gap method. In both methods,

the tooth surface is ﬁrst heated with the induc-

tion coil and then quenched with a specially

adapted spray system.

Figure 14 shows the shape and position of the

massive inductor placed around the gear tooth

(Ref 27). The coil is shaped so that the gap

between the coil and the tooth surface varies.

Only in this way is it possible to ensure a uni-

form thickness of the layer in the middle and on

the edge of the tooth (Ref 33–37).

Difﬁculties with this method of gear wheel

hardening occur when the tooth gaps are too

small and the coil heats up the adjacent ﬂank.

This method of induction surface heating of gear

teeth is often not appropriate, since after heating

the adjacent tooth, the next step does not ensure

the desirable hardness of the adjacent tooth. To

avoid this, special protection made of thin cop-

per sheets is used to prevent heating of the

adjacent tooth ﬂanks. The conditions are pre-

sented in Fig. 15(a–c) (Ref 27). In addition to

protecting adjacent tooth ﬂanks, the shape of

the coil around the tooth has other effects. By

changing the shape and position of the coil

around a particular tooth, it is possible to achieve

equal hardened layer proﬁles, as in the case of

single-shot hardening. The thickness of the

hardened layer is at a maximum at the tooth

tip and then gradually decreases toward the tooth

root. In Fig. 15(c), the coil is placed slightly

lower, heating only the tooth ﬂank along

the entire height from the root to the tip.

Figure 15(b) shows the lowest position of the

coil while still reaching below the tip of the

tooth. The coil positioned in this way does

not heat the tip of the tooth but only the surface

of the tooth ﬂank from the tip to the root and

yields maximum thickness of the hardened layer

in the middle of the tooth. In this case, too, small

gears are heated with a high-frequency current,

Fig. 10 Single-shot surface induction hardening of gear wheel tooth tips

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Fig. 11 Induction heating system for gear wheel heating by double frequency (medium/high). Source: Ref 27

Fig. 12

Tooth-by-tooth induction surface heating process of

installing the induction coil around the gear wheel

tooth for heating, followed by shifting spray quenching

Fig. 13

Gap-by-gap gear wheel induction surface heating by

moving the induction coil for heating and moving or

installing spray quenching

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whereas large-sized gears are ﬁrst heated with a

medium-frequency current, and then heating is

performed with a high-frequency current (Ref 2,

16, 18, 19, 27, 32).

The third method of induction surface hard-

ening is appropriate for large gear modules and

is known as tooth-gap hardening, which belongs

to the progressive hardening methods. In this

case, the coil is placed so that it ensures a uni-

form gap between the coil and the ﬂanks of

two adjacent teeth. The tooth-gap hardening

method is very demanding and requires much

Fig. 14

Relative position of the massive induction coil to the

gear wheel tooth at induction heating. Source: Ref 27

Fig. 15 Inﬂuence of induction coil height on proﬁle of induction heating surface layer in an individual tooth. Source: Ref 27

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experience and knowledge to achieve the

desirable properties of the gear.

This method is also known as contour hard-

ening. It is an ideal method for heat treatment of

gears, because it increases the hardness on the

tooth surface only slightly while decreasing the

load-bearing capacity in the root of the tooth.

Gears heat treated in this way exhibit very good

behavior in operation, because compressive resi-

dual stresses are present in the root of the tooth.

Gears with induction-hardened ﬂanks, given that

the dimensioning is carefully carried out, can

achieve the highest fatigue strength. To verify

the results of induction surface hardening, it is

necessary to take certain measures for control-

ling the quality of the hardened layer. For this

purpose, hardness and microhardness measure-

ments, supported by microstructural analysis,

are commonly used. A disadvantage of this pro-

cedure is that, due to the method of heating and

quenching (nonuniformly overheated left and

right tooth ﬂank), slightly higher dimensional

deviations may be obtained than in the case of

simultaneous hardening of both ﬂanks of the

same tooth (Fig. 15d) (Ref 27).

Metallurgical Aspect of Induction Surface

Heating. Prior to transformation hardening,

an operator should calculate the processing

parameters for the given power system. The pro-

cedure is as follows. Some of the processing

parameters are chosen, some calculated. The

choice is usually left to the operator and his

experience. Optimization is then based only on

the selection of power density and scan speed.

The correctly set parameters of transformation

hardening ensure the right heating rate, heating

to the right austenitizing temperature, T

, and a

sufﬁcient austenitizing time, t

. Consequently,

with regard to the speciﬁed depth of the hard-

ened layer, a temperature a little higher than the

transition temperature, T

, should be ensured.

Because of a very high heating rate, the equili-

brium diagram of, for example, steel, is not

suitable; therefore, it is necessary to correct the

existing quench temperature with reference to

the heating rate. Thus, with higher heating rates,

a higher austenite transformation temperature

should be ensured in accordance with a time-

temperature-austenitizing (TTA) diagram.

The diagram in Fig. 16(a) is such a TTA

diagram for 1053 steel in the quenched and

tempered state, whereas Fig. 16(b) is for the

same steel in the normalized state (Ref 38).

Because the steel shows a pearlitic-ferritic

microstructure, a sufﬁciently long time should

be ensured to permit austenitizing. In fast heat-

ing, austenitizing can be accomplished only

by heating the surface and subsurface to an

elevated temperature. For example, with a

heating time, t, of 1 s, for total homogenizing,

a maximum surface temperature, T

, of 880



should be ensured in the ﬁrst example and a

much higher surface temperature, 1050



C, in

the second example. This indicates that ap-

proximately 170



C higher surface temperature,

, should be ensured in the second example

(normalized state) than in the ﬁrst example

(quenched and tempered state).

Figure 17 shows a space TTA diagram

including numerous carbon steels with different

carbon contents. The TTA diagram gives par-

ticular emphasis to the characteristic steels, that

is, 1015, 1035, 1045, and 1070 steels, and their

variations of the transition temperature, T

with reference to the given heating rate and the

corresponding heating time (Ref 38). Such a

temperature difference ensures, with regard to

the heating and cooling conditions of the speci-

men, the time required for austenite homo-

genizing, t

, in the given depth.

Figure 18 shows a shift of the transformation

temperature, which ensures the formation of

inhomogeneous and homogeneous austenite

within the selected interaction times (Ref 39).

A shorter interaction time will result in a

slightly higher transformation temperature, T

and also a higher transformation temperature,

Fig. 16

Time-temperature-austenitizing diagram for steel

1053 in various states. Source: Ref 38

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. To ensure the formation of homogeneous

austenite with shorter interaction times, con-

siderably higher temperatures are required.

Figure 18(a) shows a temperature-time diagram

for austenitizing of Ck 45 steel.

The isohardnesses obtained at different in-

teraction times in heating to the maximum

temperature ensure that partial or complete

homogenizing of austenite is plotted.

Figure 18(b) shows the same temperature-time

diagram for austenitizing of 100Cr6 hyper-

eutectoid alloyed steel.

The diagram indicates that with short inter-

action times, which in laser hardening vary

between 0.1 and 1.0 s, homogeneous austenite

cannot be obtained; therefore, the micro-

structure consists of austenite and undissolved

carbides of alloying elements that produce a

relatively high hardness, even up to 920 HV

0.2

After common quenching of this alloyed steel

at a temperature of homogeneous austenite, a

considerably lower hardness, only 750 HV

0.2

but a relatively high content of retained austenite

were obtained. Retained austenite is unwanted,

since it will produce unfavorable residual

stresses and reduce wear resistance of such a

material.

Fig. 18 Temperature-time-austenitizing diagrams with lines of resulting hardness for various steels. Source: Ref 39

Fig. 17

Inﬂuence of heating rate and carbon content

on austenitic transformation temperature. Source:

Ref 38

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The distribution of residual stresses in heat

treatment procedures where only the surface of

the workpiece is heated (induction hardening,

ﬂame hardening) differs greatly from the pro-

cedures where heating is performed throughout

the entire volume (nitriding, cementation). In

nitriding and cementation, the aforementioned

second layer in the subsurface does not appear at

all, because the direction of the heat ﬂow is

opposite to the direction of the heat ﬂow in

induction and ﬂame hardening. The resultant

operating tensile stresses on the surface or in the

surface layer can thus be considerably smaller.

Due to the surface hardness, induction and ﬂame

hardening lowers the fatigue strength of mach-

ine components; therefore, care should be taken

to diminish all detrimental effects in the surface

layer.

A typical example of induction surface hard-

ening is surface hardening of gears that are

heated with a low heating rate and relatively low

current frequency. The outer hardened zone in-

cludes almost the entire height of the gear teeth,

whereas the second zone is in the tooth root area.

A gear heat treated in this way will meet the

wear-resistance requirements expected of the

gear tooth, while the strength of the other part of

the tooth is of minor importance. The fatigue

strength in this case will be relatively low due to

high tensile residual stresses in the tooth root,

that is, in the second zone where the operating or

load tensions and the tensile residual stresses are

summed up. Figure 19 shows another example

of an induction-hardened gear where the fre-

quency of the current was so high that the gear

tooth is heated along the ﬂank surface and tip, as

is the case in cementation. The energy input in

heating a gear tooth or the whole gear was such

that the second zone has not shown up. A similar

heat treatment can be applied to the spline inside

the gear. A gear heat treated in this way is more

resistant to wear and corrosion and should have

high resistance to fatigue in bending because of

a smaller thickness of the layer in the second

zone.

Many induction-hardened gears are treated in

the tooth gap, that is, in simultaneous heating

and subsequent quenching of two adjacent

ﬂanks of the left and right tooth of the gear with a

hardened root area. In view of the variety of

methods to induction harden gears and the other

possible ways of gear hardening, it is unwise to

make a hasty decision in selecting the procedure.

It is necessary to make a thorough analysis,

including answers about the expected quality of

the hardened layer and analysis of the operation

loads of the machine components.

Highly loaded gears can be successfully

induction hardened if a high-frequency current

and high-input power are used. High frequency

is necessary in order to obtain a sufﬁcient thick-

ness of the hardened layer on the tooth ﬂanks and

a ﬁne hardened layer in the tooth root area. A

high-input power is necessary to increase the

heat gradient, which makes the size of the sec-

ond zone smaller, resulting in a thinner second

layer with tensile residual stresses.

On gears with a small diameter, the induction-

hardening equipment should be able to harden

the gear throughout its volume. Induction hard-

ening of entire gears has some advantages, such

as rapid heating, no danger of decarbonization

and oxidation, high productivity, and repeat-

ability in the gear quality. Thus, the hardened

layer is only deﬁned by the ﬁrst zone to the depth

that is greater than the height of the gear tooth.

Fig. 19

Typical examples of induction surface hardening of (a) carbon steel and (b) alloyed steel gears produced from carbon steel (a)

and alloyed steel (b)

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In induction hardening and quenching, lowering

or even the disappearance of compressive resi-

dual stresses is achieved in the tooth root area,

causing a considerable decrease in fatigue

strength and a higher propensity for fatigue.

In ﬂame heating, lower temperature gradients

are reached than in induction heating. This

results in increased thickness of the layer in the

second zone. Possible harmful effects due to the

disappearance of the second zone of the hard-

ening layer can be avoided by heating the entire

gear, which is possible to do in commonly used

furnaces.

When the induction coil has stopped heating,

an austenitic microstructure in the surface layer

should be obtained. Then, the cooling process

for the austenitic layer begins. To accomplish

martensite transformation, it is necessary to

ensure a critical cooling rate that depends on

the material composition. Figure 20 shows a

continuous cooling transformation diagram

for EN19B steel, including the cooling curves

(Ref 38).

Because carbon steels have different carbon

contents, their microstructures also show differ-

ent contents of pearlite and ferrite. An increased

carbon content in steel decreases the tem-

perature of the beginning of martensite trans-

formation, T

as well as of its ﬁnish, T

Figure 21 shows the dependence between

carbon content and the two martensite transfor-

mations (Ref 38).

Consequently, an increase in carbon content

in steel results in the selection of a lower critical

cooling rate. In general, the microstructures

formed in the surface layer after transformation

hardening can be divided into three zones:

 A zone with completely martensitic micro-

structure

 A semi-martensitic zone or transition

microstructure

 A quenched and tempered or annealed zone

with reference to the initial state of steel

Transformation hardening of steel starts

from its initial microstructure, which is ferritic-

pearlitic, pearlitic-ferritic. or pearlitic. In steel

heating, transformation into a homogeneous

austenitic microstructure should be ensured.

Figure 22 shows the dependence of the maxi-

mum surface temperature obtained in induction

heating with a machine part made of steel with

0.8% C and a pearlitic microstructure (Ref

40, 41). How the through-depth heating of the

machine part will proceed depends on the maxi-

mum surface temperature obtained and the

power density, which means that the through-

depth hardness proﬁles will differ.

Fig. 20 Continuous cooling transformation diagram of EN19B steel. Source: Ref 38

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With the same power density, this means that

a higher maximum surface temperature will be

accomplished with a longer heating time. Since

the heating times are usually short, austenitic

grains have little time left to grow. It can be

assumed that the grains remain ﬁne and do not af-

fect through-hardenability, that is, the through-

depth hardness proﬁle. The diagram shows the

heating conditions that provide, in all cases, a

completely homogeneous austenitic microstruc-

ture to a certain depth and, consequently, con-

stant hardness. Then follows a transition zone

consisting of homogeneous austenite and some

inhomogeneous austenite and pearlite. Conse-

quently, hardness in the transition zone will

gradually decrease to that of the parent metal. It

will be approximately 240 HK. It is important

that the transition zone consists of a mixture

showing different ratios of the microstructures

concerned. The different microstructure ratios in

the transition zone, however, deﬁne the hardness

proﬁle in this zone. Thus, at the maximum sur-

face temperature, a maximum hardness of ap-

proximately 700 HK, without a constant part

with homogeneous martensite, is obtained, and

then it decreases immediately to the hardness of

the parent metal.

With a maximum temperature of 800



there will be constant hardness to a depth of

0.6 mm; from that point to a depth of 0.85 mm,

the hardness will slowly decrease to that of the

parent metal, that is, 240 HK. The highest

hardness, 850 HK, is obtained with the max-

imum austenitizing temperature, 850



C, and is

found to a depth of 1.10 mm. From that point

to a depth of 2.70 mm, the hardness slowly

decreases to that of the parent metal. With higher

maximum surface temperatures, the lowest

Fig. 21

Inﬂuence of carbon content in steel according to start

and ﬁnish temperature of martensitic transformation

Fig. 22

Hardness proﬁles for an induction-hardened 0.8% C steel for various maximum temperatures. The initial microstructure of

the steel was pearlite. Source: Ref 40, 41

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hardness is obtained at the surface, and then,

with a greater depth, it slowly decreases. With

steel having a pearlite-ferrite or ferrite-pearlite

microstructure, a microstructure consisting not

only of inhomogeneous martensite but also of

pearlite-ferrite grains will occur in the transition

zone. Such a microstructure results in a stronger

decrease in hardness than with the steel having a

pearlite microstructure. It is essential for efﬁ-

cient induction surface hardening that constant

hardness is obtained to a sufﬁcient depth and that

the hardness proﬁle of the transition zone is

adequate. It is only in this way that notch defects

in the hardened layer may be prevented and

better operation of the machine part under

dynamic load may be ensured.

Magnetic Flux Concentrators

In induction surface heating, unwanted areas

on the workpiece are often heated or even hard-

ened. This type of problem appears when the

shape of the product surface is very complex,

and it is therefore difﬁcult to adjust the coil for

local heating and quenching. For more de-

manding shapes, shields from materials with

good heat conductivity, for example, copper, are

often used on the product to prevent heating of

the workpiece. Heating the copper shield pre-

vents undesirable heating of certain areas as well

as loss of heat. Such an example can be seen in

Fig. 15, which shows the protection of two ad-

jacent heat ﬂanks while heating the tooth in the

middle with an induction coil (Ref 15, 19–21,

28, 42).

Heat losses occur due to local heating of the

workpiece surface that is not to be hardened;

however, this is done because it is not possible

to adjust the shape of the induction coil.

The heating temperature is higher than the

hardening temperature, so that after quenching,

unwanted increased surface hardening is ob-

tained. In cases of treatment of two local surface

areas on the workpiece that are in direct pro-

ximity, if heat is applied two times in sequence,

the heating of the second area may result in

tempering of the previously treated area. In these

cases, the area hardened ﬁrst may have a tem-

pered microhardness with a lower hardness.

Therefore, research has been done on how to

form and adapt the coil to offer a more con-

centrated magnetic ﬂux. By adjusting the con-

centration of the magnetic ﬂux, it is possible to

achieve localized heating of only those areas on

the workpieces that are to be hardened. In the

last decade, the development of induction coils

has been directed toward achieving localized

concentration of magnetic ﬂux (Ref 19, 21). The

purpose of magnetic ﬂux concentration is to

improve the efﬁciency of surface heating and

reduce heat losses. The use of a magnetic ﬂux

concentrator enables selective local heating on

workpiece/product areas with complex geo-

metry. Figure 23 shows a straight conductor with

current density distribution in points “A” and

“B” for three cases (Ref 19, 42):

 Current density distribution in a straight

conductor (Fig. 23a)

 Current density distribution in a straight

conductor when the conductive material

(workpiece) is approached (Fig. 23b). The

current density is greater in point “A”, closer

to the workpiece material, than in point “B”.

As a result, the workpiece is locally heated

over a longer length—longer than the width

of the conductor. This is referred to as current

redistribution due to the proximity effect.

Fig. 23

Current distribution in an inductor without/with a magnetic ﬂux concentrator and its effect on the heating proﬁle of the

workpiece. Source: Ref 19, 42

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