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

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ductile fracture, or could this be a brittle frac-

ture? Imagine that only this one photograph is

available, and it must be determined whether

this was a ductile or brittle crack.

Clearly, the steel that was used to make the

link has the capability of being deformed. In

other words, the steel is ductile at the conditions

present at the time of bending. The analyst must

not be satisﬁed with that answer, though. The

proper analysis includes a determination of

when the deformation happened. Postfracture

deformation does not make the crack event itself

ductile. The macroscale deformation must occur

during and as an inseparable part of the fracture

process for the crack itself to be a ductile crack

event.

For those unfamiliar with this methodology,

imagining that the link broke suddenly while it

was under load can be helpful. If the crack were

ductile, one would likely see some necking at the

crack location, since chain links are generally

loaded in tension. No such localized deforma-

tion is observed in this case. On closer exam-

ination, one can see a tiny shear lip at the top

edge of the fracture surface. That makes this

crack, for the most part, a brittle crack at the

macroscale, despite the presence of available

ductility in the material. The visible deformation

near the center of the lower (originally straight)

portion of the link happened after the crack was

completely formed.

In doing fracture analysis, it is important

to distinguish the capability of ductility in the

material from the behavior at the time of

the crack event. Despite the material ductility,

the crack happened in a brittle way. Closer

examination of other views not shown provide

clear evidence that this was a fatigue crack.

Beach marks were visible. Fatigue cracks grew

below the yield strength of the component,

creating macroscale brittle features. To clarify

one other potential source of confusion, it is

important to remember that tensile refers to a

loading geometry. Fatigue is a type of crack

path. In this case, a fatigue crack grew due to a

tensile load in the horizontal portions of the link

(as shown).

To add a few more details to this case study,

the chain was in service at a plant that processes

meat, and strong acids were used to clean the

conveyor systems. This crack actually initiated

at a corrosion pit on the inside surface. The

cleaner reached the inner surfaces, but the em-

ployees may not have rinsed the chain very well.

This allowed a corrosion pit to form, which then

allowed a fatigue crack to grow. Again, it is

important to understand that the material itself is

ductile; there is nothing wrong with the material.

People involved with failure analysis need to

keep in mind that material behavior is or at

least may be different from material capability.

People doing failure analysis work need to be

able to distinguish inherent capability and actual

behavior.

To underscore the importance of separating

the behavior from the capability, imagine the

potential corrections that may be considered if

someone found this to be a ductile overload

fracture. The “cure” may be to make it harder. In

the case of the acid cleaning, harder steels are

often more susceptible to stress corrosion than

softer steels. If the “harder-is-the-answer” the-

ory were put into practice, an undesirably short

life may become a horribly short life. It is

important to be sure that a crack that is diag-

nosed as ductile is really ductile and one that is

diagnosed as brittle is really brittle. Finally,

returning to the heat treating issues, the fact that

someone misdiagnosed this crack as a ductile

fracture may lead to the heat treater being

blamed for overtempering or inadequate hard-

ening. In fact, until now, the potential blame or

innocence of the heat treater has not been

investigated in any thorough manner. It is poss-

ible that poor heat treating or poor material

manufacture contributed to the ease of corrosion

attack, and the meat processing plant employees

were blamed incorrectly. Further examination of

the microstructure is required to reveal the root

physical cause of the fracture and its timing.

Finally, there is the shaft fragment shown in

Fig. 5. The shape is cylindrical. The image

shows one fracture face. No necking or reduc-

tion in area is visible at the fracture face location.

However, this is a ductile fracture. It is necessary

to know that this shaft broke in torsional

loading. In torsion, the shear stresses are in

the transverse orientation to the length of the

shaft. To best understand macroscale ductile and

Fig. 4

Example of a macroscale brittle fracture in tensile

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brittle fracture, one must be familiar with normal

and shear stresses. On a simple level, normal

stresses cause macrobrittle cracks due to crack

opening forces created at the crack tip. Shear

stresses allow slip and are the basis of the

deformation that creates the ductile crack event.

So, even though there is no necking, this is a

ductile crack.

Often, it is relatively easy to see some evi-

dence of twisting on the side of a ground shaft

that has failed from ductile fracture by torsional

forces, which would lend more credibility to this

diagnosis of ductile fracture. However, this shaft

was extremely smooth, and it required a long

etching time in a heated acid solution to reveal

permanent twisting on the original cylindrical

surface.

Another possible source of conﬁrmation that

this is a ductile crack is the classical smear

features on the fracture face. It has been argued

that these smeared features could be a result of

postfracture damage. While this is a possibility

that should be considered, since the background

evaluation revealed that the shaft was loaded in

torsion, and there are no crack opening stresses

operating on the transverse planes, this must be

a macroductile crack. A macrobrittle crack in

torsion is helical.

In closing this section on fracture, note how

important it is to follow the advice of the many

authors and teachers who state that background

research is step one of a failure analysis. The

possible loading geometries that could have

created the fracture must be reviewed to make a

proper determination of whether or not the crack

is macroscale ductile or brittle. That determina-

tion cannot be made without assessing what the

loading geometries may have been. If this frag-

ment had been totally covered with red rust, it

would have been even more difﬁcult to deter-

mine the basic ductile or brittle behavior of the

material without knowledge of the loading

geometries and expected fragment shape.

Stress versus Strength. Almost all real

loading geometries cause the stress to be highest

somewhere along the part surface. If the strength

is uniform, for example, if there is a piece of hot

rolled 1050 steel that does not have any de-

carburization or carburization and has not been

shot peened, the crack initiation is expected

somewhere at the original part surface. In the

presence of any type of bending or torsional

stresses, the highest stress will be at the surface

of the part. In the presence of pure tensile

loading, theoretically the crack could start any-

where in the cross section. Such pure tensile

loading is rare. Imagine the case of a hydraulic

cylinder rod. Even here, there must be a section

change, a ﬁllet, at some point. The loading at the

ﬁllet is not uniform; there is a stress concentra-

tion. Even a tensile test coupon that is forced to

break in an area of nominally uniform strength

and stress is not totally uniformly loaded. Most

tensile test coupons are tapered so that the stress

is slightly higher at the center of the gage length.

This brings the discussion to what is so useful

about heat treating steel. Many types of steels

and heat treatments create harder or stronger

layers at the surface. Heat treating allows

the strength to be increased where it is useful.

Figure 6 is from Ref 1with annotations. This

ﬁgure shows a subsurface crack initiation along

the boundary between the induction-hardened

case and the softer core. A second initiation

appears to be inside the induction-hardened

case. It must be recalled that cracks can happen

whenever the local stress is higher than the local

strength. Normally, the stress is expected to be

highest at the surface, and the stress decreases

toward the center of the part or the center of the

cross section. In this case, the ratio of the stress

to the strength was higher below the surface than

it was at the surface, so the crack initiated sub-

surface. It is important to note such an unusual

situation, where the location of the ratio of the

stress to the strength was highest at a subsurface

position.

Fig. 5

Example of a macroscale ductile fracture in torsional

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Imagine a simpler case of a cylindrical com-

ponent (Fig. 7). The surface of the part is shown

along the left side of the graph, and the center-

line is shown at right. For such a cylindrical

component that is loaded in either bending or

torsion, the stress will be highest at the

outside surface, and at the centerline, or neutral

axis, it will be nominally zero. A carburized or

induction-hardened material is actually stronger

at the surface layers where the stress is highest.

The y-axis, instead of being the stress level, can

be conceptually viewed as either the hardness or

some other measure of the strength of the com-

ponent. If the part has a heavy case, then the

strength follows the dashed line. In this case,

high strength levels go in deep toward the core.

At some point, the strength and hardness drop

off to a lower level. In this situation, if the solid

line represents the stress and the dashed line

represents the strength, this part should not have

a subsurface crack initiation. Everywhere, the

stress is lower than the strength. If the case is

too thin for the application in question, and

the strength drops off as the dotted line shows,

the stress is higher than the strength within a

subsurface band, which allows subsurface crack

initiation.

This ﬁgure shows a powerful technique for

speciﬁcation of case depths, which has the

potential to complement the usual experiential

method of case depth speciﬁcation. Anyone

doing fracture analysis on a case-hardened part

can also use this information to obtain an idea

about the appropriateness of the hardening spe-

ciﬁcation.

It is important to realize that there is one

other case where a subsurface initiation may

occur that is not related to the heat treating or

speciﬁcation quality. Imagine the presence of

a subsurface discontinuity, such as an inclusion,

a void, or a tiny crack. Even if the steel has

Fig. 7

Stress and strength as a function of position in a

cylindrical component loaded in torsion. Fracture ini-

tiation may be at either the surface or subsurface. Subsurface

initiation depends strongly on the hardness proﬁle from surface to

center if loading is in bending or torsion.

Fig. 6

Fracture features of an induction-hardened shaft (1541 steel) after fatigue testing in rotary bending. A, B, fracture origins.

Adapted from Ref 1, with annotations by W.T. Becker

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constant strength all the way through its cross

section, an inclusion that is big enough to locally

increase the stress above the fracture strength of

the part makes it possible to create a subsurface

initiation (Fig. 8).

Another potential cause of subsurface fracture

initiation is contact loading. A well-lubricated

bearing without any friction has the highest

stresses in its subsurface layers. Thus, fracture

initiation will be at a subsurface location where

contact loads are the predominant source of

stress. Inclusions can thus be very damaging in

contact-stress applications, such as bearing balls

or other rolling elements and races.

Understanding that most loading geometries

create the highest stresses at the surface also

allows one to understand why decarburized

layers can be so damaging. A decarburized layer

is softer and lower in strength than the material

with the desired nominal amount of carbon.

Decarburization can even occur on carburized

steel. Figure 9 shows a metallographic cross

section of a piece of steel that is carburized and

has quite a bit of retained austenite. Note the

dark constituent at the surface (arrows). On

carbon steels in the medium-carbon range, de-

carburization usually looks white, but here it

looks dark due to the presence of pearlite. The

decarburization affected the hardenability as

well as the hardness in this case. There was not

enough carbon to form martensite at the surface

when the part was quenched in heat treatment.

Very ﬁne pearlite was formed instead. The

pearlite structure is not as strong or fracture

resistant as the martensite structure that is

expected in the absence of the decarburization.

Thus, this part could be more susceptible to

fracture because of the decarburization during

heat treatment.

To complete the discussion of fracture, the

previous is summarized by emphasizing that the

macroscale features reveal the loading con-

ditions. Fracture analysts must start with the

macroscale, or the big picture. Many people start

with the details, or the little picture, and move on

to the big picture, but this can be a problem that

facilitates mistakes on the part of the analyst.

Microscale Fracture Features. Scanning

electron microscopy can be used to reveal the

microscale fracture features. Figure 10 shows a

Fig. 8

Stress and strength as a function of position in a

cylindrical component loaded in torsion with subsur-

face discontinuities. Surface conditions may include: inadvertent

decarburization, typically thin and may not be easy to ﬁnd; deep

case from induction or carburization; nitrided, thin case, often

not more than 5–10 mils. Part of the case may be ground off in the

ﬁnishing operations. The defect could be a faceted inclusion

(nitride) in a low-ductility matrix. Nondeformable nitride causes

stress concentration in the matrix.

Fig. 9

Decarburization of carburized steel. Each small scale

division is 2 mm.

Fig. 10

Scanning electron micrograph of microvoid coales-

cence

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ductile crack path, or what is often called ductile

dimples, microvoids, or microvoid coalescence

(MVC). The MVC is a characteristic fracture

morphology that indicates a component was

subjected to stresses in excess of the nominal

ultimate tensile strength of the component

material.

The MVC is often an indication that the heat

treated steel did not have a gross problem with

the heat treating that caused or contributed to an

embrittlement problem. Brittle fracture features,

including cleavage and (most) intergranular

features, are often indicative of a heat treating

problem. The arrows in Fig. 10 show the non-

metallic inclusions that initiated the void for-

mation.

Brittle fractures are often unexpected and

occur suddenly without any prior warning.

Ductile fracture by MVC is typically accom-

panied by prior plastic deformation, which gives

advanced warning of the impending fracture

event. This prior warning makes MVC the pre-

ferred mechanism if fracture occurs.

While MVC is generally desirable, it can

indicate that the material is too soft if a high-

strength material is in question. The MVC can

also reveal that the heat treater made a mistake,

such as no heat treatment, despite the often

preferred MVC fracture path.

Classical microscale brittle crack paths are

along the grain boundaries (intergranular),

(Fig. 11) or cause the grains themselves to split

(transgranular or cleavage). Many heat treating

and other processing problems can cause unde-

sirable intergranular cracking at the microscale.

To have intergranular cracking, something may

either cause a low-strength condition at the grain

boundaries or cause the stresses at the grain

boundaries to be higher than in the core of the

grains. Refer to the previous concept regarding

the relationship between the local stress and the

local strength.

One mechanism where intergranular fracture

occurs is quench cracking. When a piece of steel

is quenched to harden it, martensite will gen-

erally start forming near the surface, because that

is where it cools off fastest. As anyone familiar

with heat treating of steel knows, each deeper

layer of grains will subsequently transform to

martensite. On transforming to martensite, the

material expands. As the material continues

to cool, it contracts. So, there are grains that

undergo expansion during transformation

while the grains next to them contract. Could

that create a shear stress at the grain-boundary

location? The situation is obviously more com-

plicated than just described; obviously, the grain-

boundary strength is a function of temperature,

which is rapidly changing, but it is helpful to

think about what could be causing that inter-

granular crack and why quench cracks are gen-

erally intergranular.

Note also that carburized steel often will have

intergranular cracking in fatigue. Sometimes,

even the best metallography cannot show any

problem at the grain boundaries, that is, no

grain-boundary carbides, oxides, nitrides, poro-

sity, and so on. It is true that there are many

heat treating problems that can facilitate, or be a

factor in, intergranular cracking at lower stress

levels than the part usually sustains. However, it

is important to note that just because there is an

intergranular crack, it does not mean there is

surely a heat treating problem.

Hydrogen embrittlement is often revealed in

part by its intergranular crack habit, particularly

in steels heat treated to high strength levels.

Hydrogen embrittlement is not always purely

intergranular; sometimes, there will be tiny,

shallow microvoids on the grain-boundary sur-

faces. Hydrogen embrittlement could be the heat

treater’s fault (if the hardness is too high), and it

could be an issue with the plating (if records do

not show proper baking); it could be an inter-

action between these two factors.

The important point is not to confuse identi-

ﬁcation of the crack path with the cause. There

is a difference between identiﬁcation of the

physical shape of the crack and the physical

characteristics relating to the crack event. These

do not automatically lead to the cause.

Fig. 11

Scanning electron micrograph of intergranular

cracking

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The last microscale crack path is cleavage,

and Fig. 12 shows a classical view. Ferrite

cleaves readily at low temperatures. If a part is

not supposed to have any ferrite in it and there is

a large amount of cleavage, then that may be a

clue to look carefully for ferrite during micro-

structural analysis. The classical way to recog-

nize cleavage is the presence of patterns that

look like riverbeds with multiple tributaries.

The arrows in Fig. 12 show these river line

features.

In closing this section, fatigue microscale

features in heat treated steels are often not very

interesting or classical. The experienced analyst

can recognize them, but they are difﬁcult to

describe. They rarely have the textbook striation

features that are commonly shown for super-

alloys or aluminum alloys in published micro-

fractographs. Striations may be visible in a

low-carbon annealed steel, particularly in steels

with ferrite as part of the microstructure.

Beginners must be careful to distinguish pearlite

platelets from striations. Pearlite stops at grain

boundaries, and striations may cross grain

boundaries. If there are questions, pearlite spa-

cing can be examined on a cross section at a later

time in an effort to distinguish one from the

other. It is quite rare to ﬁnd striations in any kind

of hardened steel. Beach marks are often visible

at the macroscale, but striations are very un-

common as microfractographic features.

Summarizing the differences between macro-

and microscale features: The macrofeatures

show the loading geometry. The microfeatures

show the result of the microstructural interaction

with the environment, and mechanical and che-

mical aspects may inﬂuence the way the crack

interacts with the microstructure.

Wear

The original shape of the object shown in

Fig. 13 was a gear with normal-shaped teeth. It is

severely worn. No judgments can be made about

the cause with this one image.

Wear has many similarities to fracture and

deformation. Wear is basically deformation and

fracture going on at a microscale, and it can

continue until the point that macroscale damage

is present. Scanning electron microscopy (SEM)

is very helpful in understanding how wear

happens. Wear specialists have identiﬁed many

different wear mechanisms. However, even

without that specialized knowledge, the SEM

can reveal useful information for diagnosis and

prevention. In Fig. 14, there is smeared material.

This solid steel has now ﬂaked and smeared to

the point that it is present as thin platelets, which

are breaking off and allowing material loss. This

is one example of a combination of deformation

and fracture.

Fretting is a common type of wear that is

almost never related to heat treating or any

problem with heat treating but rather is related to

the geometry of the assembled parts. Fretting in

steel will generally produce a reddish, iron oxide

powder, and it roughens the surface. Figure 15

is an SEM image that just barely reveals

the initiation of a crack. Fretting often produces

a crack in an area that is thought to have

low stress.

Contact forces can cause surface damage due

to the action of Hertzian stresses. Figure 16

Fig. 12 Scanning electron micrograph of cleavage cracking

Fig. 13

Cross section of worn gear teeth. Approximate width

of steel segment shown is 23 mm (0.9 in.).

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shows a bearing race and a bearing ball. The ball

is being pressed into the race. This ﬁgure is

a rough schematic, conceptually showing an

exaggerated view of the most highly stressed

area due to the elastic deformation of the two

components. Without the load, there is essen-

tially a point. Under load, the contact area

becomes circular or elliptical; in other words,

there is a contact footprint. The actual high-

stress location is just below the surface. In the

presence of friction, the highest stresses are

moved toward the surface. An important part of

bearing wear failures is determining whether the

crack initiated at the surface, showing the pos-

sibility of a lubrication issue, or if it was truly a

subsurface initiation, in which case it may be a

microstructure problem, an inclusion, or another

anomaly. Bearings are generally loaded to very

high stress levels. The quality of the steel and, in

particular, minimization of inclusions are very

important in this type of application.

Another problem is that can cause surface-

initiated cracks grinder burn, which can create

high tensile stresses at the surface. Regardless of

what the service load is, a stress ﬁeld has been

created with a very high tensile stress at the

surface, the most undesirable location.

To close this section on mechanical damage,

it is often important to determine if there was a

service problem, such as abuse, or misuse, such

as using a screwdriver as a pry. Understanding

loading geometries and related fragment shapes

can shed light on this type of question.

Figure 17 shows a steel bar with a threaded

portion that is much smaller in diameter than the

rest. Note that the crack is in the large-diameter

portion. Could this have been caused by

the user?

To answer the question, one would have to do

a large amount of background information col-

lection. The important point to realize is that the

crack location is totally unexpected, and it is

difﬁcult to think of something that could have

happened in service to create a weak spot at this

hefty location.

If it cannot be qualitatively demonstrated that

somebody abused something, then it must be

quantitatively demonstrated. This is often difﬁ-

cult. It is important to know what the material

strength, fracture toughness, and other material

properties were at the time of the damage. It is

important to determine what the load or loading

geometry was that caused the damage. One must

Fig. 14

Scanning electron micrograph of a worn piece of

hardened medium-carbon plate showing details of

the wear mechanism

Fig. 15

Scanning electron micrograph of fatigue crack initi-

ating on worn carbonitrided steel. Original magni-

ﬁcation: approximately 4000 ·

Race

Bearing ball

Stress

concentrations

Fig. 16

Rough schematic of stresses in contact loading of a

bearing ball on a race

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consider how the suspected loading geometry

differs from the actual design intent. The analyst

again must remember that just because it is

broken does not mean it was abused. Even if it

meets the speciﬁcation, it does not prove that the

user abused the product.

Corrosion and Environmental Damage

Figure 18 shows an example of a 300-series

stainless steel that was probably not heat treated

in the most ideal manner, since small pre-

cipitates can be seen along the grain boundaries.

Note the crack location, which seems to be

seeking the grain boundaries. To create a crack,

there must be a load or a stress. There is corro-

sion present too, so this may be a stress-

corrosion crack. Did the nonideal heat treating

condition cause the crack? In this case, a chain

of reasoning cannot directly link the speciﬁc

heat treating problem with the crack, because

there is no information about the stress

levels. Stress-corrosion cracking, by deﬁnition,

requires a threshold level of corrosion and

stress. In this case, there is no evidence that the

crack was caused by bad heat treatment. This

seemingly subtle distinction may be very

important in the case of a catastrophic failure

event.

Figure 19 shows an example that involves

another stainless steel weld. Welding is a kind

of heat treatment, although not as controlled as

an intentional heat treatment. An acid sub-

stance, polythionic acid, was in contact with

the weldment. There are some cracks on one

side of the weld, while the other side is free of

cracks. What kind of cracks are these? The

crack path is intergranular (Fig. 20). The

micrograph in Fig. 21 was taken after an

ASTM International test, and it shows ditching

Fig. 17

Steel bar with crack in unexpected position. Orig-

inally shock loaded in compression. Threaded

portion diameter is approximately 2 cm.

Fig. 18

Grain-boundary precipitates in a 300-series stainless

steel

Fig. 19 Stainless steel weldment

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characteristics. This indicates that whatever the

heat sequence from the weld, it did create this

condition that appears to have facilitated

intergranular corrosion. However, steel would

have been considered to pass the test for free-

dom from sensitization. Despite this, the ther-

mal experience of the material can be directly

linked to the form of the crack. Although this is

not a heat treating example, this demonstrates

the line of reasoning that is required to deter-

mine cause. It is interesting to note that one

side cracked and the other side of the weld did

not. It is likely that the side that did not crack

was an L-series, a low-carbon series, speciﬁ-

cally made to minimize the chance of cracking

in weld heat-affected zones in stainless

steels. The cracked material was probably not

an L-series. This is an example of a classical

damage mechanism, stress-corrosion cracking.

When the regular 300-series stainless heated

up, the chromium and carbon combined and

precipitated along the grain boundaries, taking

the chromium out of solution and making the

material less corrosion resistant. Frequently,

such situations lead to pit formation, which

then allows the crack to propagate from the

stress concentration at the pit. This is an

example of a complex damage mechanism.

Another commonly named damage mech-

anism is corrosion fatigue. It must be under-

stood that a corroded part that broke due to

repeated crack extension under load did not

necessarily experience the mechanism called

corrosion fatigue. Corrosion fatigue is a dam-

age mechanism that is studied in the laboratory.

The named mechanism is invoked when it can

be demonstrated quantitatively that the crack is

growing much faster under the same loading

conditions than it would in the absence of the

corrosive substance. In a real component out in

the ﬁeld, such as a heavy off-road vehicle

application, it is very difﬁcult to obtain an

accurate service history day-by-day. Going

back to published research data for standard

test coupons and proving that a particular

situation is or is not corrosion fatigue will

likely prove very difﬁcult. In Fig. 22, a shaft is

Fig. 21

ASTM International sensitization test results showing

ditching characteristics

Fig. 20

Scanning electron micrograph of stainless steel

weldment with intergranular cracking

Fig. 22

Cracked shaft used in a corrosive environment.

Diameter of the shaft is approximately 10 cm (4 in.).

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obviously corroded. The band highlighted

by the arrows was covered with a layer of red

rust. Figure 23 shows the crack created by the

corrosion and stress combination. Note that

there are also secondary cracks. The tip of this

crack looks like a witch’s broom or the trails of

a sparkler. This is not a normal fatigue crack. A

normal fatigue crack does not branch out and

have multiple tips. In this case, the corrosion

deﬁnitely has some kind of signiﬁcant impact

on the damage mechanism. In many cases,

examination of ﬁeld failures is likely to leave a

question mark regarding the quantitative eva-

luation of how much faster the crack is growing

because of the corrosion.

To demonstrate that corrosion is the cause

or the fault of a failure, it is not enough simply

to say the component was in a corrosive en-

vironment and it cracked because of that. It must

be assured that the speciﬁc conditions, the spe-

ciﬁc material, and the speciﬁc process condition

of the material were in the realm that has been

demonstrated to be a problem for the damage

mechanism invoked.

For example, concentration of an aggressive

substance, the threshold stress level, and the

temperature may be required to be in a restricted

range before a particular mechanism can be

properly said to have been acting.

Factors Contributing to Poor Response

from Heat Treatment

Raw Material Characteristics That

Can Contribute to Poor Response

from Heat Treatment

What are the raw material characteristics that

can contribute to poor heat treating outcome?

One very important characteristic is composi-

tion. There will be a range of values for each

type of atom that is speciﬁed for the grade in

question, as well as for unspeciﬁed elements.

A heat treater may receive material of an iron

matrix that could, with the same name, have a

very wide range of responses to the heat, heating

rate, heating dwell time, cooling rate, and so on.

Lean and rich alloy content can have a strong

inﬂuence on whether or not quench cracks occur.

Lean and rich compositions also strongly inﬂu-

ence how readily the hardness speciﬁcation is

attained. For a hardness speciﬁcation that is

toward the upper limit of what can be reliably

obtained for a particular grade, it may be difﬁ-

cult to meet the speciﬁcation for a given lot if all

the elements are on the lean end. If the part(s)

crack on quenching, an important task in the

troubleshooting process is to determine whether

it is due to a lean or rich (more likely) alloy

Fig. 23

Crack proﬁle and adjacent secondary crack tip. Original magniﬁcation: 50 · . Inset is of a different secondary crack tip.

Original magniﬁcation: 500 ·

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