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

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composition that is facilitating martensitic trans-

formation. This must be differentiated from the

situation where the major cause of the crack is

that the heat treater quenched the part(s) more

severely than usual.

It is difﬁcult to distinguish quench cracks due

to extreme quench severity from those due to

rich composition. However, it is easier to de-

termine whether the crack has an expected

geometry for a quench crack. In Fig. 24, the

crack is at the section change, which is a prime

location for a quench crack, as are sharp corners.

When examining a part to determine if rich

alloy/quench severity issues were at stake,

examine the crack surfaces for traces of temper

colors, including blues and browns. A dark

matte gray or black surface may indicate the

presence of an oxide-ﬁlled discontinuity that

simply opened due to thermal stresses rather

than a rich alloy/quench severity problem.

If temper colors are visible at the portion of

the crack surface nearest the part surface, it is

likely that one or more of the following factors

was present:

 Quench was more severe than was appro-

priate

 Alloy was richer than usual

 Section change was more severe (for ex-

ample, the ﬁllet radius was too sharp)

To conﬁrm the suspicion of a “pure” quench

severity/rich composition-related quench crack,

it is advisable to conﬁrm that the microscale

crack path is intergranular (usually SEM is

required). If a seam had been found at the

quench-opened crack, it would not be correct to

blame the heat treatment. Figure 25 shows an

interesting crack. There is a very heavy oxide

layer revealed by the cross section. The inset

shows a higher-magniﬁcation view of the seam

detail. There is a rounded particle that is totally

covered by a heavy, rounded oxide layer. The

crack surface does not have blue or brown

temper colors but was found to be a dark char-

coal black. This type of heavy oxide is unlikely

to have happened between when the part was

quenched (and presumably cracked) and when it

exited the temper furnace. Another unusual

feature of this crack is that it changed direction

multiple times (large arrows show crack growth

direction changes). The fact that these are not

predominantly intergranular cracks deals the

ﬁnal blow to any theory stating that this crack

was due to a heat treating problem.

Some people wonder why it was not possible

to see the seam before it went into the heat

treating process. Many seams are tightly closed

or smeared over until the part experiences the

stresses of the heat treating operation. Limita-

tions of nondestructive testing methodology

may also play a role. In the case of a part with a

quench crack in a location that would not be

expected to have high stresses in quenching due

to differential cooling rates, the experienced

Fig. 24 Quench crack with typical geometry

Fig. 25

Oxide layer along a seam most likely present in the raw material. Original magniﬁcation: 50 · . Inset original magniﬁcation:

200 ·

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analyst looks for some type of discontinuity. The

discontinuity does not have to be very thick. This

example was particularly heavy, but even a very

thin oxide layer is enough to be the predominant

cause of the crack.

Macro- or microsegregation (otherwise

known as banding) are other raw material char-

acteristics that can interfere with the expected

outcome of a heat treating process. Figure 26

is a longitudinal cross section from a medium-

carbon piece of steel. If the low-carbon layer is

right at the surface, it may be difﬁcult to meet a

minimum hardness speciﬁcation.

Banding, or microsegregation, is not always

bad. It can make it easier for the crack to grow in

a particular direction, and that may be an

advantage for a particular application. Samurai

and Damascus swords from antiquity had basi-

cally banded microstructures with very desirable

characteristics. While banding is not necessarily

bad, it can cause some variation in the response

to the heat treatment.

If a material has intermittent coarse grains, it

may be easier to form martensite in the large

grains and pearlite, ferrite, or bainite in the

surrounding ﬁne grains. Can the heat treater

create such a grain size distribution? This may

be possible by overheating.

However, it is also possible that the coarsen-

ing came from a subcritical amount of cold work

stored in the material. Normalizing the steel

prior to hardening may eliminate the nonuni-

form response to hardening, but the added cost

of normalization is often objectionable (Fig. 27).

Another type of raw material characteristic

that can cause problems in some applications is

heavy bands of stringers. Figure 28 shows a

piece of steel that has long sulﬁde stringers in it,

which can act like a seam.

Figure 29 is of a wire product that was used in

a coil spring. The material was subject to tor-

sional loading. A longitudinal discontinuity in a

material that is subject to torsional loading can

create very high local shear stresses in the

longitudinal or radial directions. This is one

reason that seams are not allowed in critical

applications for spring wires. Decarburization is

not detected on the surface, but there is a heavy

decarburized layer on either side of an inclusion

of the shape characteristic of a seam. The heat

treatment is unlikely to create such a varying

thickness layer of oxide.

Fig. 26

Longitudinal cross section showing microsegreg-

ation. Original steel segment shown is approximately

2 cm wide.

Fig. 27

A few coarse grains in the core of a ﬁne-grained

material that has been carburized are the only por-

tion of the core able to form martensite. Original magniﬁcation:

100 ·

Fig. 28

Scanning electron micrograph of sulﬁde stringers in a

piece of bar stock

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Component Characteristics

Figure 30 shows a cast steel product that was

quite uniformly carburized, except for the white

script features. The pattern looks very similar to

microsegregation between the dendrites and the

matrix. It would be very difﬁcult for the heat to

overcome the initial segregation in the raw

material. This component may not perform as

well as a component with a uniform case. These

script features may make the case even more

brittle than usual. The heat treater can control the

carbon potential and the heating cycle. How-

ever, the heat treater cannot locally put fewer

carbon atoms into the steel at the locations that

already have too many. When performing fail-

ure analysis of steel components, analysts must

Fig. 29

Optical micrograph of an oxid e-lined seam in a

piece of steel wire

Fig. 30

Cast steel after carburizing. Original magniﬁcation:

100 ·

Dark etching

area possible

indication of

heat treating

problem

Unusual crack

shape

Oxide filled crack

Evidence of

pre-existing

discontinuities

Note multiple

cracks in one

thread root

Fig. 31 Thread root of a steel fastener. Original magniﬁcation: 100 ·

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always be on the lookout for the whole history of

the part that led up to the heat treating event.

Forging discontinuities are another situation

that could cause a problem. Figure 31 shows a

cross section of a threaded fastener with a locally

different compositional steel inclusion. Some-

thing unusual happened to create this feature.

Other problems relating to poor heat treating

or poor service outcome include many different

types of design details. Other chapters in this

book address these issues in greater detail. To

summarize, some areas of common oversight

include:

 Materials selection

 Heat treating process selection

 Hardness level speciﬁcation and range and

position on part for hardness test

 Process details (batch or continuous

oven, etc.)

 Heat treater’s familiarity with the size and

complexity of the part and the quality level

needed

 Distortion control

 Testing competence

 Hardness scale selection

 Frequency of part testing

Figure 32 shows two micrographs at the same

magniﬁcation from the same component,

25 mm (1 in.) apart from each other. One of

them is virtually all martensite. In the other

location, there are wide grains of pearlite inter-

spersed between the martensite grains. There

was as much as 12 Rockwell C points difference

between the two microstructures. The designer

never indicated where to test the component!

This is a typical design issue.

Another common cause of disputes between

purchasers and providers of heat treating ser-

vices is that designers specify Rockwell tests

when there is no way to perform anything but a

Knoop or Vickers test. There are still many

newly minted engineering prints with case

depth speciﬁcations that are very unclear. There

are standard methods for specifying carburized,

induction-hardened, or carbonitrided case

depths, and it is helpful to use an industry

Fig. 32

Medium-carbon steel microstructures from the same component at two locations separated by approximately 25 mm (1 in.).

Each small scale division is 5 mm.

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standard when possible. The automotive manu-

facturers have done a good job of providing a

range of methods at a range of ease of testing. If

a standard method is not being used, it is often

difﬁcult to determine the designer’s intent.

The speciﬁcations for 400-series stainless

steel can be particularly difﬁcult for the average

mechanical designer to write. Many designers

specify 400-series stainless steel because they

want stainless, but they want to be able to heat

treat it to obtain higher hardness than standard

300-series annealed bar stock. For a number of

the 400-series grades, one must determine in

advance whether maximum strength or max-

imum corrosion resistance is desired. Two

totally different heat treating processes attain

those goals. The heat treater has no means of

guessing which characteristic is required.

What Are the Things That Can Go Wrong

in the Heat Treating Process?

This chapter has attempted to cover all the

aspects for which the heat treater should not be

blamed. What are the aspects for which the heat

treater may or should bear responsibility?

One approach is to say that heat treaters must

take responsibility for those aspects that are

speciﬁc to their equipment. These are details that

only the heat treater could know. Design engi-

neers specify materials and thus need to know

how to specify the type of testing and evaluation

required for application. A design engineer

cannot know how fast a load of parts will be

heated in a particular company’s individual

furnace. The design engineer cannot be expected

to know what type of ﬁxturing may be necessary

to maintain required distortion levels in the

part. These are aspects the heat treaters must

know. The heat treaters must know what load

size can be treated in their own furnaces and how

the load should be distributed. The heat treaters

must understand the characteristics of the

interactions between their equipment and the

full range of part sizes and load sizes they are

processing.

Heat Treating Errors. Excessive heating

rate, excessive time at temperature, and exces-

sive temperature can lead to excessive distor-

tion. Excessive temperature can cause problems

with excessive autotempering. If a massive part

is heated to 75, 100, or more degrees hotter than

it needs to be to obtain the uniform austenite

required before quench, then all the extra

heat must be removed, which can make it difﬁ-

cult to create the desired martensite at all. Fur-

thermore, the extra heat may act to partially

temper the martensite that is present. Excessive

Fig. 33

Undissolved ferrite and martensite in improperly speciﬁed and improperly induction-hardened medium-carbon steel part.

Each small scale division is 5 mm.

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heating can cause trouble in obtaining minimum

speciﬁed as-quenched hardness values. These

low as-quenched hardness characteristics may

or may not make a difference in the component

performance. However, when as-quenched

hardness tests are required, it is important to

know that excessive temperature may be a

cause.

Inadequate Heating Rate, Inadequate

Time at Temperature, and Inadequate Tem-

perature. Figure 33 shows the microstructure

of a part that was supposed to be induction

hardened. There is some martensite. The light

constituent is ferrite that never went into sol-

ution. The initial microstructure was probably a

mixture of ferrite and pearlite. The pearlite

transformed into martensite, but little of the

ferrite did. On quenching, the material produced

islands of martensite with islands of ferrite. The

irregular shapes of the ferrite islands are classi-

cal undissolved ferrite. This is not a typical

shape of ferrite grains formed on cooling.

Because of the severity of the consequences,

most of the common problems in heat treating

are related to hardening. Annealing can also be

done incorrectly. Figure 34 shows the micro-

structure of a steel that was supposed to be

spheroidized annealed. Spheroidization is a pro-

cess that may take 12, 15, or even 20 h at 600 to

700



C. Spheroidized annealed steel is generally

quite expensive. Its applications are usually

reserved for severe forming operations where

the added ductility is a necessity. The annealing

line management may be tempted to cut down

the process time to save money. The minimum

spheroidization that the material processor

believes will work is what is often provided.

In the case of varying incoming microstructures,

the part may not be as ductile as usual. Examine

the situation where the forming process creates a

crack that was undetected, and then the part is

heat treated for hardening. If the crack remains

undetected, there is now a part with a dis-

continuity due to the spheroidizing being done

poorly. This type of situation may be difﬁcult to

ﬁgure out, especially if the failure happens some

years after the fact.

Insufﬁcient time or temperature could apply

to formed parts requiring stress relief. Even at

relatively low hardness values, excessive resi-

dual stresses can make the part sensitive to

hydrogen embrittlement. A low-carbon steel

part that has been heavily deformed and

improperly stress relieved can crack after a very

short service life or even while sitting on a stor-

age shelf. Stress relief is often used on weld-

ments, and if it is not done properly, fatigue

cracks can initiate more readily at weld toes.

Machined parts that are improperly stress

relieved can distort or crack at a later time,

because the stresses are higher than one may

think.

Cooled Too Fast. The part that is cooled too

fast due to cold quenchant or excessive quen-

chant agitation may crack or suffer excessive

distortion. Undesired microstructures may also

result from excessively fast cooling. Bainite

may be desired, but the process formed mar-

tensite.

Cooled It Too Slowly or Cooled to the

Wrong Temperature. This can be a result of a

delay in moving the parts into the quench tank.

Alternatively, the composition of the polymer

quench tank may be improperly maintained.

Slow cooling can be a problem, because the

crack resistance of a microstructure with excess

ferrite may be lower than a properly hardened

and tempered martensitic structure.

Improper Atmosphere. Decarburization can

result from low carbon potential in the atmo-

sphere surrounding the parts. Carburization

can occur if there is too much carbon in the

atmosphere. Retained austenite in undesirable

amounts can also result from excessively rich

carbon in the atmosphere. There are still heat

treaters in business who believe that it is good to

use some ammonia when carburizing, even if the

Fig. 34

Incompletely spheroidized annealed steel. Each

small scale division is 2 mm.

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customer did not ask for carbonitriding. These

heat treaters may believe that the customer is

happy to get the lower price due to a faster

process that meets a surface hardness speciﬁ-

cation. The problem is that if the stress state

requires a certain level of strength at a certain

depth, the faster ammonia-enhanced process

may be inadequate.

Retained austenite in greater amounts than

normally found in parts that are straight

carburized can also be a problem. Porosity can

also be created in the case with nitrogen atmo-

spheres. This can be a problem if the surface

hardening is desired for strength rather than just

scratch resistance.

Figure 35 shows an example of bad carbur-

izing. There is a signiﬁcant fraction of retained

austenite in this case-hardened part, as well as a

large, chunky, unusually shaped “puzzle piece”

carbide. This feature could be a problem for

some applications of carburized parts and may

be the result of excessive carbon potential.

Concluding Comments

For those who do failure analysis of heat

treated steel parts, it is important to understand

what microstructure is expected, given the heat

treating process that is speciﬁed. For example, in

a 1050 steel that is 100 mm in diameter and

100 mm long and water quenched, should it be

martensite? There probably will be some mar-

tensite near the surface, but it may not be a very

deep layer. The exact depth will depend on how

hot it was heated prior to quench, the details of

the quench tank design, and many other factors

that may not be readily apparent.

It is important to have a large amount of

reasonably deep knowledge to be able to make a

fair and correct determination of where there

may have been a problem in the entire process of

designing a part, procuring material, and making

a component. This knowledge base includes

failure analysis, fracture analysis, and micro-

structure analysis and interpretation in order to

“read” the process history. Simply checking the

hardness and the composition to see if they meet

the speciﬁcation is not failure analysis. Failure

analysis includes a determination of the loading

geometry and background information, at the

very least, in addition to the basic certiﬁcation

conformance tests.

Some people legitimately perform internal

process failure analysis on a part that never left

the door of the manufacturing plant. In failure

analysis of a ﬁeld return, even from a non-end-

user assembly problem, it is important to do

more than simply look at the composition and

the hardness. The type of damage needs to be

identiﬁed, as well as the possible sources of that

Fracture #2 has both large chunky and script carbides

and large amounts of retained austenite.

01020304050

Fig. 35

“Puzzle piece” carbide microstructure in carburized steel, possibly due to excessively high carbon potential. Each small

scale division is 2 mm.

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type of damage. A review of the comprehen-

siveness of the design process may be in order. If

recurrence prevention is a goal of the failure

analysis, the damage specialist may lend some

understanding to the design engineers to help

them clarify the requirements of the component

characteristics.

Failure analysis can be very routine, or it can

be extremely involved. This chapter has con-

sidered only a few categories of the analysis

procedures and some of the reasoning involved

in determining what went wrong and at what part

of the life cycle the problem initiated.

A decision must be made at the beginning

of the analysis about how detailed the project

will be. If there is a single component, espe-

cially, or a very limited number of failed

parts, inadequate planning can leave inadequate

specimen material for testing in the case of

unanswered questions at the end of the project. It

is difﬁcult to overemphasize the importance of

spending enough time initially ﬁguring out

exactly what the goals of the failure analysis

project are and how much detail is required.

ACKNOWLEDGMENTS

The author thanks Mrs. W.T. Becker for permission

to use the copyrighted material of William T. Becker

in Fig. 6 to 8.

REFERENCE

1. D. Wulpi, Understanding How Components

Fail, American Society for Metals, 1985

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General Aspects of Failure Analysis

Waldek Wladimir Bose-Filho and Jose´ Ricardo Tarpani,

Universidade de Sa˜o Paulo

Marcelo Tadeu Milan, Instituto de Materiais Tecnolo´gicos do

Brasil Ltda.

FAILURE ANALYSIS is the process of

collecting, examining, and interpreting damage

evidence. The objective is to understand the

possible conditions leading to a failure and

perhaps prevent similar failures in the future.

A failure analysis should provide a well-

documented chain of evidence that either

excludes or supports possible interpretation of

the damage evidence. Clear-cut conclusions

do not always occur, and the tendency of

developing preconceived interpretations should

be avoided.

Various publications (e.g., Ref 1–6) describe

the guidelines and methods of failure analysis,

and this chapter brieﬂy outlines some of the

basic aspects of failure analysis. The ﬁrst section

describes some of the basic steps and major

concerns in conducting a failure analysis. This

is followed by a brief review of failure types

from fracture, distortion, wear, and corrosion.

Fracture is a common damage feature, because

the vast majority of mechanical failures involve

crack propagation—typically classiﬁed as duc-

tile, brittle, and fatigue, as brieﬂy described

in more detail. Distortion, wear, and corrosion

also can be important damage factors in failure

analysis.

General Guidelines of Failure Analysis

For a complete evaluation, the sequence of

stages in the investigation and analysis of fail-

ure, as detailed in Ref 5, is as follows (Ref 2):

1. Collection of background data and selection

of samples

2. Preliminary examination of the failed part

3. Nondestructive and mechanical testing

4. Selection, identiﬁcation, preservation, and/

or cleaning of specimens

5. Macroscopic examination and analysis and

photographic documentation

6. Microscopic examination and analysis

7. Selection, preparation, examination, and

analysis of metallographic specimens

8. Determination of failure mechanism

9. Chemical analysis

10. Fracture mechanics analysis

11. Testing under simulated service conditions

12. Analysis of all the evidence, formulation of

conclusions, and writing the report

These stages or steps are brieﬂy outlined as

follows.

Collection of Background Data and

Selection of Samples. There are basically

three fundamental principles to be carefully

followed when collecting damage evidence

from a fractured material (Ref 2):

 Locate the origin(s) of the fracture. The

whole fracture surface should be visually

inspected to identify the location of the

fracture-initiating site(s) and to isolate the

areas in the region of crack initiation that

will be most fruitful for further micro-

analysis. Where the size of the failed part

permits, visual examination should be con-

ducted with a low-magniﬁcation wide-ﬁeld

stereomicroscope having an oblique source

of illumination (Ref 3).

 Do not put the mating pieces of a fracture

back together, except with considerable care

and protection. Protection of the surfaces is

particularly important if electron micro-

scopic examination is to be part of the pro-

cedure (Ref 2). Appropriate packaging of

failed components for shipping is equally

important. Wrapping them directly into a

plastic bag, or placing pieces directly into

a plastic bottle or container, can intro-

duce unwanted hydrocarbon contaminants.

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Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 111-132

DOI: 10.1361/faht2008p111

www.asminternational.org

Fingerprints on the failed surfaces can also

introduce contamination (Ref 4);

 Do not conduct a destructive testing without

considerable thought. Alterations such as

cutting, drilling, and grinding can ruin

an investigation if performed prematurely.

Destructive testing must be performed only

after all possible information has been

extracted from the part in the original con-

dition and after all signiﬁcant features have

been carefully documented by photography

(Ref 2).

Preliminary Examination of the Failed

Part. In addition to locating the failure origin,

visual analysis is necessary to reveal stress con-

centrations, material imperfections, presence of

surface coatings, case-hardened regions, welds,

and other structural details that contribute to

cracking. A careful macroexamination is neces-

sary to characterize the condition of the fracture

surface so that the subsequent microexamination

strategy can be determined. Corrodents often

do not penetrate the crack tip, and this region

remains relatively clean. The visual macro-

analysis will often reveal secondary cracks that

have propagated only partially through a crac-

ked member. These part-through cracks can be

opened in the laboratory and are often in much

better condition than the main fracture (Ref 3).

Nondestructive and Mechanical Testing.

A wide variety of nondestructive testing is

available, including dye penetrant, ultrasonics,

x-ray, and eddy current, which can help in

the failure analysis task in order to unveil

even subtle and/or internal defects in a part.

Mechanical property tests are also ready to use,

ranging from a sample hardness test to elevated-

temperature tensile and impact testing. These

tests are often used to determine if degradation is

related to fabrication or to the service environ-

ment. Sometimes, a standard test can be adapted

to simulate manufacturing or in-service condi-

tions more closely (Ref 4).

Selection, Identiﬁcation, Preservation,

and/or Cleaning of Specimens. Unless a

fracture is evaluated immediately after it is

produced, it should be preserved as soon as

possible to prevent attack from the environment.

The best way to preserve a fracture is to dry it

with a gentle stream of dry compressed air, then

store it in a desiccator, a vacuum storage vessel,

or a sealed plastic bag containing a desiccant.

However, such isolation of the fracture is often

not practical. Therefore, corrosion-preventive

surface coatings must be used to inhibit oxida-

tion and corrosion of the fracture surface. The

primary disadvantage of using these surface

coatings is that fracture surface debris, which

often provides clues to the cause of fracture, may

be displaced during removal of the coating.

However, it is still possible to recover the sur-

face debris from the solvent used to remove

these surface coatings by ﬁltering the spent

solvent and capturing the residue. In regard to

cleaning techniques, fracture surfaces exposed

to various environments generally contain un-

wanted surface debris, corrosion or oxidation

products, and accumulated artifacts that must be

removed before meaningful fractography can

be performed. Before any cleaning procedures

begin, the fracture surface should be surveyed

with a low-power stereobinocular microscope,

and the results should be documented with ap-

propriate sketches or photographs. Low-power

microscope viewing will also establish the

severity of the cleaning problem and should also

be used to monitor the effectiveness of each

subsequent cleaning step. It is important to

emphasize that the debris and deposits on the

fracture surface can contain information that is

vital to understanding the cause of fracture. The

most common techniques for cleaning fracture

surfaces, in order of increasing aggressiveness,

are (Ref 3):

 Dry air blast or soft organic-ﬁber brush

cleaning

 Replica stripping

 Organic-solvent cleaning

 Water-based detergent cleaning

 Cathodic cleaning

 Chemical-etch cleaning

Macroscopic Examination and Analysis

and Photographic Documentation. More

often than not, the investigation starts with a

low-magniﬁcation, if any, observation of the

failed part. This visual examination can often

quickly answer questions such as: What was the

mode of failure? Did it crack, or was there a

uniform or pitting corrosion failure? Did the

protective oxide ﬁlm break down? Were the

welds visibly contaminated? A variable magni-

ﬁcation stereoscope equipped with a ring light

and directional ﬁberoptic lighting is a powerful

tool for macroscopic visual examination.

Contemporary stereoscopes can operate over a

range of 2.5 to 50 · (Ref 4).

Microscopic Examination and Analysis.

Once the area of interest is isolated, a smaller

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