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

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a discussion of all materials. For almost all

applications, the high hardness of cold working

tool steels is normally achieved from low tem-

pering treatments. AISI D and O steels normally

have low secondary hardening. In fact, the cold

work application is dictated by this situation.

Because the materials show a low potential

for hardness retention at hot conditions, their

application is limited to low temperatures,

typically room temperature.

In practical terms, the aforementioned curves

show that tempering must normally be per-

formed at temperatures near 200



C, if a hard-

ness of approximately 60 HRC is desired. AISI S

grades, herein represented by S1 steel, are also

tempered in low temperatures, approximately

300



C. The chemical composition of these

alloys is specially designed for this condition,

with a high silicon level, because this element

is known to dislocate temper embrittlement to

higher temperatures (Ref 4). A distinct behavior

is presented by the newcomer 8% Cr steels.

Besides lower carbon and chromium, this class

also has higher molybdenum contents than AISI

D grades. This enhances secondary hardening,

which enables tempering at higher than 500



and obtains hardness as high as 62 HRC. A

substantial improvement in toughness and sur-

face treatment behavior is obtained through this

alternative tempering, which is discussed sub-

sequently.

Some heat treating failures are observed

immediately after heat treatment and appear as

small or (usually) large catastrophic cracks.

However, other failure types related to heat

treatment are only observed during tool use,

when one notices premature failure or a lower-

than-normal performance.

Fig. 6

Microstructures of cold work tool steels. (a) AISI D6, which is similar to D3. (b) AISI D2. (c) An 8% Cr tool steel with brand name

VF800AT. (d) AISI O1. Regions are typical for midradius of a 63 mm (2

in.) bar after hardening and tempering to 60 HRC.

(a–d) Etched with 4% nital for the same amount of time. Original magniﬁcation: 100 · . Source: Ref 3

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For simpliﬁcation issues, heat-treating-

related failures are divided by topics that rep-

resent the main cause. However, in many cases,

this division is not possible. That is because

several causes can and do act in synergy,

amplifying their effects and thus leading to the

observed failure. Nevertheless, the division is

kept. It is up to the reader to combine the pre-

sented information, keeping in mind the possi-

bility for interaction when solving or analyzing a

speciﬁc troubleshooting case.

Design-Related Failures. The previous dis-

cussion of cold work tool steel metallurgy and

characteristics explains why this class of mate-

rials is so prone to fracture and cracking. Except

for AISI S grades, all other materials are very

brittle. This is due to their intrinsic nature—the

combination of high hardness and primary

carbides—and also because cold work steels

are used predominantly at room temperature,

where fracture toughness of steels is naturally

reduced (Ref 5). This fact is illustrated in Fig. 8,

where the lower toughness of A, D, and O grades

in comparison to H or S steels is obvious.

Cold work tool steels are thus prone to failure

under stress concentrators, also called stress

raisers, that are imposed by tool design or

machining. Today (2008), modern software is

able to calculate stresses and tool working con-

ditions and can help to reduce stresses and

especially localized stresses under some regions

of tools. However, several tools are still

designed based only on previous experience.

Design faults may cause failures in heat

treatment but also during tool use, leading to

short service life. Failures just after heat treat-

ment normally occur in the presence of some

of the following features: heavy sections adja-

cent to light sections, sharp corners, stamp

marks, blind holes, and improperly spaced holes

(Ref 7). Several of these faults are illustrated in

Fig. 9. Large section-size variation caused the

failures shown in Fig. 9(a) and 9(b), while the

presence of sharp corners or closely spaced

holes is shown in Fig. 9(c).

Other stress-concentration effects can also

cause or even facilitate cracking. One example

Fig. 8

Comparison of longitudinal Charpy V-notched impact

toughness for various tool steel specimens taken from

89 mm square stock and tested at working hardness. Source:

Ref 6

Fig. 7

Tempering curves for most common tool steels used in cold working. Tempering curves are obtained after hardening

small (25 mm or 1 in.) specimens of all materials with the usual hardening temperature: 920



C for S1, 800



C for O1, 940



for D6 (similar to D3), 1010



C for D2, and 1030



C for the 8% Cr steel called VF800AT.

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is presented in Fig. 9(d), showing cracking

produced by stamp marks. It is not evident

in Fig. 9(d), but this failure also had a contri-

bution of poor machine ﬁnishing, because the

deep tool mark also acts as an important stress

concentrator. Figure 9(e) shows a typical sharp-

corner crack. In this case, the corner was ﬁlled,

but there was a nick in the corner where the

cracking began. The shape of this ﬁxture is also

poor for steel that must be oil quenched. As in

the case of Fig. 9(a), thinner outer regions cool

more rapidly, forming martensite ﬁrst, while the

more massive central regions cool more slowly.

In some cases, it is not possible to eliminate

all the stress-concentration effects from a tool

design. However, they can be minimized if heat

treatment and service failure issues are con-

sidered prior to tool design. In other words,

the designer should foresee possible heat treat-

ment or service problems at the beginning of

tool design. As a result, several failures can

be avoided, and service life may be enhanced.

In this context, some basic advice is given in

Fig. 10.

Another possibility for solving heat treatment

or service cracking is tool steel selection. Instead

of using water-hardening grades, oil-hardening

ones are preferred in situations sensitive to

quench cracking. In some circumstances, it is

possible to apply air-hardening grades, such as

Fig. 9

Examples of heat treatment cracking caused by design faults in hot work tool steels. (a) Cold work punch, made of a high-speed

steel, that cracked because of the large difference in section. Source Ref 1. (b) The same for a D2 die, also assisted by poor

machine ﬁnishing. Source: Ref 8. (c) O-type steel die cracked through the sharp corners Source: Ref 8. (d) Failure of die caused by stress-

concentration effect of deep stamp marks. Source: Ref 1. (e) Fixture made from AISI O1 tool steel that cracked during oil quenching. A nick

in the ﬁllet region helped to initiate cracking. Original magniﬁcation: 0.75 · . Source Ref 9

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series A or the new grade type 8% Cr. For

example, this could be a solution for the failure

shown in Fig. 9(e). Even D grades can be air

quenched, depending on die section. However,

the continuous cooling transformation diagram

of the cooling material should be analyzed for

the hardening condition and also the possibility

for carbide precipitation on the grain boundaries.

Before ﬁnishing this subject, one further point

should be considered. Stress raisers increase

failures in cold work tool steel, mainly due to

the intrinsically low toughness of such grades.

However, almost any tool displays some loca-

lized stress. Depending on working conditions,

one or more cracks can be initiated and propa-

gate throughout, fracturing or spalling the tool.

Therefore, cold work steels are sensitive to over-

load failure. In many situations, no problems

exist in the steel, the design, or even in the heat

treatment; the only cause may be excessive

stressing of the tool due to its incorrect use.

Surface Damage by Grinding or Electrical

Discharge Machining. In the previous section,

the intrinsic brittleness of cold work tool steels

was discussed, as well as the correlation of tool

failures to poorly designed tools (regarding stress

raisers). This section discusses surface defects

introduced in tool manufacturing by grinding or

electrical discharge machining (EDM). Never-

theless, these processes can introduce more than

the macroscopic stress raiser effect due to two

major factors, described as follows.

First, both grinding and EDM cause local

heating in the tool surface that, depending on

operational conditions, induces local tempering

or, far worse, reaustenitizing, quenching, and

hardening. The high carbon of these grades,

normally more than 1%, promotes high hard-

ness, more so than in other lower-carbon tool

steels (Fig. 11). As a result of heating and mar-

tensite transformation, small cracks, normally

hard to see with the unaided eye, may also be

formed, acting as stress raisers during tooling

and thus enabling premature cracking. Sec-

ondly, after such a metallurgical transformation

on the tool surface, the microstructure will be

predominantly untempered martensite (also

known as fresh martensite). This microstructure

is very brittle, especially in high-carbon steels

such as the cold work grades. The pre-existing

cracks or other cracks formed during tool

operation are much more prone to propagate,

thus accelerating tool failure.

Grinding and EDM cause heating and actually

act as a heat treatment applied to tool steel. In the

next sections, grinding and EDM are treated

separately, with some advice for avoiding

problems.

Incorrect Grinding. Hardenable steels are

more prone to grinding cracks than low-carbon,

low-alloy steels. Cold work tool steels and high-

speed steels are the most sensitive tool steel

grades to such problems due to the high hardness

Fig. 10

Simple possibilities for avoiding (a) sharp corners

and (b) large variation in section. Source: Ref 10

Fig. 11

Effect of carbon content on the hardness of differ-

ent microstructures. Martensite hardness increases

rapidly with carbon content. Reaustenitizing and quenching,

which can occur in the surface of ground or electrical discharge

machined tools, can cause high hardness and brittleness in high-

carbon grades such as cold work steels, leading to tool failure.

Source: Ref 1

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of as-quenched martensite and its low toughness.

Examples of grinding cracks are shown in

Fig. 12.

The formation of grinding cracks can be

explained as follows (Ref 11). Almost all the

energy used in grinding is converted into heat,

partly through pure friction and partly as a result

of deformation of the material. If a correct

grinding wheel has been chosen, most of the heat

will be removed in the chips, with only a smaller

part heating up the workpiece. Incorrect grinding

of a hardened tool steel can result in such a high

temperature at the ground surface that the tem-

pering temperature of the material is exceeded.

This results in a reduction in the hardness of the

surface, causing low performance when the tool

is used in ﬁeld applications. However, in addition

if the temperature is allowed to rise further, the

hardening temperature of the material can be

reached, resulting in rehardening.

Rehardening during the grinding operation

produces a mixture of nontempered and

tempered martensite in the surface layer, toge-

ther with retained austenite, as shown in

Fig. 13(a). The affected layer normally shows

white under optical microscope examination

(after metallographic preparation and acid

etching); this denotes the presence of untem-

pered martensite, which is more corrosion

resistant than tempered martensite. The diagram

in Fig. 13(b) shows the hardness proﬁle through

the surface of a cold work tool steel, incorrectly

ground in such a way as to produce rehardening.

The surface exhibits a high hardness due to

the untempered martensite. An overtempered

zone occurs just below the surface, where the

hardness is lower than the basic hardness of the

workpiece.

The following hints may provide a solution

to grinding problems. Incorrect grinding, result-

ing in a modiﬁed surface layer, often reveals

itself through burn marks—discoloration of the

ground surface (as indicatedted in Fig. 12b). In

order to avoid burning and grinding cracks, it

is necessary to keep down the temperature of

the ground part, for example, by means of good

cooling, and to employ properly dressed grind-

ing wheels that cut the material with sharp cut-

ting edges instead of simply generating heat

through friction (Ref 11).

The majority of grinding operations leave

residual stresses in the ground surface, usually

being at a maximum close to the surface. The

ﬁrst and most common effect of such stresses

is the occurrence of cracks. Stresses can cause

permanent deformation of the ground part when

grinding thin materials. This may be accom-

panied by retained austenite formation, which

Fig. 12

Examples of grinding cracks. (a) Two views of an S1

tool cutter die cracked and spalled after grinding. As-

received (left) and after magnetic particle testing (right), accent-

uating the cracks Source: Ref 9. (b) A D2 die that cracked due to

incorrect grinding (arrow indicates grinding marks) Failure was

also assi sted by closely spaced holes and electrical discharge

machining procedures. Generally, grinding cracks are not as easy

to see as this. It is usually necessary to examine the part under a

microscope or with magnetic powder inspection in order to see

the cracks. Source: Ref 8

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enhances deformation and the possibility for

cracking.

Three avenues are available to reduce grinding

assisted failures. First is to control the heat

emerging from the grinding operation by use of

a proper cooling process. Secondly, the effect

of grinding stresses and problems can be reduced

by stress-relief tempering after grinding. This

also involves the tempering of some regions of

untempered martensite, if present. The treatment

temperature should be approximately 30



below the previous tempering temperature to

avoid any risk of reducing the hardness of the

workpiece. Third, another way of reducing

grinding stresses is to tumble or blast the ground

parts (Ref 11). Obviously, if the heat damage is

too high, that is, cracks, stress relief may not help.

Incorrect EDM. Electrical discharge mach-

ining is often used in the production of cold

work tools for various reasons. Cold work tools

have an intrinsic high wear resistance and are

normally difﬁcult-to-cut materials under regular

machining processes, such as milling. Finishing

the die making with EDM may be an interesting

solution, especially for complex-shaped tools.

However, new developments in high-speed

machining that feature low stock removal and

high frequency have been used on dies as hard as

60 HRC.

The use of EDM on hardened steels, however,

can produce a shallow, rehardened layer of

rapidly quenched as-cast structure and untem-

pered martensite at the surface, beneath which

is a layer of tempered martensite (Fig. 14). The

EDM surface layer is known as the white layer

because of its lighter appearance under optical

microscope observation of etched samples (this

is caused by the higher corrosion resistance of

untempered martensite). Normally, the white

layer contains microcracks that can grow into

serious cracks when the tool is loaded in service

(Ref 7).

When used on hardened steel, EDM also adds

surface stresses to the already established resi-

dual stresses; the origins of such stresses are

the thermal and phase transformation dimen-

sional variations that occur in EDM surfaces.

The temperatures developed in such regions are

so high that local melting and resolidiﬁcation

occur, as shown in the upper-left microstructure

in Fig. 15(c).

In summary, the EDM white layer has four

major problems that can enable or accelerate

die failures: high hardness, residual stresses,

Fig. 14

Electrical discharge machining (EDM) white layer

found on a die surface made of AISI D6 (similar to

D3) tool steel. Note the white aspect of untempered martensite

caused by the EDM process and the presence of small cracks in

this layer. Original magniﬁcation: 500 · . Courtesy of Villares

Metals

Fig. 13

(a) White layer on a tool surface rehardened by an

incorrect grinding procedure. (b) Typical hardness

proﬁle in regions close to cracks. Source: Ref 11

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Fig. 15

(a) A2 tool steel blanking die, 63 mm (2

in.) in diameter by 13 mm (

in.) thick, that cracked in service because of a brittle

zone that had formed during electrical discharge machining (EDM) of the cavity at center. Arrows point to cracks emanating

from the cavity. Source: Ref 7. (b) Tool failure due to the same reason, where the 3.2 mm (

in.) holes were produced by wire-EDM.

(c) The effect of EDM on surface microstructures and approximate hardness of the tool shown in (b) are presented. Etched with 3% nital.

Central image in lower magniﬁcation; all other images in the same magniﬁcation. Source for (b) and (c): Ref 9

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coarse (as-cast) microstructure, and frequent

precracked regions.

Some typical examples of die failures assisted

by incorrect EDM are shown in Fig. 15. In

Fig. 15(a), a 63 mm (2

in.) diameter by

13 mm (0.5 in). long blanking die for a small

part cracked from the corners of the EDM cavity

after the die had produced 20,000 pieces. The

die was made of heat treated A2 tool steel. In

Fig. 15(b), the surface of an AISI A4 cup plate is

shown, with spalling at one of the holes, which

were made by EDM. A laboratory investigation

of this failure led to the typical appearance of

EDM-assisted failures: a coarse, white surface

layer that is very high and brittle, due to the

presence of an untempered martensitic matrix

and net carbides. Below, unquenched martensite

is observed, followed by a region of overtem-

pered martensite, after which the normal (core)

microstructure is observed.

In many situations, the affected regions are not

exposed as clearly as in Fig. 15(c). However, the

typical white layer is always present on the tool

surface, as shown in Fig. 14. The coarser this

layer, the higher the probability for tool failure,

due to its brittleness and the fact that the EDM

white layer likely possesses cracks.

Three major practices are recommended for

avoiding premature cracking caused by EDM:

 Reduce the stock removal when ﬁnishing the

EDM process (if low-frequency EDM was

used for roughing, high-frequency should be

used for ﬁnishing). This is helpful for mini-

mizing the depth of the rehardened white

layer.

 The white layer should be eliminated or

minimized by light grinding or lapping. This

procedure is time-consuming but, in many

situations, can lead to an impressive exten-

sion of tool life, especially when white layers

are thick and the tool is crack-sensitive.

 For relieving stresses in EDM-processed

dies and increasing the toughness of the re-

maining white layer, a new tempering treat-

ment should be performed. Its temperature

should be 30 to 50



C below the maximum

tempering temperature used in the heat

treatment, to avoid hardness loss. Normally,

this procedure is easy to apply and therefore

is highly recommended.

Although both grinding and EDM can damage

the tool surface, EDM problems are much

more common in industrial tool failures. This

occurs in particular for cold work tool steels and

high-speed steels, where the surface white layer

has high brittleness and the base material (i.e.,

the tool steel) has low resistance to crack pro-

pagation. However, EDM-assisted failures are

also observed in hot work dies.

Also important to mention here is the effect of

incorrect EDM in plastic molds. Although the

mechanical stressing is normally low, surface

ﬁnishing (by polishing or texturing) is crucial in

this application because plastic injected parts are

able to reproduce any problems on the mold

surface. Thus, EDM defects may cause serious

quality problems to injected parts, impairing the

mold application.

Failures due to the Heat Treating Proce-

dure of Cold Work Steels. The heat treating

procedure can itself deeply change the micro-

structure and properties of all tool steels, not

only the cold work grades. This may occur even

if the speciﬁed hardness is obtained. This section

deals with failures caused by improper heat

treating procedure and is divided into the three

most common causes in cold work tool steels:

the use of incorrect temperatures, the use of

excessively short tempering times (or even no

tempering at all), and the formation of excessive

amounts of retained austenite, caused either by

improper hardening or incorrect tempering.

Incorrect Hardening or Tempering Tem-

peratures. As for other tool steels, the same

class of cold work tool steels may present im-

portant differences in the indicated heat treat-

ing temperatures. If the temperature is higher

or lower than that indicated for a certain grade,

mechanical properties may be altered, especially

for toughness. Thus, the die performance is also

strongly inﬂuenced.

This section describes this effect in a speciﬁc

grade—the 8% Cr tool steel, which has been

highly employed in tools that traditionally use

grades from the D or O series. As discussed

previously, the 8%Cr-0.8%C steels have a dis-

tinct combination of toughness and wear resis-

tance that makes these grades very suitable for

cold work tooling. However, their heat treating

temperatures are considerably different from

that used in the usual grades.

To illustrate this effect, an 8% Cr steel was

chosen (commercial name VF800), and various

temperatures were used for its heat treatment.

The composition of this grade is shown in

Table 2. Such conditions were analyzed in

the laboratory in terms of microstructure and

mechanical properties (measured by a bend

test, Ref 12). Four conditions were applied, as

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follows, and summarized in Fig. 16. For all, the

hardness was maintained at 60 HRC:

 Condition 1: standard-condition VF800AT

grade, with hardening temperature approxi-

mately of 1030



C and tempering at high

temperatures, 540



C (twice, for 2 h)

 Condition 2: low hardening and low tem-

pering temperatures, 970 and 200



respectively. This condition is typical for

high-chromium and high-carbon D grades,

such as D3 and D6.

 Condition 3: typical hardening but lower

tempering temperature. Although not indi-

cated for 8% Cr steels, this condition is

typical for D2 steel, a well-known grade for

heat treaters. This is used in D2 for attaining

the 60 HRC level, due to the weak secondary

hardness of this grade. However, the 8%

Cr grades normally have higher alloy con-

tent in terms of molybdenum or vanadium,

allowing 60 HRC to be obtained after

high-temperature tempering. In the case of

VF800AT, up to 63 HRC is possible, de-

pending on the hardening condition (Ref 13).

 Condition 4: hardening temperature higher

than normal, using a condition typical

for high-speed steels (when treated to

60 HRC)—hardening at 1150



C and

tempering at 570



The results of mechanical properties and

microstructures for all conditions are shown

in Fig. 16. A substantial reduction is observed

for conditions 2 to 4 compared to the material

treated under normal conditions (1).

This difference in mechanical properties may

be understood based on the relative micro-

structure for each condition. The ﬁrst condition

has a relatively dark martensitic matrix and

dispersion of primary carbides, undissolved

during the hardening treatment. This is typical

for this material. The dark matrix indicates high-

temperature tempering, where stress relief of

martensite transformation is well performed; the

dispersion of primary carbides is important for

wear resistance.

In the other conditions, the microstructures

show a different aspect. In conditions 2 and 3,

tempering at low temperatures is denoted by less

intense etching, converting to a lighter matrix.

In these cases, hardness is produced by a highly

unstable and stressed martensitic structure

instead of the secondary hardening of high-

temperature tempering (adequate condition).

This reduces the toughness to the observed

levels. The last condition, 4, produced the lowest

toughness values. This is due to the intense

grain growth produced by the high hardening

temperatures, which are typical for high-speed

steels but not applied for this grade. In this

microstructure (Fig. 16e), the coarse martensite

plates reﬂect coarse austenite grains. The 8% Cr

cold work steels, as with other cold work steels,

have much lower alloy content than high-speed

steels. This reduces the pinning effect of carbide

precipitation on grain boundaries, thus causing

rapid grain growth when high-speed steel hard-

ening temperatures are used.

Excessive high hardening temperatures are

also common problems in heat treating high-

speed steels. Hardening temperatures for these

steels are close to the solidus temperature (less

than 50



C, 90



F), above which liquid forma-

tion starts within the microstructure (in carbide

rich areas), leading to expressive embrittlement.

Hardening temperature control is thus very

important. For example, M2 high-speed steel

hardening temperature is about 1200



(2192



F), but exceeding 1220



C (2228



may cause loss of toughness without beneﬁts

to hardness, and above 1240



C (2264



F),

liquation is likely to occur (Ref 38).

Figure 17 shows two cases of an incorrect heat

treatment procedure applied to an 8% Cr tool

steel, for two punches and a cutting blade, that

cracked prematurely. The microstructure ob-

served was close to condition 3 of Fig. 16, but

the microstructure was very difﬁcult to observe

after regular (nital) etching. A stronger etching

condition was applied. Besides tempering at low

temperature, there were evidence of hardening

overheating (coarse austenite grain sizes). This

is therefore a combination of two incorrect

situations—conditions 3 and 4.

Excessive Retained Austenite Content.

Hardening of tool steels involves the transfor-

mation of an initial phase, austenite, formed

during heating (austenitizing treatment). The

following transformations are directly depen-

dent on austenite composition. Martensite for-

mation is of particular interest, because this is

the expected phase after quenching. Martensitic

transformation is distinct from the usual solid-

phase transformations, because it does not

occur by the diffusion process. The formation

of martensite depends on the temperature

attained; therefore, two important temperatures

are deﬁned: the start and ﬁnish of martensite

transformation, determined by the M

and M

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Fig. 16

(a) Bend strength and fracture energy (energy necessary to fracture the specimen) obtained in a static bend test. Four-point

bend test with specimens of 5 mm (thickness) per 7 mm (width) cross section. Tested material is an 8% Cr cold work steel

(brand name VF800AT, Ref 13), heat treated to 60 HRC under four different conditions, 1 to 4 (see text). The legend indicates the

hardening (hard.) and tempering (temp.) temperatures, all for 30 min and twice for 2 h, respectively. (b) to (e) Respective microstructures

for conditions 1 to 4 after etching with 4% nital for 10 s. All regions refer to the midradius of a 60 mm bar. Source: Ref 12

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