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

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temperatures, respectively. If no transformation

takes place during quenching, martensite is

formed gradually after the M

temperature is

reached, and total transformation of austenite

to martensite takes place when the part reaches

the M

temperature.

In tool steels, most compositions have M

values below room temperature; as a result, part

of the austenite is not transformed to martensite.

This austenite fraction is called retained auste-

nite. It is distributed within the material micro-

structure; after etching, it is observed as a light

matrix crossed by plates or laths of martensite

(Fig. 18). As is discussed later, higher harden-

ing temperatures (overheating) lead to larger

amounts of retained austenite. In the micro-

structure, it becomes more evident due to the

presence of large martensite plates (resulting

from larger austenite grain sizes) crossing the

austenite phase within the matrix (Fig. 18b).

Retained austenite content is directly depen-

dent on the chemical composition, because it

determines the M

and M

values. The following

equation (Ref 14) quantiﬁes this dependence

for M

, since it is also a similar rate for M

.Itis

important to observe that all the alloy elements

(with the exception of cobalt) reduce the M

values, especially carbon, thus increasing the

amount of retained austenite:

(



C)=539 423%C  30:4%Mn  12:1%Cr

 17:7%Ni 7:5% Mo

(all elements in weight percent)

(Eq 1)

Consider again the phase transformation tak-

ing place, keeping in mind the effect of alloy

composition on M

values. As explained, aus-

tenite will be transformed to martensite because

of the rapid cooling of the quenching process.

Fig. 17

Examples of failures in an 8%Cr-0.8%C tool steel, caused by an incorrect heat treating procedure. (a) Punches and (b) their

microstructures. (c) Cutting blade and (d) the microstructure observed in its failure analysis. The primary cause of failure is

low-temperature tempering and high-temperature hardening, a combination of incorrect conditions 3 and 4 shown in Fig. 16. Micro-

structures etched with Villella’s reagent. Courtesy of Villares Metals

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Therefore, the composition that affects the M

value is the austenite composition at the mo-

ment of martensite transformation, not the alloy

composition. For monophase steels, such as

low-alloy, low-carbon grades, the austenite

composition is practically the alloy composi-

tion. However, in tool steels, especially cold

working tool steels and high-speed steels, high

amounts of carbon and alloy elements are

trapped in the undissolved carbides. The release

of these elements is only possible through car-

bide dissolution, which depends on time and

particularly on the austenitizing temperatures

involved. The higher the austenitizing tempera-

ture (hardening temperature), the larger the

amount of carbon and alloy elements that go

into solid solution, lowering the M

temperature

(according to Eq 1) and thus increasing the

amount of retained austenite. Examples are

shown in Fig. 19 for a chromium high-carbon

tool steel and for D2. As the hardening tem-

perature increases, the M

value decreases

(Fig. 19a), and the amount of retained austenite

increases (Fig. 19a, b).

Although the basic mechanism of retained

austenite in tool steels has been described, its

effect on materials properties has not been

explained. In general, retained austenite is un-

desirable (Ref 17). First, it is softer than mar-

tensite, due to the crystallographic nature of

austenite (face-centered cubic lattice). How-

ever, the effect of the amount of retained aus-

tenite on the as-quenched hardness is normally

only a few HRCs, unless a strong deviation in

heat treating procedure has occurred. Secondly,

the retained austenite may cause lower tough-

ness. Due to its unstable nature, austenite at

room temperature tends to transform to mar-

tensite if adequate thermodynamic conditions

are established; one possibility, besides the

temperature reduction, is stress. Such conditions

may be imposed during tool work that involves

elevated localized stress. In this condition,

austenite transforms to martensite (untempered,

of course), leading to brittleness. Because

retained austenite is distributed along all regions

of the steel microstructure, normally close to

carbides (microsegregated areas), its transfor-

mation to martensite may deeply embrittle

working regions, causing cracks or fracture

failures. Third, this transformation of retained

austenite also causes a volume increase, due

to the lattice difference between austenite

(more compact structure) and martensite. This

impairs dimensional stability, which is critical

for several tools, especially those that work

with small clearances, such as ﬁne blanking

dies.

Fig. 18

Retained austenite in two cold work tool steels after

hardening and tempering. (a) D2, with 60 HRC. (b)

O1, with 54 HRC. For both, the retained austenite content is

higher than expected (due to overheating in the hardening

treatment). (c) O1 punch from which the microstructure in (b) was

obtained. Cracked in service after short life. Retained austenite

and carbides are lighter in the microstructure after etching, but

they can be differentiated; carbides are either faceted or round,

but retained austenite has no delineated area, being within the

matrix. In both images, martensite plates that cross retained

austenite regions are clearly observed. In (a), the retained auste-

nite content is high enough to reduce the desired hardness from

60 to 54 HRC. Compare to the usual microstructure of O1 and

D2 in Fig. 6.

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Besides these three undesirable effects of

retained austenite, the cause of excess retained

austenite should also be considered when ana-

lyzing a failure. As discussed and shown in

Fig. 19, the increase in retained austenite content

in cold working tools is normally caused by

exceeding the recommended hardening tem-

perature. However, the retained austenite con-

tent may also be reduced when it is converted to

martensite (or bainite) after the ﬁrst tempering.

Transformation of retained austenite depends on

the tempering temperatures, as Fig. 20 shows for

A2 tool steel. For the purpose of converting

retained austenite, tempering must be carried out

immediately after quenching to avoid stabiliza-

tion of retained austenite (Ref 15) and a second

tempering must always be applied. At the second

tempering temperature, this untempered mar-

tensite (also known as fresh martensite)

or bainite is tempered, being the ﬁnal micro-

structure free from hard brittle phases.

For some cold work steels, such as AISI A or

8% Cr steels, tempering can be conducted at

temperatures (4500



C) that practically elimi-

nate retained austenite. However, in AISI O or

D steels, tempering to 60 HRC or more is nor-

mally done at lower temperatures (5350



C),

where some amount of retained austenite will

exist after tempering; in these cases, the control

of retained austenite content should be done

by not exceeding the hardening temperatures.

Nevertheless, in some cases, it is possible to lose

some hardness by using higher tempering tem-

peratures to reduce the amount of retained

austenite. This can be done for D2 steel if tem-

pered at approximately 520



C (Fig. 7); hard-

ness would be up to 58 HRC, but the amount of

retained austenite decreases from approximately

15% to less than 5% (Ref 18), considerably

improving toughness (Ref 19).

Application of cryogenic treatments is also

a possibility to reduce retained austenite

content, especially in cold work tool steels that

(a) (b)

Fig. 19

(a) Inﬂuence of austenitizing temperature on martensite transformation of a tool steel containing 1.1% C and 2.8% Cr. Higher

austenitizing temperatures lower M

temperatures and increase the amount of austenite retained at room temperature.

Source: Ref 15. (b) Amounts of microconstituents in D2 tool steel as a function of austenitizing temperature. Specimens austenitized for

30 min at temperature and oil quenched. Composition: 1.60% C, 11.95% Cr, 0.33% Mn, 0.32% Si, 0.79% Mo, 0.25% V, 0.18% P and

0.010% S. Source: Adopted from Ref 16

Fig. 20

Hardness and retained austenite as a function of

tempering in A2 tool steel. Source: Ref 18

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are tempered at low temperatures. In cryogenic

treatments, tools are led to very low tempera-

tures, approaching or crossing the ﬁnal M

temperature. They are normally performed after

a stress relief (at about 150



C) to avoid tool

cracking. After the cryogenic treatment, tem-

pering is also necessary to avoid brittleness from

the just formed fresh martensite.

In summary, the presence of retained auste-

nite in unusually large amounts is an indication

that either the hardening or tempering treatment

has been inadequately conducted. Other sources

of brittleness can emerge in such situations,

besides the retained austenite itself. For

example, higher hardening temperatures pro-

mote coarse grain sizes and also may increase

the potential for carbide precipitation on auste-

nite grain boundaries; both effects cause intense

embrittlement (condition 4, Fig. 16). Incorrect

tempering, with shorter times (see next section)

or incorrect temperatures (previous section),

changes the stress relief of the martensite

structure and the whole strengthening mecha-

nism, thus affecting material toughness as well

(condition 2, Fig. 16).

Therefore, several examples are observed

of industry failures assisted by retained

austenite that emerged from incorrect proce-

dures. Figure 21 presents two examples. The

ﬁrst, (Fig. 21a, b) shows an AISI O6 tool that

cracked after limited service. Retained austenite

is clearly observed in its microstructure. The

second tool in Fig. 21 is an AISI S7 die. This

grade has lower carbon (~0.50%) and much

lower undissolved carbides in comparison to the

O- or D-series steels. It should be less prone to

retained austenite formation and to the effect of

incorrect heat treating conditions. However, this

tool was carburized, and a surface pickup of

carbon took place, leading to a reduction of M

and M

and thus causing the high amount of

retained austenite. In both cases, overaustenitiz-

ing conditions were employed, enabling the

existence of such high retained austenite content

and leading to embrittlement.

Excessively Short or Absent Tempering.

“Tempering, the ﬁnal heat treatment step

applied to tool steels, is deﬁned as the heating

of a martensitic or hardened steel to some tem-

perature below A

temperature (initial tempera-

ture of austenite formation); this step produces

the ﬁnal structure and mechanical properties of

a hardened steel.” This citation, from Ref 1,

brieﬂy explains the importance of tempering

treatment. However, in practical situations, this

is not so obvious. After austenitizing and

quenching, the steel is hardened to a very high

hardness—in many cases, the highest hardness

possible to attain for a given steel. After tem-

pering, no signiﬁcant differences can be ob-

served in hardness measurements, especially for

cold work steels, which have a work hardness

very close to the as-quenched hardness. This can

lead to several problems regarding the embrittle-

ment of a tool caused by poor tempering prac-

tice.

During tempering, several solid-state pheno-

mena occur simultaneously in the steel micro-

structure. Depending on the alloy content of a

tool steel, the tempering curve presents a dif-

ferent aspect, as shown in Fig. 22 by a division in

classes. Class 1 is typical for high-carbon, low-

alloy tool steels, class 2 for high-chromium cold

work steels, class 3 for high-speed steels with

strong secondary hardening, and class 4 for hot

work tool steels. Typical cold work steels, from

AISI D or O series, will have curves close to

classes 1 or 2.

As shown and discussed in Fig. 22 and

quantitatively in Fig. 7, both AISI O and D

grades must be tempered at lower temperatures

to attain the highest hardness levels—60 HRC

or higher. Tempering is typically conducted

at temperatures of approximately 200



C. In

practical situations, this is not that simple. The

heat ﬂux at such low temperatures is also low,

and thus, tempering of dies may take several

hours. As a consequence, it is common to ﬁnd

examples of tools that were insufﬁciently tem-

pered. Two cases are shown in Fig. 23.

Avoiding these kind of failures is not techni-

cally difﬁcult, but it is time (and money) con-

suming. A basic rule should be observed: Any

operation should not be conducted with the steel

in the as-quenched condition. If the tempering

time is unknown, a rule of thumb to observe

is that a steel is hardly ever overtempered (if

the time is exceeded) but can be easily under-

tempered (if the time is too short). This happens

because the tempering parameter, as described

in Fig. 22, has the time in log scale. However, the

most desirable situation is to have good control

of the tool temperature during heat treating.

One example is monitoring tool heating and

treatment time with thermocouples attached

to the part; ideally, all tool regions should be

maintained at temperature for approximately

2h.

This discussion of tempering times is im-

portant, due to the intense microstructural

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transformations that occur in the initial temper-

ing stages. Carbon rearranges into martensite

crystals, and transition carbides are precipitated

that are only 2 to 4 nm in size. These transfor-

mations considerably enhance material tough-

ness, but hardness decreases only slightly from

the initial as-quenched hardness. In conclusion,

low-temperature tempering must not be sup-

pressed based on hardness but kept due to the

necessary changes in steel microstructure that

it promotes.

Heat Treating Failures of Hot Work Tools

Chemical Composition and Main Charac-

teristics of Hot Work Tool Steels. Heat

treating of hot work tools is usually more critical

than for cold work tools. Hot work tools are

normally larger and have higher machining

costs, besides being applied to high-demand

applications. Although cold work tools may also

be critical, such as for drawing dies and indus-

trial cutting blades, they usually do not exceed

Fig. 21

(a) AISI O6 graphitic tool steel punch machined from centerless-ground bar stock that cracked prematurely. (b) Micro-

structural examination revealed an overaustenitized structure consisting of appreciable retained austenite and coarse plate

martensite. (c) Failed AISI S7 jewelry striking die showing cracks (arrows) that formed shortly after the die was placed in service; (d) Its

microstructural examination revealed that the surface was slightly carburized and the die had been overaustenitized. Note coarse plate

martensite and unstable retained austenite. Source: Ref 9

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more than 500 kg. As discussed in the previous

section, it is useful to ﬁrst describe the heat

treating conditions of the common grades and

then to discuss failure analysis. For this reason,

Fig. 24 shows tempering curves for the most

common grades, whose chemical compositions

are shown in Table 3.

By analyzing chemical compositions, one can

anticipate several characteristics of hot work

tool steels and their differences from cold work

steels. First, hot work steels normally have lower

carbon, leading to lower as-quenched hardness;

also, undissolved carbides will be much lower or

even nonexistent in these grades. Thus, abrasive

wear resistance, which depends on hardness

and coarse primary carbides, is much lower in

hot work steels. Final hardness in hot work

tool steels is normally determined by desired

toughness instead of wear resistance. In most

situations, hardness is between 40 and 50 HRC.

Other properties, in addition to mechanical

strength at room temperature and wear resis-

tance, are important for hot work tool steels. The

main metallurgical properties are toughness

and strength at elevated temperatures; this last

property is dependent on the hardness stability

at high temperatures.

Toughness in hot work tools is an essential

property for avoiding cracks and fractures that

can be common in normal working conditions.

Unexpected and very unstable in cold work

tools, cracks may be encountered after a given

operation time for hot work dies. The most

important example is ﬁne crack networks, large

in number but small in length, observed on a tool

surface after the tooling operation. This kind of

crack is known as heat checking and is typically

found in die-casting dies (Fig. 25) as well as in

forging dies or other hot work tools. The cause

is thermal fatigue at the tool surface, caused by

repeated temperature ﬂuctuations (heating and

cooling) during tool operation. A full explana-

tion of this behavior is found in the literature,

for example, Ref 20 to 22. Depending on the

intensity, heat checking can lead to tool failure,

which is typically the case in die-casting cavities

(Fig. 25). In such situations, the casting metal,

normally aluminum, enters into the heat check-

ing cracks, making part extractions difﬁcult or

impairing part surface quality. In cases of a lack

of a toughness, gross cracking may also occur,

leading to complete loss of the working tool.

High mechanical strength at high temperature

is also fundamental in hot working steels. High

temperature means temperatures that can affect

steel microstructures, normally above 500



Depending on time and stress conditions, such

high temperatures lead to a decrease in hardness

that accelerates several die failure mechanisms,

especially hot wear, plastic deformation at

working temperatures, and heat checking. The

ability of a given grade to maintain hardness at

prolonged time at high temperatures is referred

to as tempering resistance. A simple (basic)

comparison of tempering resistance may be

done by analyzing tempering curves. Higher

tempering resistance is related to curves dis-

located to the right (Fig. 24).

As shown schematically in Fig. 26, such high-

temperature hardness is only possible by a very

important straightening mechanism—secondary

hardening, promoted by the precipitation of

ﬁne alloy carbides. The stronger the secondary

hardening (meaning more intense carbide pre-

cipitation), the higher the tempering resistance

of hot work tool steels. Such precipitation

intensity depends on the amount of alloy ele-

ments in solid solution, which is related to the

alloy composition and heat treating practice.

Figure 27 explains this effect by comparing the

addition of different alloy contents to carbon

steels; a stronger precipitation hardening is

Fig. 22

Schematic diagram of hardness versus tempering

temperature (assuming constant time at each tem-

perature) or versus a time-temperature tempering parameter for

four major types of tempering response in tool steels. Classes 1 to

4 are typical, respectively, for low-alloy tool steels, cold work

chromium steels, high-speed steels, and hot work tool steels.

Source: Ref 1

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obtained by molybdenum, vanadium, or tung-

sten alloying.

Therefore, the desired mechanical properties

of hot work tool steels are only attained if a

proper heat treatment is applied. For example,

adequate secondary hardening will only be

present if alloy elements are in solid solution in

the as-quenched structure (usually martensite).

This is only possible through an adequate aus-

tenitizing treatment for hardening, capable of

dissolving the alloy elements present in the form

of carbides in the initial (annealed) state. In

addition, a proper hardening procedure should

avoid excessive grain growth and grain

boundary embrittlement by carbide precipita-

tion. Tempering, on the other hand, should

eliminate retained austenite and promote

adequate precipitation of the alloy carbide. In

summary, tool steel properties and the expected

performance are only possible after a quality

heat treatment. Otherwise, failures may occur,

reducing die life and increasing tooling costs.

The following sections describe some typical

failures of tool steels after heat treatment and

the main mechanism that caused failure. The

main mechanism is described and divided sys-

tematically, but in practical situations, several

mechanisms as well as the tool use should be

(a)

(b)

Fig. 23

(a) Tool called a triturating wheel made from AISI D6 (similar to D3) that had poor performance due to a premature crack.

Tool diameter of approximately 300 mm (~11.5 in.). The identiﬁed cause was the absence of tempering. The hardness

measurement was 65 HRC, the usual value for the as-quenched condition of this grade. Courtesy of Villares Metals. (b) D2 dies cracked

during ﬁnish grinding operation. Cracks emerged due to the as-quenched condition of the microstructure (not tempered). Source: Ref 9

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investigated to discover the root cause of a

failure.

In specialized literature, it is less common to

discuss failures of hot work tools than of cold

work. This is because of the intrinsic lower

brittleness of hot work tool steels. Nevertheless,

the subject is very important for these materials,

due to the high value of a tooling set for hot

working and also because of the large pro-

duction performance expected from it. Even if

no catastrophic failure occurs, the reduction of

hot work tool performance can cause serious

damage in terms of cost and time.

Premature Cracking Caused by Excessive

Hardness. A wide range of hardness levels can

result from hardening and tempering a hot work

tool steel. In Fig. 24, this can be observed for

high tempering temperatures, but the variation

is even higher if the whole tempering curve is

observed (see the tempering curve for H13,

Fig. 28, for example). Normally, it is possible to

attain values between 30 and 58 HRC in the most

common hot work steels. However, the steels are

not used within this hardness range. Although

some variation may exist, hardness levels higher

than 50 HRC or lower than 40 HRC are not

typical.

To improve wear resistance, one can propose

an increase in hardness. However, two aspects

should be considered before such a decision.

First and most obvious is the toughness nece-

ssary for a given application. Reduction of

Fig. 24

Tempering curves for the most common hot work tool steels. Tempering curves are obtained after hardening small (25 mm,

or 1 in.) specimens of all materials with the usual hardening temperature: 1020



C for H13, TENAX300 (brand name of low-

silicon H11), and VHSUPER (brand name of high-molybdenum, low-silicon modiﬁed H11); and 1100



C for H21 (higher temperatures

may be used, increasing tempered hardness but reducing toughness).

Table 3 Typical chemical composition of some hot work tool steels

AISI(a) DIN UNS

Composition, wt%

C Si Mn Cr Mo W V Fe Others

~H10 1.2365 T20810 0.32 0.30 0.3 2.9 2.8 ... 0.50 bal ...

H11 1.2343 T20811 0.37 1.0 0.3 5.0 1.3 ... 0.90 bal ...

H13 1.2344 T20813 0.40 1.0 0.3 5.0 1.3 ... 0.45 bal ...

Low-Si H11;

TENAX300(b)

... 0.36 0.3 0.3 5.0 1.4 ... 0.45 bal ...

~6F3 or ~L6 1.2714 ... 0.56 0.3 0.7 1.1 0.5 ... 0.15 bal Ni = 1.7

H21 1.2581 T20821 0.32 0.3 0.3 3.5 ... 9.0 0.50 bal ...

(a)“~”, similar to but not exactly the same. (b) Trademark of Villares Metal Company, Brazil. TENAX 300 is not standardized; therefore, the brand name is given.

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toughness may accelerate several mechanisms

of tool damage, such as heat checking, gross

cracking, and wear by chipping, in addition to

the risk of catastrophic cracking.

The relationship between hardness, tough-

ness, and tempering temperature can be quanti-

tatively evaluated in Fig. 28 for AISI H13. When

the tempering temperature is far from the hard-

ness peak, toughness rapidly increases with

decreasing hardness. Temperatures of approxi-

mately 500



C can be considered to cause tem-

per embrittlement, with toughness being rather

low (Ref 24–26). Such temper embrittlement

has been studied in relation to silicon content

(Ref 27–30). It has been shown, for example,

that reducing the silicon content from 1% (usual

in H-series grades) to approximately 0.3%

causes a strong toughness increase, close to peak

hardness but also for higher tempering tem-

peratures.

Secondly, the type of wear should be con-

sidered. In cold work tooling, wear is directly

related to hardness, but in hot work tooling, the

situation is normally quite different. Wear can

occur by a combination of abrasion and adhesion

at high temperatures. The main reason to avoid

wear is the ability to keep high hardness at

working conditions (tempering resistance) as

well as adequate toughness to avoid chipping

after adhesion. One example of such an effect is

presented in Fig. 29 for a precision hot forging

punch. The traditional material for such an

application was H13 steel with high hardness

(55 HRC). However, accelerated wear was

observed. Analysis of the tool after end-life

showed a strong hardness decrease in the work-

ing regions, indicating that a higher tempering

resistance was necessary instead of higher

hardness. Such an alteration was done, sub-

stituting H13 with a higher-molybdenum

grade, the commercial brand VHSUPER (not

standardized), which led to a 50% increase in the

tool performance.

It is commonly found that excessive hard-

ness assists failures in hot work applications.

Figure 30 shows further examples. For all, low

tempering temperature, short tempering time,

or even speciﬁcation changes led to an increase

in hardness and premature failure due to the

resulting low toughness.

While excessively low hardness can lead to

problems, hardness also should not be too low.

The adequate value, as mentioned previously,

is normally between 40 and 50 HRC in hot work

tools; in some cases for warm forging, it

is possible to use tools up to 58 HRC. An inter-

esting effect of hardness and toughness in ﬁnal

performance can be discussed based on Fig. 31.

This graph shows the effect of these two prop-

erties on heat checking, focused on die-casting

applications. The higher the hardness and

toughness, the lower the heat checking damage,

measured by an appropriate chart. However, as

discussed and shown in Fig. 28, hardness and

toughness are also associated. Establishing ideal

Fig. 25

Examples of heat checking cracks on aluminum die-casting dies. Cracks are white because they are ﬁlled with aluminu m.

Courtesy of Villares Metals

Fig. 26

Schematic of hardness after tempering and the effect

of secondary hardening in high-alloy steels. Observe

that high-temperature hardness is only possible through pre-

cipitation hardening caused by alloy carbides (secondary hard-

ening). At low temperatures, hardness is less than martensite due

to the presence of retained austenite, which is eliminated after

high-temperature tempering.

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hardness thus depends on several conditions of

tool design and use and is also based on previous

experience. As a starting value, 45 HRC would

be recommended, with increases or decreases

depending on the results experienced.

Inadequate Heat Treating Procedures in

Hot Work Tool Steels. Heat treating of hot

work steels has several important parameters,

but incorrect practice does not always produce

failures observable just after heat treating. Some

examples were shown in Fig. 29 and 30; several

tools were improperly heat treated (to higher

hardness), but the problem was only observed

during tool use.

Fig. 27

Effect of vanadium molybdenum, tungsten, and chromium additions on secondary (high-temperature) hardness of medium-

carbon steels. Source: Ref 23

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