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

Подождите немного. Документ загружается.

In a recent failure analysis of a compact

vehicle suspension spring (chemical composi-

tion shown in Table 8) with a speciﬁed hardness

range of 53 to 55 HRC, a fatigue crack nucleated

from a surface defect that had grown quickly

(radial marks), as seen in Fig. 27. A scanning

electron microscopy image of the radial fracture

surface (Fig. 28) shows transgranular and

intergranular fracture paths.

The spring hardness was on the order of 54

HRC, which means that the tempering process

was carried out very close to the beginning of the

TE range. However, a signiﬁcant intergranular

crack path was found in the fracture surface.

This case shows that the fracture can assume an

intergranular path even outside of the embrit-

tlement region. The high hardness means high

yield stress, which prompts grain-boundary

decohesion.

Similar behavior was found when analyzing a

carbonitrided sample quenched from 880



and tempered at 180



C. Figures 29 and 30 show

a carbonitrided layer and core microstructure,

respectively.

The hardness of the carbonitrided layer is in

the range of 60 to 62 HRC, while the core

hardness values are in the range of 30 to 32 HRC.

The carbon-enriched surface (approximately

0.9%) shows an intergranular overload fracture

path, and the nucleus low-carbon structure

(approximately 0.2%) presents a dimpled

transgranular crack pattern (Fig. 31a, b).

The higher carbon content increases hardness

values, which can promote stresses higher than

the cohesive grain-boundary strength or even

tempering outside of the embrittling temperature

range, as mentioned previously.

ACKNOWLEDGMENT

The authors would like to acknowledge J.C. Ven-

dramim from ISOFLAMA Ind. Com Equip. Ltd.,

Brazil, for helpful suggestions in writing this chapter.

Fig. 27

Surface fracture spring. The dotted arrow shows the

fatigue crack nucleus, and the dashed arrow shows

the fatigue-to-brittle fracture transition. The solid arrow shows the

surface analyzed by scanning electron microscopy. Original

magniﬁcation: 6·

Fig. 28

Secondary electron image showing intergranular

(horizontal arrows) and transgranular (vertical

arrows) crack path

Fig. 29

Tempered martensite with some retained austenite

(approximately 20%). Etched with 2% nital

Table 8 Vehicle suspension spring analysis

Chemical composition, wt%

C Si Mn Cr S Ni P Cu

0.49 1.12 0.65 0.59 0.008 0.21 0.017 0.22

Steel Failures due to Tempering and Isothermal Heat Treatment / 305

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_285-309.pdf/Chap_09/ 18/8/2008 3:35PM Plate # 0 pg 305

Fig. 30 Base material. Low-carbon martensite (horizontal arrow) and upper bainite (vertical arrow). Etched with 2% nital

Fig. 31

Secondary electron image. (a) Intergranular crack path on the carbonitrided case. (b) Transgranular dimpled fracture in the

base material

306 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_285-309.pdf/Chap_09/ 18/8/2008 3:35PM Plate # 0 pg 306

REFERENCES

1. M. Szczepanski, The Brittleness of Steels,

John Wiley & Sons, Inc., New York-

London, New York, 1963

2. R.W.K. Honeycombe, Steel Microstructure

and Properties, Edward Arnold Ltd., 1981,

p 78, 205

3. E.C. Bain and H.W. Paxton, Alloying Ele-

ments in Steel, American Society for

Metals, 1966

4. A.V. Sverdlin and A.R. Ness, Chapter 2—

The Effects of Alloying Elements on the

Heat Treatment of Steel, Steel Heat Treat-

ment Handbook, G.E. Totten and M.A.H.

Howes, Ed., Marcel Dekker Inc., New

York, NY, 1997, p 45–91

5. G.R. Speich and W.C. Leslie, Tempering of

Steel, Metall. Trans., Vol 3 (No. 5), May

1972, p 1043–1054

6. G.R. Speich, Trans. Metall. Soc. Am. Inst.

Mech. Eng., Vol 245 (No. 12), 1969,

p 2553–2564

7. M. Narazaki, G.E. Totten, and G.M. Web-

ster, Hardening by Reheating and Quench-

ing, Handbook of Residual Stress and

Deformation, G.E. Totten, M. Howes, and

T. Inoue, Ed., ASM International, 2002,

p 499

8. A. Kelly and R.B. Nicholson, Strengthen-

ing Methods in Crystals, Materials Science

Series, Applied Science Publishers Ltd.,

London, 1971, Printed in Great Britain

by Galliard Limited, Great Yarmouth,

England

9. J. Vatavuk and H. Goldenstein, Ac¸os Had-

ﬁeld Contendo Carbonetos de Nio

bio e

Tita

nio, Anais do II Encontro de Tecnologia

e Utilizac¸a

o dos Ac¸os Nacionais, May 18–

22, 1987 (Rio de Janeiro), ABM, COPPE

(UFRJ), p. 151–169

10. Heat Treating Data Book, 8th ed., Seco/

Warwick Corporation, 2000, p 96

11. T. Swarr and G. Krauss, Effect of

Structure on the Deformation of As-

Quenched and Tempered Martensite in a

Fe-0.2%C Alloy, Metall. Trans. A, Vol 7,

1976, p 41–48

12. G. Krauss, Chapter 8—Tempering of Steel,

Principles of Heat Treatment of Steel,

American Society for Metals, 1980, p 187–

227

13. B. Liscic, Chapter 8—Steel Heat Treat-

ment, Steel Heat Treatment Handbook, G.E.

Totten and M.A.H. Howes, Ed., Marcel

Dekker Inc., New York, NY, 1997, p 527–

662

14. H.K.D.H. Bhadeshia, “Tempered Marten-

site,” University of Cambridge, http://www.

msm.cam.ac.uk/phasetrans/2004/Tempered.

Martensite/tempered.martensite.html (ac-

cessed March 2008)

15. G. Krauss, Deformation and Fracture in

Martensitic Carbon Steels Tempered at Low

Temperatures, Metall. Mater. Trans. B,

Vol 32, 2001, p 205–221

16. W.J. Nam and H.C. Choi, Effect of Silicon,

Nickel and Vanadium on Impact Toughness

in Spring Steels, Mater. Sci. Technol.,

Vol 13, 1997, p 568–574

17. W.J. Nam and H.C. Choi, Effect of Si on

Mechanical Properties of Low Alloy Steels,

Mater. Sci. Technol., Vol 15, 1999, p 527–

530

18. C. Jensen, Dimensional Changes during

Heat Treatment of Hot Work Steel, Ind.

Heat., March 1993, p 33–38

19. R.W. Hertzberg, Chapter 10: Micro-

structural Aspects of Fracture Toughness,

Deformation and Fracture Mechanics of

Engineering Materials, 4th Ed., John Wiley

& Sons, Inc., 1996, p 460–463

20. H. Ohtani and C.J. McMahon, Jr. Modes of

Fracture in Temper Embrittlement Steels,

Acta metall., Vol 23, p 377–386

21. S.K. Banerj, C.J. McMahon, Jr., and H.C.

Feng, Intergranular Fracture in 4340-

Type Steels, Effect of Impurities and

Hydrogen, Metall. Trans. A, Vol 9, 1978,

p 237–247

22. C.R. Brooks, Chapter 5—Tempering,

Principles of the Heat Treatment of Plain

Carbon an d Low Alloy Steels, ASM Inter-

national, 1996, p 127–204

23. F.A. Darwish, L.C. Pereira, C. Gattis, and

M.L. Grac¸a, On the Tempered Martensite

Embrittlement in AISI 4140 Low Alloy

Steel, Mater. Sci. Eng., Vol A132, 1991,

p L5–L9

24. I. Olefjord, Temper Embrittlement, Review

231, Int. Met. Rev., Vol 23, 1978, p 149–163

25. G. Krauss, Tempered Martensite Embrit-

tlement in AISI 4340 Steel, Metall. Trans.

A, Vol 10, 1979, p 1643–1649

26. M.A. Grossman and E.C. Bain, Chapter 5—

Tempering After Quench Hardening, Prin-

ciples of Heat Treatment, American Society

for Metals, 1964, p 129–175

27. G.E. Totten, C.E. Bates, and N.A. Clinton,

Chapter 1—Introduction to the Heat

Steel Failures due to Tempering and Isothermal Heat Treatment / 307

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_285-309.pdf/Chap_09/ 18/8/2008 3:35PM Plate # 0 pg 307

Treating of Steel, Handbook of Quenchant

and Quenching Technology, ASM Interna-

tional, 1993, p 1–33

28. V.V. Zabil’skii, Temper Embrittlement

of Structural Alloy Steels Review, Met.

Sci. Heat Treat., Vol 29 (No. 1), 1987,

p 32–42

29. S.-H. Song, R.G. Faulkner, and P.E.J.

Flewitt, Quenching and Tempering—

Induced Molybdenum Segregation to

Grain Boundaries in a 2.25%Cr-1Mo

Steel, Mater. Sci. Eng. A, Vol 281, 2000,

p 23–27

30. S.G. Chowdhury, P. Kumar, S.K. Das, D.K.

Bhattacharya, and N. Parida, Failure Ana-

lysis of High Temperature Studs, Eng. Fail.

Anal., Vol 8, 2001, p 521–528

31. Y. Ohmori, H. Ohtani, and T. Kunitake,

Met. Sci., Vol 8, 1974, p 357–366

32. G.L. Hu, Z.T. Liu, P. Wang, W.J. Hwa, and

M.K. Kang, Mechanism of Tempered

Bainite Embrittlement in Some Structural

Steels, Mater. Sci. Eng. A, Vol 141, 1991,

p 221–227

33. H.K.D.H. Bhadeshia and D.V. Edmonds,

Met. Sci., Vol 17, 1983, p 411–419

34. K.D.H. Bhadeshia and D.V. Edmonds, Met.

Sci., Vol 17, 1983, p 420–425

35. S. Raoul, B. Marini, and A. Pineau, Effect

of Microstructure on the Susceptibility of a

533 Steel to Temper Embrittlement, J. Nucl.

Mater., Vol 257, 1998, p 199–205

36. J. Vatavuk, “Estudo Sobre os Efeitos do

Nio

bio na Resposta ao Tratamento Te

rmico

de Ligas Fe-17Cr, Fe-17Cr-Ni, e do Cr e do

Ni na Solubilidade do Nb na Ferrita,” Dis-

sertac¸a

o de Mestrado apresentada a

Escola

Polite

cnica da Universidade de Sao Paulo,

1986

37. H. Kwon and C.H. Kim, Tempered Mar-

tensite Embrittlement Fe-Ni-C Steel,

J. Mater. Sci., Vol 18 (No. 12), 1983,

p 3671–3678

38. A.V. Taran, Determining Susceptibility to

Reversible Temper Embrittleness, Met. Sci.

Heat Treat., Vol 15 (No. 10), 1973, p 865–

868

39. A. Ghosh, K. Bhaduri, and K. Ray,

Characterization of Tempered Martensite

Embrittlement Using Hardness Measure-

ments, Mater. Charact., Vol 33, 1994,

p 321–327

40. N.A. Tiner, Brittle Fracture in Equipment

Failure, Mech. Eng.-CIME, Vol 112 (No. 6),

1990, p 60–64

41. M. Wang, E. Akiyama, and K. Tsuzaki,

Hydrogen Degradation of a Boron-Bearing

Steel with 1050 and 1300 MPa Strength

Levels, Scr. Mater., Vol 52, 2005, p 403–

408

42. J.H. Chuang, L.W. Tsay, and C. Chen,

Crack Growth Behaviour of Heat-Treated

4140 Steel in Air and Gaseous Hydrogen,

Int. J. Fatigue, Vol 20 (No. 7), 1998, p 531–

536

43. D. Liu, B. Bai, H. Fang, W. Zhang, J. Gu,

and K. Chang, Effect of Tempering Tem-

perature and Carbide Free Bainite on

the Mechanical Characteristics of a High

Strength Low Alloy Steel, Mater. Sci.

Eng. A, Vol 371, 2004, p 40–44

44. P.Sh. Lantsman, G.G. Vernovskaya, and

V.N. Kudryavtsev, Use of Austempering to

Reduce Hydrogen Embrittlement of Steel

65S2VA, Metalloved. Term. Obrab. Met.,

Vol 5, May 1973, p 73–74 (translated from

Russian)

45. S.P. Lynch, Metal Induced Embrittlement

of Materials, Mater. Charact., Vol 28, 1992,

p 279–289

46. P.L.J. Fernandez, R.E. Clegg, and D.R.H.

Jones, Failure by Liquid Metal Induced

Embrittlement, Eng. Fail. Anal., Vol 1

(No. 1), 1994, p 51–63

47. R.E. Clegg and D.R.H. Jones, Liquid Metal

Embrittlement in Failure Analysis, Mater.

Sci., Vol 27 (No. 5), 1992, p 453–459

48. K. Ina and H. Koizumi, Penetration of

Liquid Metals into Solid Metals and Liquid

Metal Embrittlement, Mater. Sci. Eng. A,

Vol 387–389, 2004, p 390–394

49. R.E. Clegg, A Fluid Flow Based Model to

Predict Liquid Metal Induced Embrittle-

ment Crack Propagation Rate, Eng. Fract.

Mech., Vol 68, 2001, p 1777–1790

50. D.W. Cameron, Failures in Large Gas

Turbines due to Liquid-Metal Embrittle-

ment, Mater. Charact., Vol 33, 1994, p 37–

51. Liquid Metal Embrittlement, Failure Ana-

lysis and Prevention, Vol 10, Metals

Handbook, 8th ed., American Society for

Metals, 1975, p 228–229

52. G. Nicaise, A. Legris, J.B. Vogt, and J.

Foct, Embrittlement of the Martensitic

Steel 91 Tested in Liquid Lead, J. Nucl.

Mater., Vol 296 (No. 1), July 2001, p 256–

264

53. A. Legris, G. Nicase, J.B. Vogt, and

J. Foct, Liquid Metal Embrittlement of the

308 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_285-309.pdf/Chap_09/ 18/8/2008 3:35PM Plate # 0 pg 308

Martensitic Steel 91: Inﬂuence of the Che-

mical Composition of the Liquid-Metal—

Experiments and Electronic Structure

Calculations, J. Nucl. Mater., Vol 301

(No. 1), 2002, p 70–76

54. A. Legris, J.B. Vogt, A. Verleene, and I.

Serre, Wetting and Mechanical Properties,

A Case Study: Liquid Metal Embrittlement

of a Martensitic Steel by Liquid Lead and

Other Liquid Metals, J. Mater. Sci., Vol 40

(No. 9–10), 2005, p 2459–2463

55. A Reguly, T.R. Strohaeker, G. Krauss, and

D.K. Matlock, Quench Embrittlement of

Hardened 5160 Steel as a Function of

Austenitizing Temperature, Metall. Mater.

Trans. A, Vol 35, Jan 2004, p 153–162

Steel Failures due to Tempering and Isothermal Heat Treatment / 309

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_285-309.pdf/Chap_09/ 18/8/2008 3:35PM Plate # 0 pg 309

Failure Analysis in Tool Steels

Rafael Agnelli Mesquita and Celso Antonio Barbosa,

Villares Metals

TOOL STEELS are an important class of

steels due to their distinct applications and,

especially, their speciﬁc heat treating issues.

Tool steels are used in various industrial appli-

cations that require some kind of mold, die, or

mechanical device. Tool steels are also a com-

plex class of steels with compositions close to

carbon steel or more highly alloyed grades. Tool

steels are generally classiﬁed according to three

main deﬁnitions (Ref 1, 2):

 They are used in some forming process or

forming operation for metal, ceramic, or

plastic shaping.

 Tool steel properties are only attained after

heat treating, normally performed by hard-

ening (quenching) followed by tempering.

 This class of material is produced according

to rigorous melting and processing controls.

Therefore, even for grades with chemical

compositions very close to low-alloy carbon

or engineering steels, the production prac-

tices of tool steels lead to substantial prop-

erty improvements.

According to this deﬁnition, heat treating is a

key issue for tool steels. In fact, heat treating

does have a strong effect on tool life, as des-

cribed in this chapter. In some applications,

three major points are equally related to tool per-

formance (supposing that operation is constant):

 Design and manufacture ﬁnishing

 Steel composition and its quality

 Heat treatment applied

Even though this approach shows the impor-

tance of heat treating, it may be too simple for

determining the variables related to tool per-

formances. That is, all these aspects interact with

each other and may inﬂuence the ﬁnal result.

For example, if a good design and proper heat

treatment are applied to a tool produced with an

imporperly selected steel grade, the result can be

awful. The same may happen for an incorrect

selection of heat treatment or tool design.

A helpful way to think of this synergistic relation

is to think of the factors as simple multiplying

factors. If one of them is zero, regardless which

one, the whole result would be zero. Of course,

the factors are not so distinctly separate.

Classiﬁcation of Tool Steels

Due to the large number of grades and appli-

cations of tool steels, several possible classiﬁ-

cations have arisen. This division is important

for the discussion of heat treating. One of the

most well-known classiﬁcations is made by the

American Iron and Steel Institute (AISI), divid-

ing tool steels into several classes according to

application, composition, or heat treatment. This

classiﬁcation is shown in Table 1.

Another possible classiﬁcation for tool steels

is their division into four groups according to the

ﬁnal application: hot work, cold work, plastic

mold, and high-speed tool steels. The advantage

of this division is to deal with fewer groups and

to group the diverse grades within common

aspects of each application, such as sizes, hard-

ness, operating conditions (chocks, wear, or

plastic deformation), and surface-ﬁnishing re-

quirements. This chapter follows such a divi-

sion, but the grade nomenclatures used here are

primarily from AISI.

Table 1 AISI classiﬁcation for tool steels (Ref 1)

Group Symbol

Water-hardened tool steels W

Shock-resistant tool steels S

Oil-hardening tool steels O

Air-hardening tool steels A

High-carbon and high-chromium die steels D

Tool steel for application in plastic molds P

Cr-, Mo-, or W-alloyed hot work tool steels H

Tungsten-alloyed high-speed steels T

Molybdenum-alloyed high-speed steels M

Adopted from Ref 1

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_311-350.pdf/Chap_10/ 18/8/2008 3:37PM Plate # 0 pg 311

Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 311-350

DOI: 10.1361/faht2008p311

www.asminternational.org

Cold work tool steels are employed for

tooling operations in temperatures below

200



C, typically at room temperature, and for

steel forming (Fig. 1 shows some examples).

The most common tools of this class are punches

and dies for cold drawing and stamping, knives,

thread-rolling dies, and coining or cold forging

tools. In these situations, mechanical strength

and wear resistance are the main performance-

related properties, which are only obtained

through high hardness after heat treating, nor-

mally at the 60 HRC level, and the dispersion of

coarse carbides in the tool steel microstructure.

Grades of AISI class D are the most important

examples for such tools, but A and O class

grades are also employed.

Cold work tooling is also applied to shock

operation applications, such as knives for shear

cutting thick plates (normally thicker than

13 mm, or

in.), chisels, and some powder

pressing molds or cold forging dies. In these

situations, high toughness is also important,

even if the wear resistance is reduced. Grades

of AISI class S are the most important example

in these applications. Toughness is also impor-

tant in the previously discussed application,

preventing several catastrophic failures or, more

commonly, adhesive wear failures, in which

microchipping is a very important issue. In these

situations, the advent of new materials with

lower carbon and chromium contents has shown

interesting results.

Hot work tool steels are used for applica-

tions in which process temperature is an im-

portant aspect for the working property of a

tooling material. A common limit for hot work

working temperatures is 600



C, although

lower-temperature applications may also be

classiﬁed as hot (or warm) work. In hot work

tooling, the operating temperature interacts with

the steel structure, which is very important for

materials selection.

Hot work tooling may be divided into three

major applications: hot forging, especially

for steel forgings; extrusion; and die casting

(Fig. 2). The two last groups are mainly em-

ployed for aluminum alloys and for producing

construction or automotive parts, respectively.

All applications normally employ AISI grades H

Fig. 1

Examples of cold work tools. Courtesy of Villares

Metals

Fig. 2 Examples of hot work dies for (a) press forging and (b) die casting. Courtesy of Villares Metals

312 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_311-350.pdf/Chap_10/ 18/8/2008 3:37PM Plate # 0 pg 312

steel as core tools. However, for large forging

dies and tool holders, low-alloyed materials,

such as AISI grade 6F3 (DIN 1.2714), may be

employed.

The most important properties for hot work

tooling are hot strength and toughness. Hot

strength is normally related to the operating

conditions, speciﬁcally, how long and at what

temperature the tools are exposed. This gives

rise to an important concept called tempering

resistance, which describes the steel resistance

to hardness reduction after exposure to high

temperatures. Toughness, which is related to

crack end-life situations, is also very important

in hot work tooling. Toughness requirements are

so important that hot work tool steels normally

have carbon contents close to 0.40% and hard-

ness below 52 HRC, both values much lower

than that of cold work grades.

Plastic mold steels have become a very

important division of tools due to the increase in

plastic material applications (Fig. 3). Mecha-

nical requirements for strength and toughness

are less important than in the previous applica-

tions. In general-application molds, the mold-

manufacturing-related properties are the most

important, because the end life of these molds is

rarely attained. Therefore, steel machinability,

polishability, heat treating response, and weld-

ability are of special interest. Although less

common, there are high-demand applications,

such as the processing of corrosive polymers

or reinforced plastics, where the reason for steel

selection is different, and corrosion or wear

resistance should be emphasized more than

manufacturing-related properties.

AISI P20 or P20-modiﬁed grades (mainly

DIN 1.2738) are the most employed steels in

the mold-making industry, delivered in the pre-

hardened condition with 32 HRC. Nevertheless,

more specialized applications normally require

higher-alloyed steels, such as H13, modiﬁed

martensitic stainless steel (e.g., DIN 1.2083), or

even highly alloyed powder metallurgy grades.

Currently, the advance of manufacturing tech-

nologies, especially those related to high-speed

machining technologies, has increased the appli-

cation of mold steels prehardened to high hard-

ness, such as 40 HRC or higher. This new level

improves mold quality, especially the polishing

characteristics.

High-speed steels are also tool steels, but

they have important differences from other tool

steels. They are usually used in cutting tools,

whereas the other classes are mainly employed in

forming tools. Figure 4 shows some examples of

high-speed tools. Regarding the manufacturing

characteristics, high-speed steel tools are mainly

produced by single companies and in smaller

sizes but larger quantities than molds or dies.

From the metallurgical point of view, high-

speed tool steels are deﬁnitely part of the tool

steel groups. The AISI classes for such grades

include the AISI M and T series, which corres-

pond to the main alloy element present in their

composition: molybdenum (M) or tungsten (T).

Fig. 3

(a) Plastic molds for a drilling machine body, showing a polishing operation, (b) Baby bath plastic injection mold, after ﬁnal

polishing. Courtesy of Villares Metals.

Failure Analysis in Tool Steels / 313

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_311-350.pdf/Chap_10/ 18/8/2008 3:37PM Plate # 0 pg 313

The ﬁrst high-speed steels were the T type, with

T1 being the pioneer grade. During WW II, the

supply of tungsten, mainly from Germany or

eastern Europe, was interrupted, leading to the

development of molybdenum-rich grades. The

ﬁrst developed grade was M1, which is still

employed in the United States, but M2 has

become much more important. M2 has a com-

bination of molybdenum and tungsten, in almost

equal parts, which leads to very useful properties.

This grade is used for several applications.

Today (2008), it is the most important high-

speed composition. Recently, the increase in

consumption of molybdenum, tungsten, and

vanadium has increased the cost of such alloys,

providing another driving force to the develop-

ment of new grades. In China, due to the large

amounts of tungsten ferroalloys in that country,

the use of tungsten-rich compositions has gained

new attention. Another important element is

niobium (formerly known as columbium). Its

large reserves in Brazil motivated the devel-

opment of niobium-modiﬁed grades in that

country.

High-speed tool steel properties are normally

aimed at high wear resistance, which is the main

demand in cutting operations. Wear conditions

are complex in high-speed steel tools, with

important aspects being high hardness, normally

close to 65 HRC, and the homogeneous dis-

tribution of undissolved carbides. Also impor-

tant is the material tempering resistance, in

order to retard hardness reduction at the high

temperatures developed in cutting processes.

Toughness should be of an acceptable level and

reduce chipping and cracking, but this property

is low when compared to other tool steels due

to extremely high hardness and the amount of

carbides in high-speed steel microstructures.

In the following sections, several failures and

their relation to heat treatment are described.

For better understanding, the aforementioned

division of tool steels is kept in two main groups,

cold work and hot work, which are the main

classes where failures are observed. High-speed

steels and mold steels are also discussed in some

speciﬁc cases. However, many aspects of a

given class (or for a speciﬁc failure) are also

applied to other classes; therefore, this chapter

uses references to previous or future parts of the

text and ﬁgures. Such a situation is not very

convenient for the reader but is important for

comparing diverse aspects in a given situation,

which is the ﬁnal task of any failure analysis.

Heat Treating Failures of Cold

Work Tools

Chemical Composition and Main Charac-

teristics of Cold Work Tool Steels. Typical

chemical compositions for the most common

cold work tool steels are presented in Table 2.

AISI nomenclature is used in most cases. How-

ever, some newly developed grades, not yet

Fig. 4

Examples of high-speed tools. Courtesy of Villares

Metals

Table 2 Typical chemical compositions of cold work tool steels

AISI DIN UNS

Composition, wt%

C Si Mn Cr Mo W V Fe Others

D2 1.2379 T30402 1.50 0.3 0.3 12.0 1.0 ... 0.9 bal ...

D3 1.2080 T30403 2.25 0.3 0.3 12.0 ... ... ... bal ...

D6 1.2436 ... 2.10 0.3 0.3 11.5 ... 0.7 0.2 bal ...

8%Cr-0.8%C;

e.g., VF800AT(a)

... 0.85 1.0 0.3 8.5 2.1 ... 0.5 bal Nb = 0.15

A2 1.2363 T30102 1.00 0.3 0.3 5.0 1.0 ... ... bal ...

O1 1.2510 T31501 0.95 0.3 1.3 0.5 ... 0.5 0.1 bal ...

S1 1.2542 T41901 0.45 1.0 0.3 1.4 0.2 2.0 0.2 bal ...

M2 reg. C 1.3343 T11302 0.89 0.4 0.3 4.2 5.0 6.1 1.9 bal ...

PM M3 : 2(b) ~1.3344(b,c) ~T11323(b,c) 1.28 0.4 0.3 4.2 5.0 6.3 3.0 bal ...

(a)Trademark of Villares Metals Company, Brazil. VF800AT is not standardized; therefore, the brand name is given. (b) Obtained by powder metallurgy. (c)“~”, similar to

but not exactly the same

314 / Failure Analysis of Heat Treated Steel Components

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_311-350.pdf/Chap_10/ 18/8/2008 3:38PM Plate # 0 pg 314

standardized, are referred to in the commercial

grade. The high carbon level of almost all these

grades is evident (except for S1 due to its use in

shock-resistant applications). These tool steels

have high as-quenched hardness and, when

associated with high alloy content, high harden-

ability.

The combination of high carbon content and

alloy elements, especially chromium, leads to

the formation of large (also called blocky) car-

bides within the microstructures of cold work

tool steels. Along with a high matrix hardness,

normally 60 HRC, these carbides are the main

reason for the wear resistance of cold work tool

steels. A simple mechanism for abrasive wear

is shown in Fig. 5, with an abrasive particle

grooving away portions (chips) of tool steel

material. Hardness increase reduces the particle

penetration and the wear. However, steels can-

not be much harder than 65 HRC (approximately

900 HV), which is much less than a typical

abrasive particle (between 1500 and 2500 HV).

Therefore, another mechanism takes place, this

being performed by the presence of carbides.

They can be as hard as or even harder than

abrasive particles, conferring to the material a

higher resistance to wear losses, which means

high wear resistance. In typical cold tooling, the

wear mechanism is more complicated, being

a combination of abrasive and adhesive wear.

The aforementioned mechanism is still valid,

but toughness is also shown to be important for

situations where microchipping or microcrack-

ing is present.

High-speed steels are similar to cold work

steels in this sense, with high hardness and dis-

persion of nondissolved carbides. The difference

is that high-speed steels present higher hot

resistance promoted by higher alloy content,

which leads to intense secondary hardening at

high temperatures, as discussed in later sections

for hot work steels.

Although important or even essential for

some applications, high wear resistance is nor-

mally accompanied by low toughness. High

hardness naturally causes a reduction in tough-

ness due to its effect on material fracture tough-

ness. Carbides normally act as crack initiation

sites; therefore, the higher the volume fraction

of large carbides, the lower the tool steel

toughness.

The previous discussion can be illustrated

by materials properties and microstructures.

Typical microstructures of tool steels used in

cold work tooling are shown in Fig. 6. Coarse

carbides, larger than 10 mm, are present within

the microstructures. Such large carbides are

typically the M

type, with “M” representing

the metal element that is primarily chromium.

They are typical for AISI D grades and are pre-

sent in lower amounts in 8%Cr-0.8%C new

materials, as shown for VF800AT steel (the

commercial brand name). This is the main

advantage of the higher toughness of such

grades. A combination of toughness and abrasive

wear resistance leads to important improvements

in die life for 8% Cr steels, especially in metal-

forming operations where adhesive wear is

the main operating mechanism. Lower-alloyed

grades, such as O1, have almost no carbides

in the microstructure. This is caused by the

lack of chromium content to combine with

carbon. Therefore, they are normally used in

less demanding applications in terms of wear

resistance.

Besides the microstructural aspects, a dis-

cussion about tempering curves is also important

in order to understand cold work steel properties.

Figure 7 shows the tempering curves for some

of these grades, selected because they enable

Fig. 5

Schematic showing wear caused by the movement of a hard particle through microgrooving. The base material has three

different conditions. (a) Tool steel with 50 HRC. (b) After hardness increase to 60 HRC. (c) Combination of high hardness and

presence of carbides within the microstructure

Failure Analysis in Tool Steels / 315

Name ///sr-nova/Dclabs_wip/Failure_Analysis/5113_311-350.pdf/Chap_10/ 18/8/2008 3:38PM Plate # 0 pg 315