Martensite

artensite is our friend, so sayeth the heat treater, but what is martensite, really? And why is a tem-pered martensitic structure the single-minded goal of every heat treater when hardening steel?

Lets learn more.

Martensite FormationIn order to form martensite we need to heat steel into the aus-

tenite eld (above Ac3) and quench rapidly enough from the austenite phase to avoid pearlite formation. The rate must be fast enough to avoid the nose of the Time-Tem-perature-Transformation (TTT) curve the so-called critical cooling rate for the given steel. The formation of martensite involves the structural rearrangement (by shear dis-placement) of the atoms from face-centered cubic (FCC) austenite into a body-centered

tetragonal (BCT) martensitic structure. This change is accompa-nied by a large increase in volume and results in a highly stressed condition. This is why martensite has a higher hardness than aus-tenite for the exact same chemistry. The martensite transformation, while not instantaneous, is sig-ni cantly faster than diffusion-controlled processes such as ferrite and pearlite formation that have different chemical compositions than the austenite from which they came. Thus, martensite is a meta-stable, strain-induced state that steel nds itself in. The re-sultant steel hardness is (primarily) a function of its carbon con-tent (Fig. 1).

Martensite MorphologyMorphology is a term used by metallurgists to describe the study of the shape, size, texture and phase distribution of physical ob-jects. Martensite can be observed in the microstructure of steel in two distinctly different forms lath or plate depending on the carbon content of the steel (Fig. 2). In general, lath martensite is associated with high toughness and ductility but low strength, while plate martensite structures are much higher in strength but tend to be more brittle and non-ductile.[2]

For alloys containing less than approximately 0.60 wt.% car-bon, lath martensite appears as long, thin plates often grouped in packets (Fig. 3). Plate (or lenticular) martensite is found in alloys containing greater than approximately 0.60 wt.% carbon. The mi-crostructure is needle-like or plate-like in appearance across the complete austenite grain (Fig. 4). With carbon contents between 0.60 and 1.00 wt.% carbon, the martensite present is a mixture of

lath and plate types. As the carbon content increases, so-called high-carbon mar-tensite twins begin to replace dislocations within the plates. This transformation is accompanied by the volumetric expansion men-tioned earlier, creating (residual) stress in addition to the strains due to interstitial solute atoms. At high carbon levels these stresses can become so severe that the material cracks during transforma-tion when a growing plate impinges on an existing plate.[3] Thus, coarse martensite (Fig. 5) and plate martensite are less desirable structures in most applications.

Ms and Mf TemperaturesThe martensite transformation begins at the martensite start (Ms) temperature and ends at the martensite nish (Mf) temperature and is in uenced by carbon content. Increasing the carbon con-tent of the austenite depresses the Ms and Mf temperatures, which leads to dif culties in converting all of the austenite to martensite. Ms and Mf temperatures are also important in welding, as they in uence the residual stress state.[5] Ms and Mf temperatures can be calculated, and if you need to know them for a particular steel, one source for this data is at www.thomas-sourmail.org/martens-ite.html, which lists over 1,000 different steel types.

Martensite

Daniel H. Herring | 630-834-3017 | [email protected]

The Heat Treat Doctor

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18 June 2011 - IndustrialHeating.com

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Marder (27)Hodge and Orehoski (28)Burns et al. (29)Irvine et al. (30)Kelly and Nutting (31)Kurjumov (32)Litwinchuk et al. (33)Bain and Paxton (34)Jaffe and Gordon (35)Materkowski (36)

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ness

, DPH

Carbon, wt %0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Hard

ness

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kwel

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Fig. 1. As-quenched hardness vs. carbon content[1]

Tempered MartensiteAll steels containing martensite should be tempered. As heat treaters, we need to know that martensite in steel produces a hard, brittle microstructure that must be tempered to provide the deli-cate balance necessary between strength and toughness needed to produce a useful engineering material. When martensite is tem-pered, it partially decomposes into ferrite and cementite. Tem-pered martensite is not as hard as just-quenched martensite, but it is much tougher and is ner-grained as well.

Final Thoughts about MartensiteThe heat treater might be interested to know that martensite for-mation is not restricted just to steels because other alloy systems produce crystallographic changes of a similar nature (Table 1). Learning more about martensite is an essential part of what we need to do as heat treaters since it is one of the de ning character-istics of our industry. IH

References1. Krauss, G., Martensitic Transformation, Structure and Properties in

Hardenable Steels, in Hardenability Concepts with Applications to

Steel, D.V. Doane and J.S. Kirkaldy [Eds.], AIME, Warrendale, PA, 1978,

pp. 229-248.

2. Vander Voort, George F., Martensite and Retained Austenite,

Industrial Heating, April 2009.

3. Elements of Metallurgy and Engineering Alloys, F. C. Campbell [Ed.],

ASM International, 2008, pp. 169 173.

4. Vander Voort, George F., Microstructures of Ferrous Alloys,

Industrial Heating, January 2001.

5. Payares-Asprino, M. C., H Katsumot and S. Liu, Effect of Martensite

Start and Finish Temperature on Residual Stress Development in

Structural Steel Welds, Welding Journal, Vol. 87, November 2008.

6. Zackary, V. F., M. W. Justusson and D. J. Schmatz, Strengthening

Mechanisms in Solids, ASM International, 1962, p. 179.

7. G. B. Olson and W. S. Owen [Eds.], Martensite, ASM International,

1992.

8. Krauss, George, Steels Processing, Structure and Performance, ASM

International, 2005.

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Use this Mobile Tag to view the Vander Voort article on martensite

and retained austenite

Table 1. Crystal structures formed in martensite or quasi-mar-tensite transformations[6]

Alloy system Crystal structure change[a]

Co, Fe-Mn, Fe-Cr-Ni FCC to HCP

Fe-Ni FCC to BCC

Fe-C, Fe-Ni-C, Fe-Cr-C, Fe-Mn-C FCC to BCT

In-Ti, Mn-Cu FCC to BCT

Li, Zr, Ti, Ti-Mo, Ti-Mn BCC to HCP

Cu-Zn, Cu-Sn BCC to FCT

Cu-Al BCC to HCP (distorted)

Au-Cd BCC to Orthorhombic

ZrO2 Tetragonal to Monoclinic

Notes: [a]FCC = face-centered cubic; BCC = body-centered cubic; HCP = hexagonal close packed; BCT = body-centered tetragonal; FCT = face-centered tetragonal;

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0

871

760

649

538

427

316

204

93

0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Greninger (8)Toriano and Greninger (10)Cohen et al (24)Digges (25)Greninger and Troiano (26)Kaufman and Cohen (27)Esser et al (28)Bibby and Parr (29)

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empe

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Lath PlateMixed

Carbon, wt %

Fig. 2. Formation of lath and plate martensite[1]

Fig. 3. Lath martensite example (carburized 8620)[4]

Fig. 4. Plate martensite example (carburized 8620)[4]

10m 10m

Fig. 5. Coarse martensite (carburized SAE 9310)