Martensite and Retained Austenite

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Martensite development is critical to many heat-treatment processes. This paper examines the conditions under which austenite is retained and the problems associated with its presence, with detecting it and with measuring it.

Transcript of Martensite and Retained Austenite

  • IndustrialHeating.com - April 2009 51

    epending upon the carbon content of the parent aus-tenite phase, either lath (low-carbon) or plate (high-

    carbon) martensite may form, as well as mixtures of the two. In general, lath mar-tensite is associated with high toughness and ductility but low strength, while plate martensite structures are much higher strength but may be rather brittle and non-ductile. Increasing the carbon content of the austenite also depresses the martensite start (Ms) temperature and the marten-site nish (Mf) temperature, which leads to dif culties in converting all of the aus-tenite to martensite. When this happens, we have retained austenite, which may be either extremely detrimental or desirable under certain conditions.

    HistoryUp to about 100 years ago, the heat treat-ment of steels was certainly an art as the science behind what was happening was just starting to be understood. The con-trol of grain size in carburizing was just be-coming possible by the work of McQuaid and Ehn. They discovered that small ad-ditions of aluminum would keep the grain size ne after a long exposure, generally 8-10 hours, at the carburizing tempera-

    ture. Prior to that, coarse prior-austenite grain structures would be observed in the carburized case that would initiate brittle intergranular fractures at minor loads. Next, Grossman and Bain developed the theory of hardenability where the ideal critical diameter (DI) could be calculated from the prior-austenite grain size and the composition. Then, the DIcould be used to estimate the as-quenched hardness pro le of a uniformly shaped bar given a particular strength quench. About the same time, isother-mal transformation (IT) diagrams were developed, and it became easier to identify these lesser-understood microstructures of upper and lower bainite. An IT diagram, while it is helpful in understanding microstructures and in developing annealing cycles, is not particularly useful for understanding heat-treat-ment structures. This problem was solved by developing con-tinuous cooling transformation (CCT) diagrams. Shortly before the writer joined the Homer Research Laboratories of Beth-lehem Steel, they had developed

    CCT diagrams using the arrested-Jominy-bar method a rather painful process in-deed. Dilatometer-based CCT diagrams were far easier to develop and in less time, but this equipment came later.

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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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    Fig. 1. Summary of extensive as-quenched hardness data from the literature for Fe-C alloys and steels by Krauss[2]

    Martensite and Retained AusteniteGeorge F. Vander Voort Buehler Ltd., Lake Bluff, Ill.

    Martensite development is critical to many heat-treatment processes. This paper examines the conditions under which austenite is retained and the problems associated with its presence, with detecting it and with measuring it.

    FEATURE | Materials Characterization & Testing

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    MCT-Martensite-Buehler.indd 51 3/18/09 3:25:53 PM

  • 52 April 2009 - IndustrialHeating.com

    FEATURE | Materials Characterization & Testing

    Technological AdvantagesDevelopment of thin-foil technology for the transmission electron microscope (TEM) produced a far-deeper understand-ing of the ne details of steel microstruc-tures because these features were well beyond the resolution of the light optical microscope (LOM). The development of IT and CCT diagrams had shown that martensite began transforming at temper-atures relative to the composition of the austenite, with its carbon content being most critical. Problems due to excessive retained aus-tenite had plagued the tool-steel industry since the late 19th century. X-ray diffrac-tion had been the primary tool for the study of retained austenite and certainly for its quanti cation, but it could not im-age this microstructure. LOM could not image retained austenite until it was at least present in the 10-15% range. TEM thin foils could detect and image retained austenite even at levels somewhat under 2% with careful use of dark- eld illumi-nation. The morphology of the marten-site makes it dif cult to distinguish small particles of retained austenite within the complex plate-martensite patterns. With low-carbon lath martensite, thin lms of retained austenite could be seen with very careful TEM work, but this was very dif- cult work.

    Tool SteelsIn the tool-steel industry, excessive re-tained austenite is universally considered to be detrimental. Exactly what consti-tutes excessive is dif cult to de ne as not enough data exists, and what is excessive will vary with the grade and application. For example, relatively low-carbon 5%-Cr hot-work die steels such as H11 and H13 have been used for years as guage blocks. Any dimensional change with time is to be avoided. Consequently, these steels are triple tempered at a relatively high tem-perature where retained austenite will be converted to either fresh martensite or bainite, and they will be tempered with the next tempering cycle. In other applications, any observable (by LOM) retained austenite is highly det-rimental. Service stresses will convert the retained austenite on the rst use. As the carbon content of the retained austenite is high, the martensite that forms is highly tetragonal and the resulting expansion cracks the steel because the matrix is not ductile enough to tolerate the expansion stresses. With carburized gears, on the other hand, only a very thin layer at the surface is carburized and may contain 20-25% re-tained austenite. The bulk of the gear is a highly ductile (compared to a tool steel), low-carbon alloy steel. Gears are usually

    not impact loaded like a tool-steel die, so the stresses are much lower and the retained austenite usually does not trans-form substantially during service. If the retained austenite did transform, the steel around it and below it is ductile enough to accommodate the strains without frac-ture. Retained austenite does become stable with time, and some will transform to martensite at room temperature. Samu-els[1] states that up to 5% of the austenite present after quenching and low-temper-ature tempering (0.8%, the as-quenched hard-ness drops. This is due to the presence of retained austenite, which is much softer than plate martensite. Litwinchuk et al.[3] took this curve to nearly 2% carbon which demonstrates the effect of retaining large

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    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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    Fig. 3. Relationship between carbon content, marten-site type and Ms temperature

    [4]Fig. 3. Relationship between carbon content, martensite type and Ms temperature[4]

    MCT-Martensite-Buehler.indd 52 3/18/09 3:26:43 PM

  • IndustrialHeating.com - April 2009 53

    amounts of retained austenite upon the as-quenched hardness. Figure 3, from the work of Marder and Krauss[4], shows the relationship between the type