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Martensite & Retained Austenite – (Article)

ih0409-mct-fig3-mdMartensite 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.

 

Depending upon the carbon content of the parent austenite phase, either lath (low-carbon) or plate (high-carbon) martensite may form, as well as mixtures of the two. In general, lath martensite 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. By George F. Vander Voort

 

Imaging Phases in Steels

Some techniques learned from experience can help metallographers identify certain phases in steel.

The microstructure of metals and alloys can be quite complex, particularly in certain alloy systems such as iron-based alloys. A good understanding of physical metallurgy, the role composition and processing on structure formation and modification, and an idea of the approximate expected hardness of different phases or constituents will help narrow the range of possible phases that might be present.

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Etching Isothermally Treated Steels

The microstructure of iron-based alloys is very complicated, being influenced by composition, homogeneity, processing, and section size. Microstructures of
coarse-grained steels are much easier to observe than those of fine-grained steels. Of course, steels are normally made with a fine grain size to optimize
their mechanical properties. In general, it is easiest to identify heat treated structures after transformation and before tempering. However, in most applications, hardened steels must be tempered and are usually examined in this condition. If a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature increases toward the lower critical temperature (Ac1).

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