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Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). [1] First described by E. S. Davenport and Edgar Bain, [2] [3] it is one of the products that may form when austenite (the face-centered cubic crystal structure of iron) is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite ...
Martensite in AISI 4140 steel 0.35% carbon steel, water-quenched from 870 °C. Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. [1]
As a result of the transformation, the microconstituents, pearlite and bainite, form; pearlite forms at higher temperatures and bainite at lower. TTT diagram of isothermal transformations of a hypoeutectoid carbon steel; showing the main components obtained when cooling the steel and its relation with the Fe-C phase diagram of carbon steels.
The steel is then removed from the bath before any bainite can form, and then is allowed to air-cool, turning it into martensite. The interruption in cooling allows much of the internal stresses to relax before the martensite forms, decreasing the brittleness of the steel.
The term "martensite" was originally coined to describe the rigid and finely dispersed constituent that emerges in steels subjected to rapid cooling. Subsequent investigations revealed that materials beyond ferrous alloys, such as non-ferrous alloys and ceramics, can also undergo diffusionless transformations.
A tempered outer ring of martensite, 2. A semi-tempered middle ring of martensite and bainite, and 3. a mild circular core of bainite, ferrite and pearlite. This is the desired micro structure for quality construction rebar. In contrast, lower grades of rebar are twisted when cold, work hardening them to increase their strength.
Even faster, and fine pearlite will form. If cooled even faster, bainite will form, with more complete bainite transformation occurring depending on the time held above martensite start Ms. Similarly, these microstructures will also form, if cooled to a specific temperature and then held there for a certain time. [17]
In austempering the heat treat load is quenched to a temperature which is typically above the Martensite start of the austenite and held. In some patented processes the parts are quenched just below the Martensite start so that the resulting microstructure is a controlled mixture of Martensite and Bainite.