Bainite

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, 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, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite. A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The large density of dislocations in the ferrite present in bainite, and the fine size of the bainite platelets, makes this ferrite harder than it normally would be. The temperature range for transformation of austenite to bainite (125–550 °C) is between those for pearlite and martensite. In fact, there is no fundamental lower limit to the bainite-start temperature. When formed during continuous cooling, the cooling rate to form bainite is more rapid than that required to form pearlite, but less rapid than is required to form martensite (in steels of the same composition). Most alloying elements will retard the formation of bainite, though carbon is the most effective in doing so. Aluminium or cobalt are exceptions in that they can accelerate the decomposition of austenite and raise the transformation temperature. The microstructures of martensite and bainite at first seem quite similar, consisting of thin plates which in low-alloy steels cluster together. This is a consequence of the two microstructures sharing many aspects of their transformation mechanisms. However, morphological differences do exist that require a transmission electron microscope to see. Under a light microscope, the microstructure of bainite appears darker than untempered martensite because the bainite has more substructure. The hardness of bainite can be between that of pearlite and untempered martensite in the same steel hardness.