Bainite crystal structure

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

The change of crystal structure which occurs in some materials in which different crystal structures are stable over different temperature (and pressure) ranges. In ferritic steels, the most important transformation is from the high temperature form, austenite, to lower temperature transformation products, such as ferrite, pearlite, bainite, martensite and, in weld metals, acicular ferrite and ferrite with aligned second phase. 

Numerous investigations have been done regarding the liquidus surface, die isothermal sections and the vertical sections in the stable and metastable systems. The other investigations on die ternary system concern the solubility measurements of carbon in the "y and liquid phases which go always widi activity measme-ments, the determination of the phase diagram under high pressures and die kinetics studies of die austenite transformation in martensite or bainite because these phases are important in die forecast of mechanical properties of steel. The main experimental investigations on crystal structure, phase equilibria and thermodynamics are gathered in Table 1. 

Table 1. Crystal structures of parent and bainite phases at early stages of transformation in Cu- and Ag-based alloys...

The microstructure and crystal structure of bainite crystals in Cu-Zn alloys are quite similar to those of diffusionless martensites forming at subzero temperatures in the same alloys (Comelis and Wayman, 1974 Warlimont and Delaey, 1974). Bainite plates... 

Cite the general mechanical characteristics for each of the following microconstituents fine pearlite, coarse pearlite, spheroidite, bainite, martensite, and tempered martensite briefly explain these behaviors in terms of microstructure (or crystal structure). 

Martensite—this has platelike or needle-like grains of an iron-carbon solid solution that has a body-centered tetragonal crystal structure. Martensite is produced by rapidly quenching austenite to a sufficiently low temperature so as to prevent carbon diffusion and the formation of pearlite and/or bainite. 

If austenite is cooled slowly toward ambient temperature, the dissolved carbon in excess of 0.022 weight % comes out of solid solution as cementite, either in continuous layers of FeaC (pearlite) or as layers of separated FeaC grains (bainite). In either case, the iron is soft and grainy, as with cast iron. If, on the other hand, the hot austenite is cooled quickly (i.e., quenched), the 7-Fe structure goes over to the a-Fe form without crystallization of the interstitial carbon as cementite, and we obtain a hard but brittle steel known as martensite in which the C atoms are still randomly distributed through the interstices of a strained a-Fe lattice. Martensite is kinetically stable below 150 °C above this temperature, crystallization of FesC occurs in time. 

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