Bainite temperature

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

Ba.inite. In a given steel, bainite microstmctures ate generally found to be both harder and tougher than peadite, although less hard than martensite. Bainite properties generally improve as the transformation temperature decreases. Lower bainite compares favorably with tempered martensite at the same hardness and can exceed it in toughness. Upper bainite, on the other hand, may be somewhat deficient in toughness as compared to fine peadite of the same hardness (33). 

In austempering the article is quenched to the desired temperature in the lower bainite region, usually in molten salt, and kept at this temperature until transformation is complete (Fig. 22). Usually, the piece is held twice as long as the period indicated by the isothermal transformation diagram. The article may then be quenched or air-cooled to room temperature after transformation is complete, and may be tempered to lower hardness if desired. 

In order to produce martensite and bainite the tube must have been overheated to at least the A3 temperature of 870°C (Fig. 13.4). When the rupture occurred the rapid outrush of boiler water and steam cooled the steel rapidly down to 264°C. The cooling rate was greatest at the rupture edge, where the section was thinnest high enough to quench the steel to martensite. In the main bulk of the tube the cooling rate was less, which is why bainite formed instead. 

The microstructure at position (ii) consisted of grains of ferrite and colonies of pearlite. It was noticed that the pearlite had started to "spheroidise" (see Problem 5.2). The microstructure at position (i) consisted of grains of ferrite and grains of lower bainite in roughly equal proportions. Estimate the temperatures to which the tube been heated at positions (i) and (ii). Explain the reasoning behind your answers. 

At lower transformation temperatures (<770K approx.) a second reaction, the formation of bainite intervenes. Like pearlite, the bainite constituent in steels consists of a mixture of ferrite and an iron carbide and is formed by... 

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... 

On tempering or annealing martensite, bainite or even pearlite at even higher temperatures (about 970K) a structure consisting of coarse cementite spheroids (readily visible in a light microscope) in a ferrite matrix is obtained. This is the most stable of all ferrite/cementite aggregates, and it is also one of the softest. 

Bainite a structure produced in carbon and alloy steels by rapid cooling to a temperature above that at which martensite is formed, followed by slower cooling. 

Bhadeshia (1982) associated the two sets of parameters with (1) difliisional transformations such as the formation of ferrite and (2) shear-related transformations such as bainite or Widmanstatten ferrite. Some additional conditions were made concerning the upper temperature limit to the shear C curve and the M, temperature was calculated by a thermodynamic method (Bhadeshia 1981). The calculated T il diagrams were then compared with experiment for a wide number of alloys (Figs. 

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. 

BAINITE. A product of the decomposition of austenite that usually occurs at temperatures between those that produce pearlite and those that produce martensite. Its structure consists of finely divided carbide particles in a matrix of ferrite. See also Austenite. 

The transformation of austenite to lower temperature transformation products (martensite, bainite, pearlite, ferrite etc) in a steel which is being steadily cooled, as in a weld HAZ, as opposed to being... 

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. 

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