Bainite, isothermal transformation

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. 

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. 

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... 

Bainite is a crystalline constituent that can be formed during heat treatment of steel by isothermal transformation or continuous cooling in the temperature range between those of perlite and martensite. Here, iron diffusion is no longer possible, while... 

Regarding its form, bainite is distinguished between adcular bainite (with continuous cooling and isothermal transformation) and granular bainite... 

Qui] Quidort, D., Breehet, Y.J.M., A Model of Isothermal and Non Isothermal Transformation Kineties of Bainite in 0.5% C Steels , ISIJ Int., 42(9), 1010-1017 (2002) (Calculation, Kinetics, Phase Relations, Theory, Thermodyn., 36)... 

S. A. Sajjadi and S. M. Zebarjad, "Isothermal transformation of austenite to bainite in high carbon steels". Journal of Materials Processing Technology, 189(1-3), 2007 pp. 107-113. 

Figore 10.18 Isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, including austenite-to-f)earlite (A-P) and austenite-to-bainite (A-B) transformations. 

The time-temperature dependence of the bainite transformation may also be represented on the isothermal transformation diagram. It occurs at temperatures below those at which pearlite forms begin-, end-, and half-reaction curves are just extensions of those for the pearlitic transformation, as shown in Figure 10.18, the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition that has been extended to lower temperatures. All three curves are C-shaped and have a nose at point N, where the rate of transformation is a maximum. As may be noted, whereas pearlite forms above the nose [i.e., over the temperature range of about 540°C to 727°C (1000°F to 1341 °F)], at temperatures between about 215°C and 540°C (420°F and 1000°F), bainite is the transformation product. 

Figure 10.23 Isothermal transformation diagram for an alloy steel (type 4340) A, austenite B, bainite ...

Figure 10.39 Isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy A, austenite B, bainite C, proeutectoid ce-mentite M, martensite P, pearlite.

Sri] Magnetically checking for carbide separation during quenching, TEM, electron diffraction austenized at 1250°C and water quenched specimens were isothermally held at 285°C and frnally quenched into cold water, 7.9 mass% Cr and 1.11 mass% C 3.3 mass% Cr and 0.6 mass% C Bainite transformation... 

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. 

Sch2] Schissler, J.-M., Metauer, G., Influence of the Isothermal Bainitic Transformation at 420°C on Graphitization by Ulterior Recovery in Hypereutectoid Fe-C-Si Alloys with 3.9% Siheon (in French), Compt Rend. Hebd. Acad. Set, Ser. C, 277(20), 997-999 (1973) (Experimental, Crys. Stmcture, 4)... 

Umemoto, M. Horiuchi, K. Tamura, I. (1982). Transformation Kinetics of Bainit during Isothermal Holding and Continuous Cooling. Trans. Iron Steel Inst. Jpn., Vol. 22, 854r-861. 

Bainite forms isothermally as an intermediate transformation product above but below the temperature of eutectoid decomposition in ferrous alloys. This product is a mixture of ferrite and carbide, which is distinctly different from pearlite (a eutectoid product), and is termed bainite in honor of E. C. Bain who first observed such microstructures in collaboration with Davenport (Davenport and Bain, 1930). Bainite also forms during continuous cooling. Bainite transformations occur not only in ferrous alloys but also in non-ferrous... 

The high-temperature jS-b.c.c. (A2, cI2) phase of binary and ternary Cu-Zn alloys and binary Ag-Cd and Ag-Zn alloys transforms when cooled or quenched into a metastable ordered b.c.c. phase that has either a B2 (cP2), DO3 (cF16) or L2i (cF16) structure. This metastable phase undergoes a bainite transformation during isothermal aging into a f.c.c.-based a phase (also called a, phase) having either a 9R or 18R... 

Bainite transformations occur in many ordered b.c.c. phases upon isothermal aging above and below eutectoid temperatures. Bainite plates form martensitically either immediately after, or simultaneously with, some compositional change at a local region of the matrix that has lattice defects, whose localized stress fields assist solute atoms to diffuse. Upon prolonged aging, the... 

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