Critical Driving Forces for Formation of Bainite
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BAINITE is a microstructure that often forms in steels on cooling of the high-temperature phase, austenite. It can give the steel very useful properties, and there has long been a need to predict the bainite start temperature, BS, from the composition. For Fe-C alloys one may represent the critical conditions of bainite formation from austenite with the BS line in a T,C diagram. Usually BS is described as the critical temperature of bainite formation for a fixed carbon content but could as well represent the same BS as the critical carbon content at a fixed temperature. There are thus two natural ways of representing the BS function, BS ¼ Tcrit ðCÞ or BS ¼ Ccrit ðTÞ. To distinguish between these two functions, they can be denoted as BS ðTcrit Þ and BS ðCcrit Þ. Mathematically, they may look very different but they represent the same BS line in the T,C diagram. For any point in the T,C diagram, many thermodynamic properties can be calculated from a thermodynamic database, always obtaining a unique value, e.g.,
LINDSAY LEACH, LARS HO¨GLUND, MATS HILLERT, and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellva¨gen 23, 10044 Stockholm, Sweden. Contact e-mail: [email protected] PETER KOLMSKOG is with Sandvik Mining and Rock Technology, Sandvik Industrial Area, 6480-RTDRB, 811 81 Sandviken, Sweden. Manuscript submitted March 21, 2018. Article published online July 27, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
the thermodynamic driving force for the transformation, and it is well known that bainite does not start to form until there is a substantial driving force. From experimental information on the BS line in a T,C diagram, one can thus calculate the critical driving force, DFcrit, and evaluate a function DFcrit(T) for an Fe-C alloy under the condition that the alloy is situated on the BS line. Then, a BS line can be plotted in a DF,T diagram, which represents the same critical conditions as the BS line in the T,C diagram. The function representing the BS line in this diagram will also be denoted as BS but it will be given as BS ¼ DFcrit ðTÞ or BS ¼ Tcrit ðDFÞ. This is again a case where two BS functions may look very different mathematically but actually represent the same BS line in a diagram. One could also define DFcrit(C) or BS ¼ Ccrit ðDFÞ. All these functions and corresponding diagrams with BS lines can also be constructed for any fixed alloy content in a steel. The aim of the present work was to explore the usefulness of this kind of diagram by plotting the same BS lines in these different ways. Many equations of BS temperatures have been proposed,[1–10] e.g., an early one by Steven and Haynes,[1] in which the coefficients for carbon and alloying elements were determined by linear regression analysis of experimental BS information from a group of 65 steels, mainly commercial ones. In these equations it is assumed that the effects of alloying elements are independent of carbon content, which will here have the symbol C, irrespective of in wh
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