The influence of the Si and Mn concentrations on the kinetics of the bainite transformation in Fe-C-Si-Mn alloys
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I.
INTRODUCTION
IN a previous investigation of the kinetics of the bainite reaction in an Fe-0.38C-1.73Si alloy and an Fe-0.38C3.11Mn alloy, it was demonstrated that the bainite reaction proceeds to completion, tl] In the Fe-0.38C-1.73Si alloy, the transformation to bainite is completed in approximately 40 seconds at an isothermal reaction temperature of 450 ~ No carbides are observed within the ferrite laths, but films of retained austenite are trapped between the laths of bainitic ferrite (c~b). The bainite reaction in the Fe-0.38C-3.11Mn alloy is comparatively slow; at 450 ~ the transformation reaches completion in approximately 400 seconds. Unlike the Fe-C-Si alloy, films of retained austenite are not observed and cementites are located within and between the ferrite laths. It has been suggested that because Mn reduces the activity of carbon in austenite, the segregation of Mn to a / T interfaces will diminish the driving force for, and hence the kinetics of, ferrite growth, m This behavior has been termed the "solute draglike" effect (SDLE). t2] Since Si raises the activity of carbon in austenite and, thus, does not cause an SDLE, [3] it is expected to have a rapid ferrite growth rate and, hence, a rapid rate of the overall reaction kinetics of the bainite in Fe-C-Si alloys. It should be noted that in the time-temperature-transformation (TTT) diagrams for the Fe-0.38C-1.73Si and Fe-0.38C-3.11Mn alloys (Figure 1), ]3] the incubation time for the bainite reaction is much longer in the Mn-containing alloy. Therefore, the SDLE, which is used to explain the hindering effect of some alloying elements on the growth rate of ferrite, cannot explain all the differences in transformation behavior between Fe-C-Si and Fe-C-Mn alloys. Bhadeshia and Edmonds [a] investigated the bainite S.K. LIU, Professor, and J. ZHANG, Assistant, are with the Department of Materials Engineering, Southwest Jiaotong University, Chengdu Sichuan, 610031, People's Republic of China. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Metals Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
transformation of an Fe-0.43C-3.00Mn-2.12Si alloy. One of the main results to emerge from this study was that the bainite formation rate measured at a temperature nearby B,, the temperature of the bay in the T T T curve, was very sluggish. These researchers also suggested that the incomplete transformation phenomenon, or transformation stasis, occurred in this alloy. From all the results mentioned above, it is apparent that not only is the mechanism of the effect of Si on the bainite reaction kinetics of Fe-C alloys different from that of Mn, but also the effect of Si and Mn in combination is different from the effect of Si or Mn alone; i.e., the effects of Si and Mn are not simply additive. Therefore, the effects of Si and/or Mn on th
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