低碳钢临界区加热转变动力学研究

KINETICS OF AUSTENITIZATION DURING INTERCRITICAL ANNEALING IN LOW CARBON LOW ALLOY STEELS

用金相方法研究了20钢及16Mn 钢临界区加热转变动力学。原始组织为铁素体(F)和珠光体(P)。测出了转变动力学曲线。按 Avrami公式进行处理后得三段直线。第一段对应于奥氏体迅速长入珠光体;第二段对应于奥氏体沿 F/F 界面生长;第三段对应于奥氏体长入铁素体。导出了计算激活能的公式。结果表明,所有转变均受碳原子在奥氏体中扩散所控制,与 Mn 扩散无关。

Kinetics of austenitization during intercritical annealing wasstudied in steel 20 and 16Mn,with primary microstructure of ferrite andpearlite.The location of nucleation of austenite and the growth of austenitewere observed by metallographic microscope.The kinetics of austenite formationwere measured by using quantitative microscopic techniques.The isothermaltransformation curves were disposed by Avramis equation,as a result threesfraigh tlines were obtained.The first corresponds to rapid growth of austenite intopearlite(the first stage by Speich).The second and the third to correspond toslow growth of austenite into ferrite(the second stage by Speich),in Which thesecond corresponds austenite growth along ferrite-ferrite boundaries,and thethird corresponds the slow growth into ferrite,i.e.,the second stage bySpeich can be subdivided into two stages.Kinetics of each stage were discussedand calculation of activation energy for each stage has been developed.Fromthe values of activation energy calculaled,it is shown that stage 1 and stage2 are controlled by carbon diffusion in austenite.Stage 2 was discussed withtheory of diffusion in ternary system,from which it is concluded that nomanganese diffusion is required during transformation in stage 2,whichtherefore supports the calculation of activation energy.