材料期刊网

论述了高速钢回火时奥氏体的催化作用和稳定化现象的工作,以及根据研究结果所拟定的回火新工艺.奥氏体催化作用的效果以冷却过程中残余奥氏体转变为马氏体的温度M′来表示.催化作用的效果决定于回火的溫度和时间. 先经高溫保溫又在较低溫度(但在一定溫度T_c以上)保溫(分级回火),则催化作用不但具有迭加性,或和高溫同样时间的效果相等,而且有时超过高溫保溫的效果.高溫保溫有时还能促使低溫时的催化作用.经适当分级回火后(例如先经560℃保温又在550℃保溫),残余奥氏体转变为马氏体的数量能和多次回火后相比拟,但所需总时间却能大为缩短. M′溫度随钢內残余奥氏体量的增加而降低,并随奥氏体內含碳量而改变. 在T-c溫度以下、M′溫度以上停留,奥氏体的稳定化作用(M′值的降低)极为显著.稳定化的程度(△M′)因停留时间的增长而增加,一直达到一定的饱和值.最大稳定化的程度以及稳定化开始的速度因保溫溫度的升高而减小.在M′溫度以下保溫,其最大的奥氏体稳定量因保溫溫度的升高而增加.回火时奥氏体的稳定化现象可能主要由应力的减小所引起的,这些应力在冷却时有助于马氏体核胚的扩展. 催化作用可认为是稳定化的可逆过程,它应该包括下列过程:无序分布位错的重新排列,α-γ相交界面位错

The paper discusses the conditioning and stabilization of austenite during tempering, and the suggested new tempering process based upon the results of research on high speed steels. The effect of conditioning of austenite, expressed in term of the M' temperature at which the retained austenite transforms to martensite during cooling, depends upon the temperature and duration of tempering. On step tempering, i.e., holding at a higher temperature followed by a lower one (but above a certain temperature T_c), the effect of conditioning is additive and sometimes equals or exceeds that of tempering only at the higher temperature with identical duration. The amount of retained austenite transformed after optimum step tempering (e. g., 560℃ holding followed by 550℃ holding) is similar to that after multiple tempering process, but the tempering time is shortened. The M' temperature falls with the increasing amount of retained austenite and varies with the carbon content of austenite. Holding at temperatures below T_c and above M' causes a marked effect on the stabilization of austenite. The degree of stabilization and the initial rate of stabilization decrease with the increasing holding temperature. But the maximum amount of austenite stabilized increases with the holding temperature when the holding temperature is below M'. It is suggested that the stabilization of austenite during tempering is chiefly due to the reduction of stresses that promote the growth of martensite embryos. The conditioning may be regarded as a reversed process of stabilization and the following changes are suggested: the rearrangement of random dislocations, the formation of dislocation loops at α-γ phase boundaries and the removal of atmospheres formed from vacancies and solute atoms. A new tempering process consisting of step tempering followed by a normal tempering is suggested, e.g., holding at 590℃ for 25 minutes, then at 550°C for 25 minutes, and finally tempering at 560℃ for 60 minutes. The amount of austenite transformation, the mechanical properties and the cutting abilities of the high speed steel treated with the new process are comparable with those obtained by traditional multiple tempering. An optimum tempering process for reducing the deformation of tools can also be designed by the application of the stabilization of austenite. The total tempering durations are shortened in both cases.