两相区热处理过程中回转奥氏体的形成规律及其对9Ni钢低温韧性的影响

FORMATION OF REVERSED AUSTENITE AND ITS EFFECT ON CRYOGENIC TOUGHNESS OF 9Ni STEEL DURING TWO-PHASE REGION HEAT TREATMENT

通过XRD测定了9Ni钢中的回转奥氏体含量,并采用EBSD技术观察其在基体上的分布,研究了两相区热处理后回转奥氏体含量、分布及其稳定性的变化以及这些因素对9Ni钢低温韧性的影响.结果表明:经过两相区处理后,9Ni钢的低温韧性有不同程度的改善,其中两相区处理温度为650℃时,-196℃的冲击功最高,达到177J,此时测得的回转奥氏体含量也最多,达到10.15%,表明回转奥氏体含量对9Ni钢的低温韧性有重要的影响.EBSD结果则表明:经两相区处理,回转奥氏体不仅在晶界和板条束界形成,也在晶内的板条界上形成,因此即便在其含量低于淬火+回火处理的条件下,9Ni钢的低温韧性也有明显提高,证明同转奥氏体的分布也是影响9Ni钢低温韧性的一个主要因素.对稳定性的分析显示,在本文的工艺条件下,回转奥氏体的稳定性均未达到最佳.

The changes of amount, distribution and stability of reversed austenite and their effects on the cryogenic toughness of 9Ni steel treated by two-phase region heat treatment were studied with XRD and EBSD. It is found that the cryogenic toughness of the 9Ni steel treated by two-phase region heat treatment is more excellent than that of the one treated by quenching and tempering （QT）. When temperature of two-phase region is 650 ℃, the most excellent impact toughness is observed and the impact energy at -196 ℃ reaches to 177 J, at the same time the amount of reversed austenite increases to 10.15%, which indicates that the amount of austenitic has important effect on the cryogenic toughness of 9Ni steel. Results of EBSD show that most of the reversed austenite in the steel treated by QT distribute along the grain boundary, but in the steel treated by two-phase region heat treatment the austenites distribute uniformly along all kinds of boundaries, especially also along the lath martensite boundary in the grain, so the cryogenic toughness can be also improved even the amount of austenitic is lower than that in the steel treated by QT process. The stability analysis of reversed austenite shows that the optimized stability can not be observed under the conditions provided in this work.