深冷处理最低温度对9%Ni钢力学性能及逆转奥氏体含量的影响

Effect of minimum temperatures on the mechanical properties and reversed austenite content of 9%Ni steel subjected to cryogenic treatment

摘要:

通过实验研究了深冷处理过程中的最低处理温度对9%Ni钢力学性能和逆转奥氏体含量的影响。采用了不同的深冷处理温度和保温时间,并与9%Ni钢新发展起来的热处理工艺淬火、亚稳淬火、回火(QLT)相结合。结果表明,-80℃和-110℃的冷处理对9%Ni钢的力学性能和逆转奥氏体含量没有明显影响。然而,-140℃保温24小时的深冷处理能够提高9%Ni钢的冲击韧性,其机理主要在于深冷处理使得块状的逆转奥氏体转变为条状。此外,-140℃深冷处理通过等温马氏体转变值得逆转奥氏体的含量减少。-196℃保温24小时深冷处理增加了逆转奥氏体的含量,同时细化了二次马氏体板条组织, 从而使得9%Ni钢的室温和低温冲击韧性均得到提高。其机理主要是由于深冷-196℃深冷处理促使了超细碳化物的析出,同时增加了组织内应力,从而为逆转奥氏体在回火过程中的形核提供了更多了形核位置。

Abstract:

The effect of minimum temperature on the mechanical properties and reversed austenite of 9%Ni steel subjected to cryogenic treatment was experimentally investigated in the present work. Cryogenic treatments with different temperatures and soaking time were conducted by combining with the newly developed quenching, lamellarizing and tempering (QLT) heat treatment of 9% Ni steel. The results showed that the execution of cryogenic treatment at -80 and -110 °C had no obvious influence in the reversed austenite and mechanical properties of 9%Ni steel. However, the room temperature impact toughness was improved by cryogenic treatment at -140 °C soaking for 24 hours, which was attributed to the modification of reversed austenite morphology from bulks into strips. The volume fraction of reversed austenite was reduced slightly due to the isothermal martensitic transformation at -140 °C. The conduction of cryogenic treatment at -196 °C soaking for 24 hours increased the volume fraction of reversed austenite as well as refined the secondary martensite laths, as a result of improving both the room temperature impact toughness and cryogenic ductility. This observed results were explained through the precipitation of ultra-fine carbides and increase of internal stress during cryogenic treatment which provided more nucleation points for the reversed austenite in the process of tempering.