关键词: 马氏体时效钢, 等温淬火, 超细贝氏体, 逆转变奥氏体, 力学性能

Abstract: To address the challenge of synergistic strength-toughness regulation in 18Ni(200) maraging steel, a composite process involving 1000 ℃×1 h solution treatment + 800 ℃×1 h solution treatment + 230 ℃×8 h austempering + 500 ℃×3 h aging was proposed. Through comparative studies with the traditional process (1000 ℃×1 h solution treatment + 800 ℃×1 h solution treatment + -196 ℃×1 h cryogenic treatment + 500 ℃×3 h aging), effects of austempering on microstructure and mechanical properties of the 18Ni(200) steel were investigated. The results indicate that after solution treatment, by austempering lath martensite is transformed into ultrafine bainite, further by aging, a composite structure of bainite and a small amount (2.5%) of reversed austenite is formed, accompanied by abundant nano-sized precipitates. Compared with the martensite block width (16.3 μm) in the conventional process, the bainite lath width (3.4 μm) is significantly refined. The tensile strength, yield strength, and -196 ℃ impact absorbed energy of the austempered specimen reach (1573±15) MPa, (1432±17) MPa, and (15±2) J, respectively, which are 14.5%, 18.7%, and 87.5% higher than those of traditional processes. Fracture analysis shows that the low-temperature fracture mode of the austempered specimen is quasi cleavage type. Compared with traditional process treated steel, the size and number of toughness dimples are significantly increased. The synergistic effect of bainite fine grain strengthening, reverse austenite TRIP effect, and nano precipitation is the reason for the improvement of strength and toughness of the 18Ni (200) steel.

Key words: maraging steel, austempering, ultrafine bainite, reversed austenite, mechanical properties