Influence of Solution Treatment under (Unrecrystallizing Condition for Austenite Unrecrystallization Solution Treatment) on Martensite Structure of 18% Ni Maraging Steel

Abstract

Influences of solution treatment under unrecrystallizing condition for austenite (the unrecrystallization solution treatment, URST) on micro-structures of lath martensite in 18%Ni maraging steel were investigated using optical and electron microscopes. In the present alloy, a small amount of boron was intentionally added to retard the recrystallization of the reversed austenite. Morphological changes of martensite structure by URST relate with dislocation density in austenite. In the lath martensite transformed from the recrystallized austenite, it is possible to identify the hierarchic structure which was constructed with lath, block and packet. The URST at relatively low temperature, may not only decrease the widths of laths and blocks markedly but also destroy the hierarchic synthesis of lath martensite. The present irregular distribution of block and packet is similar with those of steels containing carbon atoms more than 0.4% in mass fractions. The URST at relatively high temperature, however, realizes the fine but hierarchic structure of lath martensite, which is similar with those of steels with less than 0.3% carbon in mass fraction. Strengthening and toughening due to URST are sustained by these subdivided micro-structures.