PHASE AND STRUCTURE TRANSFORMATION OF BAINITE IN 3Cr3Mo3VNb STEEL DURING TEMPERING

The phase and structure transformation of bainite during tempering after isothermal quenching were studied on steel 3Cr3Mo3VNb. It was shown that the secondary hardening of bainite is similar to martensite, mainly formed by precipitation of fine dispersed carbides such as Mo_2C, V_4C_3, NbC, etc. The slow dissolution of coarse cemetite in bainite retards the precipitation rate of carbides during tempering. The decomposition and transformation of the retained austenite in bainite differ from that in martensite. The lower dislocation density in bainite is unfavourable for the aggregation and coarsening of carbides. As a result of the coaction of abovementioned factors, the bainite has superior secondary hardening effect, thermal strength and structural stability to martensite formed by straight tempering.