THE PRODUCTION OF DUAL PHASE STEEL STRUCTURE IN THE CENTRAL REGION OF SURFACE-CARBURIZED AISI 8620 STEEL AND EFFECT OF MARTENSITE PARTICLE SIZE AT CONSTANT MARTENSITE VOLUME FRACTION ON TENSILE PROPERTIES

摘要

In this study, the production of dual phase steel structure in the central region of surface-carburized AISI 8620 cementation (case-hardening) steel and effect of martensite particle size variation at ～25% constant martensite volume fraction on tensile properties have been investigated. For these purposes, some of surface carburized (～ 0.8% C) specimens were oil quenched from 900°C and the others were normalized from 900°C to obtain fully martensitic and ferrite+pearlite starting microstructure, respectively. Then, specimens were oil quenched from intercritical annealing temperature of 731°C to produce dual phase steel structure at the central region of specimens with different martensite particle size at constant martensite volume fraction of ～25% and fully martensitic structure at the surface. Generally specimens with dual phase steel structure at the central region exhibited slightly lower tensile and yield strength but superior ductility than those specimens with martensite structure at the central region. Tensile properties were improved with decreasing martensite particle size at constant martensite volume fraction.