Effect of tempering treatment on microstructure and tensile properties of a ferrite- bainitie- martensite triple-phase steel

Dual phase steels have unique properties such as continuous yielding, low ratio of yield strength to tensile strength, high work hardening rate and homogenous elongation percent which are applicable for vehicle industries. Triple phase steels are a developed type of dual phase steels which contains ferrite, bainite and martensite. In this study, triple phase microstructure with 34Vol.% ferrite was obtained for AISI4140 steel using appropriate heat treatment. Then, tempering was performed for triple phase microstructures at 250, 450 and 650oC temperatures for 90 min and microstructural studies and mechanical tests including tensile and hardness were used for the related microstructures. Microstructural studies using optical and scanning electron microscops revealed the three phases as well as tempered martensite and secondary carbides formed during tempering. The results of tensile test showed continuously reduction of yield strength and ultimate tensile strength and also increase of elongation with increasing tempering temperature. The fracture surfaces of tensile tested specimens showed more ductile behavior at 650oC tempering temperature which was in agreement with other mechanical properties.