Effect of banite volume fraction, resulting from Intercritical heat treatment on mechanical properties and Fracture behavior of bainite- ferrite dual phase steels

In this research, under intercritical heat treatment cycles (Austenitising at 850°C for 1 hour, Then transferring each sample to the furnace at temperatures of 770, 760 and 750°C for 15 minutes, transferring to the salt bath of 360°C for 15 minutes and finally cooling down to room temperature in air) of the AISI 4140 steel, samples of ferritic-bainitic dual phase structures with several percentages of bainite were attained. Then the mechanical properties of the samples have been studied by mechanical tests. Results show that by increasing the bainite volume fraction, the yield strength and the ultimate tensile strength are increased while the ductility and Impact energy is decreased. by investigating the fracture surfaces of the samples by SEM, it was seen that the mode of fracture in dual phase steels is like a mixture of brittle and ductile fractures, and manganese sulfide impurity particles are the places of the nucleation, growth and propagation of cracks and also with increasing the volume fraction of bainite, the ratio of brittle fracture to ductile fracture increases.