Comparing the mechanical properties of ferrite-bainitic dual-phase microstructures formed by thermomechanical processing and heat treatment cycle in AISI ۴۳۴۰ steel

In this research, ferrite-bainitic microstructue was produced by two different methods; thermo mechanical processing and heat treatment cyle. The thermo mechanical processing includes the following steps: auseniting at ۸۵۰ °C for ۲۰ min, performing ۴۵% reduction in cross-section by rolling, quenching in water, intercrtitical annealing at ۷۵۰ °C for ۳, ۵, ۱۰, ۱۵, and ۳۰ mins, austempering at ۳۰۰-۳۵۰ °C for ۶۰ and ۱۲۰ min, and quenching in water. Also, heat treatment cycle includes the following steps: austeniting at ۸۵۰ °C for ۲۰ min, intercritical annealing at ۷۵۰ °C for ۱۵, ۳۰, and ۶۰ mins, austempering at ۳۰۰-۳۵۰ °C for ۶۰ and ۱۲۰ mins, quenching in water. The initial sample was AISI ۴۳۴۰ steel in all samples. The ferritic-bainitic microstructures with different volume fraction of ferrite/bainite was formed. The microstructure of samples was studied using Scanning Electron Microscopy (SEM), and the ferrite volume fraction was analyzed using ImageJ image processing software. Then, tesnsile and hardness tests were done on the obtained samples following the standards. The results show that tensile strength and ductility in samples produced by thermo mechanical processing have higher quantity to heat treated samples. But, ductility was approximately equal in the all samples.

Keywords:

AISI ۴۳۴۰ steel , thermo mechanical treatment (TMT) , Ferrite-Bainite , mechanical properties , intercritical annealing , microstructure