Comparing the mechanical properties of ferrite-bainitic dual-phase microstructures formed by thermomechanical processing and heat treatment cycle in AISI 4340 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 850 °C for 20 min, performing 45% reduction in cross-section by rolling, quenching in water, intercrtitical annealing at 750 °C for 3, 5, 10, 15, and 30 mins, austempering at 300-350 °C for 60 and 120 min, and quenching in water. Also, heat treatment cycle includes the following steps: austeniting at 850 °C for 20 min, intercritical annealing at 750 °C for 15, 30, and 60 mins, austempering at 300-350 °C for 60 and 120 mins, quenching in water. The initial sample was AISI 4340 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.

Comparing the mechanical properties of ferrite-bainitic dual-phase microstructures formed by thermomechanical processing and heat treatment cycle in AISI 4340 steel Keywords:

AISI 4340 steel , thermo mechanical treatment (TMT) , Ferrite-Bainite , mechanical properties , intercritical annealing , microstructure