Mechanical and corrosion properties of medium carbon low alloy steel after cyclic quenching and tempering heat–treatments

Abstract

Cyclic quenching and tempering (QT) heat–treatment processes have the potential to improve the mechanical properties of steel due to the formation of fine grain size microstructure. This work aims to investigate the mechanical and corrosion properties of experimental medium carbon low alloy steel under cyclic heat–treatment processes. Cyclic single quenching and tempering (SQT), double quenching and tempering (DQT), and triple quenching and tempering (TQT) heat–treatment processes were carried out on the medium carbon low alloy steel. X–ray diffraction analysis validated the formation of fine ε–carbide (Fe2.4C) after intermediate low–temperature tempering in DQT and TQT heat–treatments. These fine carbides provided preferential nucleation sites for the formation of new austenitic grain, resulting in a refined microstructure composed of supersaturated (carbon) lath martensite (α′) and retained austenite (γ). The DQT and TQT heat–treatments improved 19% tensile strength, 100% elongation, and 95% impact absorption energy in comparison to SQT heat–treatment. DQT and TQT heat–treatments also transformed the brittle behavior of SQT steel into a ductile behavior. It was also revealed by the small number and size of dimples along with a large number of short tear ridges in fractographs. Morphology of corrosion products of heat–treated experimental steel was also studied in a 5% NaCl solution. The SQT heat–treated sample showed porous morphology of corrosion products, while the TQT sample showed morphology with less porosity on the surface. The DQT sample showed solid corrosion morphology with little porosity. As the grain size went on decreasing in the cyclic heat–treatments the corrosion rate was also dropped.