The Role of Intercritical Annealing in Enhancing Low-temperature Toughness of Fe-C-Mn-Ni-Cu Structural Steel

Abstract

In this article, an intercritical annealing (IA) process was introduced to the conventional quenching and tempering (QT) heat treatment for a Fe-C-Mn-Ni-Cu structural steel. The corresponding microstructures and mechanical properties of this steel were characterized by scanning electron microscope (SEM) equipped with electron back scattering diffraction (EBSD) and mechanical properties test. The results showed that IA process could lead to a considerable increase in low-temperature toughness for this steel. A mixed microstructure was obtained after IA process had been adopted containing intercritical ferrite and tempered martensite together with a small amount of retained austenite. This steel with mixed microstructure exhibited tensile strength of 961 MPa, relatively lower yield strength of 830 MPa, and a lower yield-to-tensile ratio (Y/T ratio) of 0.86, while a higher total elongation of 22.2 pct was achieved. The reason for this could be attributed to the multiple effect of multi-phase microstructure and deformation-induced transformation of the retained austenite during tensile deformation. The excellent low-temperature toughness was characterized by the Charpy impact energy as 183 J at 153 K (− 120 °C), which was associated with highly stable retained austenite and finer microstructure through reversed transformation during intercritical annealing treatment. These can be considered to increase the resistance to crack initiation and propagation and decrease the ductile-brittle transformation temperature (DBTT).