Effect of Cyclic Intercritical Tempering on the Microstructure and Mechanical Properties of a Low-Carbon Cu-Bearing 7Ni Steel

High strength and toughness are usually hard to obtain simultaneously because of the trade-off. In this research, cyclic intercritical tempering (IT) was applied to a low-carbon Cu-bearing 7Ni steel to pursue a better strength-toughness balance than what conventional single intercritical tempering can achieve. The mechanical properties and microstructure of cyclic IT and single IT were studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in addition to dilatometry. It was found that cyclic IT can significantly improve the strength without much sacrifice of toughness. The additional strength comes from dislocation and precipitation strengthening. The mechanism of reverse transformation was studied, and it was found that the mechanism changes from diffusional at single IT or first-cycle IT to a combination of interface-dominated and diffusional at the following cyclic IT. It was suggested that enrichment of Ni after the first cyclic IT is responsible for the mechanism change by thermodynamic calculation. Furthermore, although the Ni content is higher in fresh martensite (FM) after following cyclic IT, no distinct decrease of Ms was found, which is related to the inhomogeneous elemental distribution of FM.