A Novel Continuous Cooling Treatment Based on the Carbon Diffusion Process During Nanobainite Transformation in High‐Carbon Steel

Abstract

A novel methodology of carbon‐diffusion‐dominated continuous cooling treatment (CC) during the bainite transformation is proposed and implemented on a high‐carbon nanobainitic steel. Nanobainite (NB) microstructure composed of bainitic ferrite lath with a thickness of ≈36 nm is obtained by successive heat treatment processes, including cooling the sample from 240 to 216 °C at a rate of 0.1 °C min−1, then cooling down to 120 °C at a rate of 0.2 °C min−1 followed by austempering at 120 °C for 12 h. Several technological routines involving oil quenching + tempering, single‐stage austempering + tempering, and two‐stage austempering + tempering are compared. The effects of heat treatment processes on the microstructure and mechanical properties are unveiled. Importantly, after the CC process, the tensile strength is 2049 MPa, the yield strength is 1552 MPa, the elongation is 2.5%, the impact toughness is 11.5 J, and the hardness is 60.7 HRC. Notably, the comprehensive mechanical properties of CC‐processed samples are superior to that of other heat‐treated samples. Microstructure analyses reveal that the CC process significantly refines the bainitic ferrite lath thickness while increasing the volume fraction of NB. Thereby, effectively shrink blocky retained austenite, and finally enhancing the overall mechanical properties.