Effect of Microstructural Evolution on the Mechanical Properties of Intercritical Heat‐Affected Zone of Quenched‐and‐Tempered Ultrahigh‐Strength Steel

Abstract

In order to investigate the effect of microstructural evolution on the mechanical properties in the intercritical heat‐affected zone (ICHAZ) of the quenched‐and‐tempered ultrahigh‐strength steel, the simulated ICHAZ samples with different austenite transformation degrees (0%, 25%, 50%, 75%, and 100%) are prepared. These samples are named IC‐0%, IC‐25%, IC‐50%, IC‐75%, and IC‐100%, respectively. The fine‐grained fresh martensite (FM) is produced in the partial austenite transformation during cooling. The austenite in IC‐100% with the lowest C and Mn content is transformed into a mixture of FM and granular bainite (GB). The IC‐0% and the IC‐25% are softened by about 20 HV10 compared with that of the base metal. When the matrix is transformed into quasi‐polygonal ferrite (QPF), the large difference in microhardness between FM and QPF causes the instability of their interfaces, resulting in the deterioration of the impact toughness in IC‐50% and IC‐75%. However, from IC‐75% to IC‐100%, the impact toughness has been improved from 18.0 to 28.9 J. In addition to the less embrittlement of microstructure in IC‐100%, the lower microhardness difference between FM and GB is the main reason.