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

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.