Microstructural characteristics and impact fracture behavior of a high-strength low-alloy steel treated by intercritical heat treatment

Effects of the intercritical heat treatment (IHT) on microstructural evolution and Charpy impact fracture behavior of a high-strength low-alloy (HSLA) steel were investigated. The toughening mechanism was clarified by analyzing microstructural characteristics and crack propagation paths. Results showed that a composite microstructure of ferrite phase separated by globular, rod and irregular shape martensite was obtained by adding the intercritical quenching to the conventional heat treatment of quenching and tempering. And 3.6% retained austenite was detectable in the microstructure. The percentage content of high-angle (15° or more) boundaries reached 78.5%. It was also found that the steel had a high ratio of propagation energy (average: 152 J) to the total absorbed energy (average: 212 J) during impacting at −40 °C. Two crack propagation path models were observed: along the long axis direction of banded ferrite, and across the grains and corresponding interfaces. The improvement of impact toughness was attributed mainly to the retained austenite, the interlocking arrangement of banded ferrite and the ferrite-martensite interfaces with high-angle misorientation, which exhibited effective resistance to the crack propagation.