Simultaneously increasing the strength and impact toughness of high-strength low-alloy steel through cyclic quenching

A cyclic quenching (CQ) heat treatment process was used to improve the balance between the strength and toughness of high-strength low-alloy (HSLA) steel via grain refinement and precipitation strengthening effects. The embrittlement problem of HSLA steel (impact energy of only 65.8 J) was effectively addressed with the CQ process. After four CQ cycles, the impact energy reached 95.1 J, the tensile strength reached 1568.4 MPa, and the elongation increased to 12.84%. The improvement in the mechanical properties was attributed to the refinement of the martensitic structure and the uniform precipitation of carbides. The fine grain strengthening and precipitation strengthening mechanisms are the main reasons for the increase in yield strength. The excellent impact toughness is attributed to the high density of high-angle grain boundaries (HAGBs), which consume considerable crack extension energy, whereas the fine carbides promote the homogenization of dislocation slips and reduce local stress concentrations.