Effects of tempering temperature on microstructure evolution and mechanical properties of high-strength low-alloy steel prepared by underwater laser melting deposition

The effects of tempering temperature on the microstructural evolution and mechanical properties of underwater single-pass multilayer laser-deposited 10CrNi3MoV high-strength low-alloy steel were systematically investigated. As the tempering temperature increased from 200 °C to 600 °C, the martensite–austenite (M-A) constituents gradually decomposed, and the carbides evolved from fine Fe₃C precipitates to coarse M₂₃C₆ carbides. Meanwhile, the precipitation behavior transitioned from dispersed intragranular distribution to grain-boundary precipitation and finally to matrix-dominated precipitation. Higher tempering temperatures also increased the fraction of high-angle grain boundaries while reducing kernel average misorientation. Tempering at 200 °C produced fine dispersed carbides without significant decomposition of M-A constituents, resulting in simultaneous improvement of strength and toughness. In contrast, tempering at 400 °C caused grain-boundary carbide precipitation, leading to deterioration in ductility and impact toughness. After tempering at 600 °C, complete decomposition of M-A constituents and increased HAGBs fraction improved ductility and toughness, but reduced strength and hardness because of carbide coarsening. Among the investigated conditions, 200 °C provided the optimal comprehensive mechanical properties, with a tensile strength of 753 MPa, elongation of 7%, low-temperature impact energy of 18 J at −40 °C, and microhardness of 259 HV.