Effect of High-Temperature Tempering on Microstructure and Mechanical Strength of Laser-Welded Joints between Medium-Mn Stainless Steel and High-Strength Carbon Steel

. The strengthening effect due to high-temperature tempering (HTT) at 700 °C on the microstructure and mechanical properties of welded joints between medium-Mn stainless steel (MMnSS) and high-strength carbon steel (CS) was studied. The microstructure of the weldments was investigated using Laser and scanning electron microscopes. An Electron probe microanalyzer (EPMA) was used to assess quantitatively the elemental distribution profiles of alloying elements within the weld zone. The strengthening precipitates induced during welding and HTT were characterized by transmission electron microscopy (TEM). Uniaxial tensile tests and microindentation hardness (HIT) measurements of the weld joints were conducted to evaluate the strengthening effect. Fully fresh-martensite and fine-tempered martensitic structures were promoted in the as-weld and HTT processes, respectively. The HTT structure exhibited a remarkable improvement in mechanical properties (a better combination of yield and tensile strength together with moderate ductility) compared to its weld counterpart. TEM investigation revealed that various types of precipitates have been promoted in the structures of the weld and HTT, e.g., nanosized vanadium and chromium carbides. It is apparent that the proposed HTT of the joints is an effective treatment for improving the mechanical properties due to inducing the formation of fine interphase precipitates, resulting in enhanced mechanical strength of the joints.