Effect of post-weld heat treatment on the microstructure and mechanical properties of the 2205DSS/Q235 laser beam welding joint

Embrittlement of the 2205DSS base metal occurred at 500 °C, 700 °C, and 800 °C as the “475 °C embrittlement” and precipitation of the σ phase. The high-hardness quenched martensite in the as-welded welding seam became low-hardness tempered martensite at 500 °C and 600 °C. It contributed to the improved toughness of the welded specimen at 500 °C and 600 °C. The toughness of the welded specimen decreased at 700 °C and 800 °C because of the generation of high-hardness normalized martensite. A carbon-depleted layer and carbon-rich layer formed on the welding seam-Q235 interface following post-weld heat treatment because of the diffusion of carbon atoms. A carbon-depleted layer and carbon-rich layer formed at 600 °C, 700 °C, and 800 °C. The thickest carbon-depleted layer was obtained at 700 °C, as the fastest diffusion of carbon atoms occurred in the Q235 BM. The ferrite grain of the carbon-depleted layer also rapidly coarsened owing to the lack of a hindering effect from Fe3C on the grain boundary migration. The optimal post-weld heat treatment temperature was determined to be 600 °C.