Zone characteristics of microstructure and mechanical properties of welded joints of dual phase stainless steel with 12%Cr

Researches were carried out on the zone characteristics of microstructure and mechanical properties of welding joint of 12%Cr content stainless grade named 00Cr12Ni with dual phases composed of ferrite and martensite, and the joints were welded by metal active gas arc (MAG)welding, plasma arc welding(PAW) and high frequency induction (HFI) welding. Typical HAZ of 00Cr12Ni welding joints is subdivided into high temperature heat affected zone (HTHAZ) and low temperature heat affected zone (LTHAZ). HTHAZ is characterized by predominant coarse ferritic grains while LTHAZ is characterized by predominant fine low carbon martensitic grains. The respective temperature ranges and thermal cycle curves of HTHAZ and LTHAZ were measured through practical MAG welding procedure, and respective mechanical properties of HTHAZ and LTHAZ were acquired by thermal simulation based on the thermal cycle curves measured in practical welding. Results show that peak temperature of HTHAZ ranges from 1 200 ℃ to melting point and that of LTHAZ ranges from 800 to1 200 ℃. HTHAZ exhibits brittle at room temperature for coarse grains and LTHAZ exhibits ductile at room temperature for fine grain non-equilibrium lath martensite, however it exhibits brittle at subzero. The welding seam of MAG welded joint is composed of tough austenite and bell-shaped, so the fracture surface covers austenitic weld seam in impact test hence better impact toughness. The coarse grains in welding joint of PAW can be eliminated through austenization at 1 000 ℃ followed by fast cooling, and fine grain martensite forms thus resulting in toughness improvement. Although welded joint of HFI procedure is composed of fine grain martensite, the toughness at room temperature is low for the joint due to work-hardening during welding. The toughness of HFI joint can be recovered through relief annealing.