Study On Microstructure And Mechanical Property Of SASS 254SMo TIG Welding Joint

SASS 254 SMo has excellent pitting resistance, good toughness and high mechanical strength because of the high contains of Cr, Ni, Mo elements, and was widely used in the industries such as desalination, flue gas industries,coastal power plants and rubbish burning. Heat affected zone(HAZ) grains coarsening and second phase precipitation would be found during 254 SMo welding process because of its phase transformation characteristics and complicated alloy composition, which decreased mechanical performance and corrosion resistance of SASS 254 SMo welding joint, then shortened the lifetime of welded structures.But in China and abroad researches of 254 SMo were mainly focused on solution aging and welding process, few study were carried on the effect of heat affected zone(HAZ) microstructure on mechanical properties and corrosion resistance. Firstly, GLEEBLE3800 was used to simulate 254 SMo heat-affected zone(HAZ) in this paper, then TIG welding test based on the data, at last electrochemical corrosion test of HAZ and properties of 254 SMo joint were studied. The results was expected to provide theoretical guidance on super austenitic stainless steel 254 SMo welding and industrial applications.Firstly, 254 SMo heat affected zone(HAZ) was simulated and the effects of heat input and cooling time t12 / 8 on microstructure properties?the hardness and the second phase precipitation of heat affected zone(HAZ) were investigated by OM and SEM in the paper. The results indicated that with the enlargement of heat input, austenite grains coarsed and the microhardness decreased. When the heat input increased greater than 1.5k J / mm, Sigma phase was observed along the austenite grain boundaries gradually increased, no Sigma phase was found inside the grains. The conclusion that the heat input of 254 SMo welding should not exceed 1.5k J / mm was drawn from thermal simulation test.Secondly, 254 SMo TIG welding experiment was carried out and the influences of heat input on microstructure and mechanical properties of weldedjoints were studied by OM,SEM,microhardness and charpy impact in the paper.The results indicated that the joint existed unmixed zone(UMZ), weld solidification mode was fully austenitic solidification, which existed three types of boundaries. Microhardness decreased with increasing of heat input and then decreased, which reached top at 1.56 k J / mm. With enlargement of heat input,the average impact energy at room temperature reduced. When heat input was less than 2.04 k J / cm, ductile fracture was occurred in heat affected zone(HAZ),otherwise, quasi-cleavage fracture happened.Finally, 254 SMo simulated heat affected zone(HAZ) electrochemical corrosion behavior was tested in the paper with artificial seawater at 70 ??p H8.1 and the chloride ion content of 7% and the effects of heat input on pitting resistance of heat affected zone(HAZ) was investigated. The results indicated that when heat input was less than 2.0k J / mm, passive current density of potentiodynamic polarization curve was relatively small,while broken potential relatively high, which showed good resistance to pitting corrosion. Surface morphology showed the appearance of pits and passivation film was not obviously destroyed. When heat input was greater than 2.0k J / mm, pitting resistance dropped and the passive film was completely destroyed, pits appeared along austenite grain boundaries and even inside the grain.Take full consideration of influences that heat input on microctructure properties?mechanical properties and pitting resistance of 254 SMo welded joint,heat input between 0.97 k J / mm-1.56 k J / mm was recommended in actual TIG welding production.