Study On Microstructure And Corrosion Resistance Of SASS 254SMo Welding Thermal Simulation

254SMo is an ultra-low carbon austenitic stainless steel with a high content of Cr,Ni,Mo.It has excellent comprehensive mechanical properties coupled with excellent resistance to pitting,crevice corrosion,intergranular corrosion and stress corrosion.Therefore,it is widely used in waste gas treatment,oil platform,desalination,flue gas desulfurization and other industries.The austenite grains in the heat-affected zone are easily coarsened during the welding process of the highly alloyed 254SMo,and the higher content of Mo will promote the formation of the precipitated phase,which will affect the mechanical properties and corrosion resistance.At present,a lot of researches have been done on the formation of precipitates in solution and aging treatment process of 254SMo at home and abroad.However,there is no systematic study on the relationship among the microstructure,mechanical properties and corrosion resistance of 254SMo welding heat affected zone.In this paper,Gleeble-3800 was used to simulate the microstructure of 254SMo welding heat affected zone.And the appropriate welding heat input was selected for the shielded metal arc welding experiment through the analysis of performance of the thermal simulation sample.The research results are expected to provide a theoretical basis for optimizing the actual welding process parameters.Firstly,thermal simulation experiment was carried out in the heat affected zone of 254SMo stainless steel by using Gleeble-3800.The influence of the heat input and the number of heat cycles on the micro-structure,precipitation phase and micro-hardness of the heat affected zone(HAZ)was analyzed by means of optical microscopy,scanning electron microscopy,and hardness testers.The results showed that with the increase of heat input and welding pass,the grain structure of the heat affected zone grows obviously and the chi phase gradually increases.The micro-hardness in the heat affected zone is close related to grain coarsening and precipitation of?phase.With the increase of heat input,the grain coarsening intensified and the micro-hardness decreased.And the increase of?phase also leads to a decrease of micro-hardness.Secondly,the pitting resistance and intergranular corrosion resistance of the thermal simulation specimens were investigated by the potentiodynamic polarization test and double ring electrochemical potential reactivation test.The results show that with the increase of heat input,the breakdown potential E_b decreases,and the resistance to pitting corrosion in the heat affected zone decreases.The reactivation rate increases first and then decreases while the resistance to intergranular corrosion decreases first and then increases as the heat input increases when the number of welding passes is three.When the heat input is 2.5 kJ/mm,the reactivation rate reaches 4.79%.Therefore,the actual heat input is recommended to be about 1.5 kJ/mm.Finally,according to the rules of micro-structure,precipitation phase,micro-hardness and corrosion resistance obtained from the thermal simulation experiment,a heat input of approximately 1.5kJ/mm was selected for the actual SMAW experiment.The micro-structure,precipitation phase and the mechanical properties of the actual heat affected zone of the welding were studied.The experimental results show that with the increase of heat input,the grains in the heat-affected zone are obviously grown up.And when the heat input is 1.61 kJ/mm,the?phase is observed in the heat-affected zone,which is consistent with the results obtained from thermal simulation test.When the welding heat input is 1.46 kJ/mm,the overall mechanical properties of the welded joint are excellent.Moreover,the grain size is relatively uniform under this heat input,and no obvious precipitation phase is produced.Therefore,the actual welding heat input in SMAW welding production is recommended to be around 1.46 kJ/mm.