Microstructure Evolution And Porperties Of Super-duplex Stainless Steel SAF2507 Welds

Modern super-duplex stainless steels,which are widely used in marine and petrochemical industries due to its excellent corrosion resistance in chloride and sulfide environments,reveal a clear trend to alloy with higher elements.However,an excessive ferritization phenomenon is presented in its welds due to the higher levels of Cr and Mo.Although Tungsten Inert Gas(TIG)welding technology possesses some virtues,such as the relatively simple operation and the low cost,its weld joints has a relatively low ratio of depth to width and a wider heated affected zone.Laser beam welding(LBW)has some advantages of fast welding speed,small deformation,narrow heated affected zone as well as a relatively high ratio of depth to width.However,an excessive ferritization phenomenon and a mounts of secondary phases are heavily presented in the duplex stainless steel LBW welds.Although post-weld heat treatment(PWHT)process was an effective operation in eliminating the excessive ferritization and the secondary phase which occurred in the duplex stainless steel LBW welds,it is unpractical to perform PWHT for the large weldments.Therefore,an on-line solution treatment is recommended in workshop to meet the practical application.With specified temperature and quench conditions,this kind of solution treatment technique calls the heat treatment time for as short as possible.On the basis of background mentioned above,the effect of the short-time treatment on the microstructure evolution,mechanical property and pitting corrosion behavior of SAF2507 super-duplex stainless steel TIG and LBW welds were investigated in the thesis by morphological observation,mechanical property evaluation method and electrochemical testing.The results are as follows:(1).The effect of short-time PWHT on the microstructure evolution and pitting corrosion resistance of SAF2507 super-duplex stainless steel TIG welds was investigated using morphological observation,potentiodynamic polarization and potentiostatic critical pitting temperature techniques.The relationship between microstructure evolution,element partitioning and pitting corrosion behavior of SAF2507 TIG welds was discussed in detail.The results demonstrated that: the far deviation of the heated affected zones from the equilibrium state reduced the corrosion resistant of the whole weld joint.The ferrite phase in the as-welded heated affected zones was the weaker phase for corrosion pits to attack.After short-time heat treatment,the phase-stabilizing elements of SAF2507 TIG welds gradually returned to their own phases with an obvious decline in the ferrite content until the heated affected zones and welded metal reached their equilibrium state.The heated affected zones and welded metal reached their equilibrium state at 1080 and 1050?,respectively.Since then,the austenite content shrank again.The highest pitting corrosion resistance of SAF2507 TIG weldments was obtained after heating at 1080? for 3 min.(2).On the basis of(1)mentioned above,the effect of the same heat treatment on the mechanical property of SAF2507 super-duplex stainless steel TIG welds was investigated using mechanical property evaluation method.The results demonstrated that: The far deviation of microstructure from the equilibrium state made the as-welded heated affected zones harder than other zones.Similarly,this deviation worsened the impact toughness of the whole welds.With a similar hardness in the BM,heated affected zones and WM,the specimens heat-treated at 1080? for 3 min achieved the highest impact toughness.(3)The effect of short-time PWHT on the microstructure evolution and pitting corrosion resistance of SAF2507 super-duplex stainless steel LBW welds was investigated using morphological observation,potentiodynamic polarization and potentiostatic critical pitting temperature techniques.The results demonstrated that: the phase equilibrium and the alloying elements distribution in the SAF2507 LBW welds were heavily disturbed by the rapid thermal cycle of LBW process.After LBW welding,amounts of nitrides precipitated in the ferrite phase.The elements of Cr,Mo and Ni were almost distributed homogeneously between ferrite and austenite.Therefore,the as-welded ferrite phase was predicted to be easily attacked by corrosion pits.Nitrides in the as-welded ferrite phase were dissolved into ferrite matrix again after PWHT at 1020? for 3 min.The pitting corrosion resistance of SAF2507 LBW welds could be effectively elevated by this heat treatment process.The highest pitting corrosion resistance of SAF2507 LBW weldments was obtained after heating at 1100? for 3 min.