Effect Of Heat Treatment On Microstructure And Mechanical Properties Of Low Transformation Temperature Weld Metals

It has been investigated that low transformation temperature?LTT?welding materials are beneficial to the fatigue performance of welded structures.However,the comprehensive properties of LTT weld metals cannot meet industrial requirements,especially toughness.Therefore,it is of great engineering significance to explore techniques for improving the comprehensive mechanical properties of LTT weld metals.In this paper,two types of LTT weld metals with different chrome/nickel equivalence ratios were developed,namely,13Cr-10Ni and 12Cr-6Ni LTT weld metals.Accordingly,both LTT weld metals were tempered at 300?for 2 h,and 620?for 0.5,1,2 and 4 h,respectively.And then the effect of five different heat treatments on microstructure and mechanical properties of LTT weld metal was studied;the relationship between the content of reversed austenite and carbides and the mechanical properties of LTT weld metals was analyzed;and the reversion mechanism of reversed austenite was dissected.The experimental results indicated that the microstructures of both as-welded LTT weld metals were quenched martensite and retained austenite,but 13Cr-10Ni LTT weld microstructure contained more retained austenite up to 20.2%.Concerning with microstructure and mechanical properties,the difference between two types of LTT weld metals was significant when LTT weld metals were tempered at different temperature.When both LTT weld metals were tempered at 300°C for 2 hours,there were M₃C?0.6%?and M₂₃C₆?0.8%?carbides precipitated for 13Cr-10Ni and 12Cr-6Ni LTT weld metals,respectively.Meanwhile,the mechanical properties of both LTT weld metals had been improved a little under such heat treatment.In addition,when the tempering temperature rose to620°C,it was found that reversed austenite was formed in both LTT weld metals,and the content of carbides was on the rise as the tempering temperature increased.However,the mechanical properties of 13Cr-10Ni LTT weld metal after tempering at620°C for 2 hours,was lower than that after tempering at 300°C for the same time in that more carbides?6.3%?precipitated at elevated temperature.Conversely,the toughness of 12Cr-6Ni LTT deposited metal dramatically improved after tempering at620°C for 2 hours,but its obdurability was unsatisfactory.In terms of microstructures and mechanical properties,the difference between two types of LTT weld metals was evident when LTT weld metals were tempered at620?with tempering time varied.To begin with,the content of carbides in13Cr-10Ni LTT weld metal increased gradually during 0.5-2 hours tempering,resulting in a decrease in the thermal stability of reversed austenite;and therefore the content of the reverse austenite was reduced.However,after 4 hours tempering,the carbide content became stabilized,and the volume fraction of reverse austenite?18.5%?rebounded.As regard the 12Cr-6Ni LTT weld metal,the content of reversed austenite,after 1 hour tempering,increased dramatically,while continuous prolonging of tempering time had little impact on the content of the reversed austenite?7.1%?.On the contrary,the amount of carbides in the 12Cr-6Ni LTT weld metal kept an upward tendency with the tempering time increased.Combined with mechanical properties,the change trend of impact toughness of 13Cr-10Ni LTT weld metal decreased firstly,but then increased later after 4 hours tempering,and it had the best obdurability after4 hours tempering.When it comes to the 12Cr-6Ni LTT weld metal,it enjoyed the perfect obdurability after 1 hour tempering.In addition,the reversion mechanism of reversed austenite in both LTT weld metals can be explained by diffusion theory.And the diffusion of Ni element played a key role in the nucleation and growth of reversed austenite.What's more,it had been shown that the reversed austenite precipitated at the boundaries and interiors of lath tempered martensite and around the carbides.