| In recent years,the dissimilar welding of ferritic steel/austenitic steel,which is an available connection method for steel structures meeting both performance and economy,has been applied widely to industrial domains such as petrochemical engineering,nuclear power and offshore engineering.So far,one of crucial obstacles for dissimilar welding needing to be overcome is that the chemical compositions in weld metal and heat affecting zone are different so that the post-weld heat treatment is difficult to improve properties of the two zones simultaneously.Aiming at overcoming the obstacle,the dissimilar welding of low activation ferrite/martensite steel(the indigenous CLAM steel)and austenitic stainless steel(316L)was realized by electron beam autogenous welding and electron beam welding with fillers.Through improving on welding techniques and post-weld heat treatment procedures,the mechanical properties of both weld metal and heat affecting zone were optimized.An in situ observation on formation and evolution of microstructures during welding and post-weld heat treatment has been carried out.The following results has been obtained.(1)The K-S orientation relationship and the irrational orientation relationship between parent phase ? and product phase ? can simultaneously occur in the same?-ferrite grain in the weld metal of dissimilar stainless welding.A partial recrystallization mechanism involving synthetic action of strong thermal activation and rapid cooling has been put forward to explain this phenomenon.The further observation showed that ? phase will transform into complete lath-like martensite during subsequent ??? transformation when the K-S orientation relationship occurs between ? phase and y phase,while y phase tends to transform partially into polygonal a phase during subsequent ??? transformation when the irrational orientation relationship occurs between ? phase and ? phase.The latter phenomenon can be used to reduce the hardening tendency of weld metal.(2)There is no heat affected zone in the austenite side of dissimilar welding joint,while heat affected zone occurs in the ferrite side.The hardness variation of heat affected zone and weld metal with temperature is different during post-weld heat treatment.The hardness of heat affected zone decreases with increased tempering temperature.Meanwhile,the hardness of weld metal arrives at the minimum when tempered at 620?.Tempered at temperatures below or above 620?,the hardness increases gradually with decreased or increased temperatures.A two-step heat treatment including critical aging(680?/4h)plus critical tempering(600?/4h)is put forward to improve mechanical properties of both heat affected zone and weld metal.It was found that the critical aging as the first step treatment reduced the hardness of heat affected zone in CLAM side to the most favored value,while the critical tempering as the second step treatment resulted in more remained austenite and critical ferrite contained in weld metal.Meanwhile,heat affected zone in CLAM side was not affected by the second treatment.(3)When welding with fillers,it was found that composition fluctuation in weld metal with addition of 310S stainless steel is less than that in weld metal with addition of pure Ni,while the layered microstructures occur mainly in weld metal with addition of 310S stainless steel.A mechanism involving influence of Ni on transformation paths of weld metal was put forward to explain the abnormal phenomenon about inconsistence between composition fluctuation and microstructure fluctuation.A heat treatment scheme consisting of high temperature normalizing plus tempering was proposed,which reduced effectually the quenching hardening of heat affected zone in CLAM side and the sensibility of layered microstructure in weld metal to thermal disturbance.It was found that the impact toughness of weld metal with addition of pure Ni is even lower than that of weld metal through autogenous welding,due to lacking of available grain boundaries and TRIP effect. |
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