Effect Of Low Transformation Temperature (LTT) Wire On Microstructure And Properties Of CLAM Steel Welded Joints

China low activation martensitic?CLAM?steel with a low coefficient of thermal expansion,good thermal conductivity and high corrosion resistance is the preferred choice of test blanket module?TBM?for China's participation in the International Thermonuclear Experimental Reactor?ITER?test program.However,weld is also a necessary process in the preparation of TBM.TBM is often subjected to high temperature alternating load.If the interior of TBM possess large residual stresses superimposed with the alternating load,the safe employment of CLAM welded joint is seriously threated.Low transformation temperature?LTT?materials can decrease the martensitic transformation temperature?Ms?,which plays a role in reducing residual stress by means of low temperature martensitic transformation expansion to offset the thermal contraction during the post-weld cooling process.In this paper,the effect of alloying elements on the martensitic transformation temperature is analyzed.LTT welding wires are prepared in combination with Schaeffler phase diagram,which are used to reduce the residual stress of CLAM welded joints.LTT and ER90S-G welding wires are used to conduct comparative welding tests on CLAM steel,microstructures,conventional mechanics,phase transition temperatures and difference in weld distortions of two welded joints are compared by the use of optical microscope?OM?,scanning electron microscope?SEM?,X-ray diffraction?XRD?and other analytical testing techniques.In addition,the high-temperature creep test of LTT joints is carried out,the creep mechanism and damage mechanism are discussed and long-term fracture strength is predicted.The main results are as follows:?1?The weld mainly consists of lath martensite and a small amount of retained austenite,however,coarse acicular martensite is observed on ER90S-G weld and Cr,Ni and other alloying elements near LTT joint fusion line occur diffusion.The content of retained austenite is 7.6%in the LTT weld,ER90S-G weld consists of mainly martensite.The microhardness of LTT weld is slightly lower than that of ER90S-G weld,the impact toughness of LTT joint is better than that of ER110S-G joint and tensile strengths of them are comparable.?2?The Ms of LTT weld is 237°C,the linear expansion of LTT weld is 0.43%during the phase transformation period.The Ms and Mf of ER90S-G weld are 519°C and 400°C,respectively,the linear expansion of ER90S-G weld is 0.31%during the phase transformation period,thermal contraction of joint region continues after the phase transformation.The surfaces of ER90S-G joint and LTT joint mainly exist residual tensile stress and residual compressive stress,respectively,the value of which are 283.2 MPa and-158.6 MPa,respectively.LTT welding wires have obviously positive effect on reducing welding residual tensile stress at the joint.The angular distortion of LTT butt welded plate is 0.0417 rad,which is 45.5%of that of ER90S-G butt welded plate.?3?The high temperature creep of CLAM LTT joint in 550°C consists three typical creep stages.The joint stress exponent is 12.3,which shows creep mechanism is the dislocation-controlled.The SEM and TEM analysis of joint creep fractures indicate that typical ductile fracture is characterized with applied stresses under 550°C.The part of subgrain boundaries and dislocation on the matrix disappear and the M₂₃C₆ second-phase particles with the size of 50-100nm precipitate at grain boundaries of prior austenite,which lead to high temperature creep resistance of joint decrease.According to the double logarithm isotherm extrapolation and Larson-Miller parameter methods,the fracture strength of CLAM LLT joints at service conditions of550°C/10⁵ h is predicted to be 156.2 MPa and 155.6 MPa,respectively.