A Study On Stress Field Simulation,Aging Microstructure And Mechanical Properties Of Laser Arc Hybrid Welded T91 Heat-resistant Steel

T91 heat-resistant steel has been widely used in power plant boilers.Generally,TIG welding is used to make the root pass,and SMAW welding is used to make the filling passes.Compared with the traditional arc welding method,the laser arc hybrid welding has a lot of advantages,such as high energy density,low heat input,small heat affected zone,small welding residual stress and deformation,and excellent mechanical properties of welding joint.Therefore,it is of great significance to develop laser arc hybrid welding technology with high efficiency and excellence for T91 heat-resistant steel.According to the characteristics of laser arc hybrid welding double heat source,a combinated heat source model with cone body heat source and double ellipsoid heat source was developed.The hybrid welding temperature field and stress field of different preheating temperature was simulated.The results of temperature field simulation showed that the welding heat cycle will change with the increase of preheating temperature,especially the cooling rate is gradually reduced,which will reduce the hardening tendency of welding joints.Thus it is beneficial to reduce the crack tendency and improve the performance.The stress field simulation results showed that the stress and distribution of the welded joint can be improved by preheating.With the increase of preheating temperature,the maximum tensile stress in the weld gradually reduced and the width of tensile stress distribution in the weld gradually decreased,which is beneficial to reduce the formation of the crack.The laser hybrid welding process parameters of T91 heat-resistant steel were optimized.The welding parameters of laser arc hybrid welded T91 heat resistant steel are as follows:laser power was 2 kW,arc power was 3.8 kW,welding speed was0.8 m/min,and the preheating temperature was 200?.The T91 welding joint obtained had good shape,no defects,suitable microstructure and good comprehensive properties.The specimens with preheating temperature of 200?were stretched at room temperature,600 and 900?.The fracture positions were all at the base metal.The number and size of precipitated phase of the base metal zone for the high temperature tensile were larger than that of the weld zone.Moreover,the precipitation phase distribution of the base metal zone was not uniform.The high temperature tensile test results at 900?showed that because the dynamic recrystallization occurs,the grain size of the base metal zone was smaller,which will cause the recrystallization softening phenomenon.However,the deformation of the weld zone was very small and dynamic recrystallization did not occur.Therefore,all these will cause that the tensile properties of the base metal are poorer than those of the weld zone.The microstructure and properties of welded joints at different aging time under high temperature were analyzed by means of OM,SEM,EDS,TEM,hardness and tensile test.It was found that the grain size gradually became larger with the increase of aging time.The lath martensite gradually became wide and fused to form massive martensite.Then the martensite was broken and decomposed into a new substructure,and some martensite transformed into lamellar ferrite.With the increase of aging time,the precipitates gradually gathered the boundaries of grain boundaries and martensite lath and began to coarsen.The number of the precipitates firstly increased and then decreased.The hardness of welded joint decreased.The tensile strength of the welded joint guadually decreased while the fracture locations were all at the base metal.The precipitated phases formed by aging treatment are mainly composed of three precipitates,namely,M₂₃C₆ phase,M₇C₃ phase and Laves phase.