Study On Welding Thermal Simulation And Post-weld Heat Treatment Of 1.25Cr-0.5Mo Steel Pressure Vessel Steel

1.25Cr-0.5Mo heat-resistant pressure vessels steel is widely used in thermal power plants and petrochemical industry due to its excellent performance.Welding and post-weld heat treatment(PWHT),which are invariably necessitated to manufacture such a large-scale equipments,not only affect the reliability of weldment,but also affect the service life of the large devices.During the service,the type ? cracking of the the heat-affected zone(HAZ)results in the premature failure of the 1.25Cr-0.5Mo welding joints.The soft zone formed during the PWHT is the type ? cracking sensitive zone.Therefore,it is of great significance to study the formation and evolution of the soft zone during the PWHT.In this study,the welding thermal simulation is used to amplify HAZ to locate the subzone accurately.Simulated welded HAZ(SW)and actual welded HAZ(AW)were obtained by welding simulation and shielded metal arc welding,respectively,under the same heat input.Then,SW and AW were subjected to PWHT at 690? for 6 hours.The microstructure and mechanical property of S W and AW before and after PWHT are compared.It is found that the width of each sub-zone of SW is 3.4 times than that of AW.The microstructure and hardness distribution characteristics of each sub-zone are identical.At the same time,both SW and AW show the soft zone where the hardness value is lower than base matel(BM)in the fine grained heat-affected zone(FGHAZ)near the intercritical heat-affected zone(ICHAZ)after PWHT at 690? for 6 hours.The precise location of the amplification and soft zone of HAZ is realized by welding thermal simulation.Moreover,SW is subjected to PWHT for 1 h,6 h,48 h and 480 h at 690?.It found that the soft zone first appears in FGHAZ close to ICHAZ.With the increase of holding time,the soft zone gradually widens.Combined the results of SEM and EPMA,the microstructure of corase grained HAZ(CGHAZ)and FGHAZ in SW is lath martensite,while ICHAZ is mainly composed of bainite.The composition of alloying elements is uniform in CGHAZ of SW,while carbide particles rich in Cr,Mo and residual austenite protrusions exist in FGHAZ of SW.During PWHT,carbon atom diffusion in FGHAZ formed carbide with Cr,Mo element in the residual austenite and martensite recrystallization,forming some coarse recrystallization grains with low dislocation density and poor alloying elements,resulting in the excessive decrease of hardness and the formation of soft zone.