Comparative Studies On Microstructures And Properties Of 12Cr1MoVG Small-diameter Medium-thick-wall Steel Tube Welded Joints With/without Post Weld Heat Treatment

12Cr1MoVG steel tube components are widely used in on-serving and on-constructing thermal power generation units in our country.Currently,post-weld heat treatment(PWHT)of the steel tubular weld joints is required,according to the technical specifications of the power supply construction industry.Due to heavy workload,low production efficiency and poor operation environment,power construction enterprises strongly request abolition of the operation.If so,how is the structure,performance and safety and reliability of the welded joint changed.No one knows it clearly.Therefore,in-depth research on this issue is necessary to provide a scientific guidance to the engineers to make new welding specifications in future.In this paper,12Cr1 MoVG small-diameter medium-thick-wall steel tube welded joints with or without PWHT were taken as the research objects to study the different characteristics of microstructures,mechanical properties at room and high temperature and residual stress distribution of the welded joints.The Goldenberg model was used to predict the durability and reliability of 12Cr1 MoVG steel tube welded joints.MSC.Marc finite element modeling simulation software and birth-death unit method were used to simulate the residual stress field in the welded joints.The simulating results were then compared with the experimentally tested axial residual stresses in the inner/outer walls of the welded joints.PWHT cannot cause a significant change in microstructure of 12Cr1 MoVG steel tube welded joints,but leads to the dissolution of granular pearlites and the precipitation of carbides,resulting in the decrease of yield strength and tensile strength of the welded joints and hardness of the weld seam,and also the increase of impact toughness and elongation of the welded joints and hardness of the base metal and HAZ.The heat treatment also causes the circumferential residual stress in the weld and HAZ changed from the tensile state to the compressive state,meanwhile,the stress is obviously reduced.Finite element simulation shows that the residual stress in the outer wall of the weld seam and near area of the welded joints is compressive,and the variation of residual stress in the base metal is smaller than that in the weld seam.The residual stress in the inner wall of the weld seam and the HAZ is compressive,but the residual stress of the base metal is tensile.These simulation results fit very well with the test results of the axial residual stress distribution in the outer/inner walls of the 12Cr1 MoVG steeltube welded joints.With the increase of the test temperature,the high-temperature tensile strengths/elongations of the welded joints with/without PWHT decrease/increase.Meanwhile,the stress tri-axiality of the welded joints is also increased,resulting in that fracture of the welded joints with/without PWHT gradually changes from the shear mode to the normal mode.The lower limits of the creep rupture strengths of the welded joints with/without PWHT at 510°C were derived to be 61 MPa and 86 MPa,respectively,which were larger than the rated vapor pressure of a power generation boiler(30.35 MPa).Although the 12Cr1 MoVG welded joints with PWHT have lower tensile properties,they still own higher long-term service reliability due to their higher creep rupture strength.