Study On Post Welding Heat Treatments And Reheat Cracking Of TP347 Welded Joints

TP347 is a stable austenitic stainless steel containing Nb.It can be selected for high temperature services due to its high creep and intergranular corrosion resistance and can better satisfy with the requirements for high parameter and large-scale development of modern industry such as hydrogenation plants.However,reheat cracks are easy to appear on the welded joint of TP347 steel during post-weld heat treatment or under high-temperature service,which becomes a major risk threatening the longterm safe operation of hydrogenation plant.At present,both in China and abroad,whether to apply post-weld heat treatment on TP347 welded joints is still in disputing;A clear understanding of the generation mechanism of reheat cracks in the welded joints of TP347 steel has not been formed.Based on the actual welded joint of TP347 and the simulated microstructure of the heat affected zone,combined with macro performance characterization and micro mechanism analysis,this paper describes in details of the influence of stabilizing heat treatment on the microstructure and properties of welded joints on TP347,studies the relationship among welding process,heat treatment standard conditions--microstructure--reheat crack sensitivity and formation mechanism.Through slow tension test and dead load test,the sensitivity and formation mechanism of reheat cracks are evaluated and analyzed,and the preventive measures of reheat crack are finally put forwards.The main contents include the following five aspects:1.Effect of solution and stabilizing heat treatment on microstructure of welded joints on TP347 steelThe changes of microstructure in different areas of welded joints on TP347 steel after solution heat treatment at 1050℃ and stabilizing heat treatment at 900℃ were analyzed to study the necessity of post-weld heat treatment of welded joints.The results show: the solution heat treatment and stabilizing heat treatment can minimize the possibility of sensitization or intergranular corrosion,and therefore effectively improve the intergranular corrosion resistance of the materials.2.Effect of different welding process and heat treatment specifications on microstructure and properties of welded joints on TP347.The tensile,bending,impact,hardness,intergranular corrosion and other properties of the as welded and post heat treated welded joints of thick-wall and thinwall TP347 steel pipes were tested.The microstructure evolution of the welded joints was analyzed by metallographic microscope(OM),scanning electron microscope(SEM)and energy spectrum analysis(EDS),and the corresponding microstructure characterization related to these heat treatments was established.The purpose is to develop a set of optimal stabilizing heat treatment process,reduce reheat crack sensitivity and improve the possibility of the wide application of TP347 steel.The results show: the welded joints on thick-walled TP347 steel are prone to reheat cracks after stabilizing heat treatment,while the welded joints on thin-walled TP347 steel have no reheat cracks before and after stabilizing heat treatment;900 ℃ constant temperature for 3-4 hours plus air cool is the best heat treatment process to improve the intergranular corrosion resistance of TP347 steel pipe welded joints.3.Reheat crack sensitivity evaluation and formation mechanism of welded joints on TP347 steelThe elevated temperature slow tension test and elevated temperature dead load test were carried out on the actual tensile sample of welded joints on TP347 steel under different conditions.The results show: the section shrinkage is the lowest at 900℃,indicating that 900 ℃ is the sensitive temperature for reheat crack formation.The critical fracture stress of the reheat crack of the welded joints at 900℃ is 24 MPa,which is much lower than the elevated temperature yield strength of 80% of the base metal,indicating that the reheat crack of the welded joint is highly sensitive.The microstructure of the heat affected zone of TP347 steel was prepared by simulating the heat cycle of the heat affected zone of welding with Thermorestor-W heat confinement simulator,and the sensitivity of the reheat crack in the heat affected zone was evaluated by a series of elevated temperature and slow speed tension tests.The nucleation and propagation of reheat cracks were studied by microscopic characterization techniques,such as SEM,EDX,TEM and EBSD to reveal the mechanism of reheat cracks.The results show: the temperature range of higher reheat crack sensitivity is 700℃-800 ℃,and the reheat crack sensitivity is the highest at750 ℃;At the stabilization heat treatment temperature,the main reason for the reduction of creep plasticity in HAZ is the strengthening of the crystal caused by the precipitation of intragranular carbides and the weakening of the interface caused by the precipitation of grain boundary carbides,which is the micro mechanism of reheat cracking.4.Microstructure evolution and failure analysis of welded joint on TP347 steel in serviceThe microstructure analysis of welded joints on TP347 steel with cracks after8,520 hours’ service in a hydrogenation unit of an oil refinery was carried out by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The failed welded joints were compared with the prepared welded samples to determine the cause of failure of the welded joints.The results show: instability caused by carbides precipitation in the heat affected zone,plastic deformation and tensile residual stress caused by solidification shrinkage of the outer weld bead,are the cause for welded joint crack.5.Preventive measures of reheat crack of welding joints on TP347 steelBased on the comprehensive analysis of the above research results,the measures to prevent reheat cracking of welded joints of TP347 steel pipe are put forward from the aspects of optimizing pipeline structure design,strictly controlling raw materials,optimizing welding and heat treatment process,controlling welding and heat treatment process,optimizing pipe installation method,optimizing nondestructive testing method and promoting the application of automatic welding technology.The technical and economic benefits of all position automatic welding technology are compared and analyzed.