Research On Stabilizing Heat Treatment Failure Of TP347's High Pressure Thick Wall Pipe After Welding

TP347(Chinese brand 06Cr18Ni11Nb),is to add Nb element in 304 stainless steel to increase its resistance to intergranular corrosion and high temperature.It is an austenitic stainless acid resistant steel which widely used in aviation,petrochemical,food,paper and other industries.Now it is still controversial at home and abroad whether austenite stainless steel materials need stabilizing heat treatment after welding.Cracks occurred after stabilizing heat treatment in the weld joint of TP347 pipe of a 2 million tons per year residual hydrogenation unit in a petrochemical plant,which had a serious influence on the security of the whole system.In this paper,the reason of reheat cracks was found by comparing the welding joint between the failure parts and the normal parts with finite element analysis and experimental study,and corresponding improving measures were put forward.In this paper,the development of austenitic stainless steel is introduced and the research of stabilizing heat treatment of austenitic stainless steel is summarized.The welded joint was obtained through the welding experiment of TP347 stainless steel thick wall pipe under the same welding process.A series of basic tests and microscopic tests were carried out on the welded joints before and after 900?/3h heat treatment,such as chemical composition analysis,bend test,intergranular corrosion test,and electron microscope test.The results show that the stable heat treatment at900 /3h does not cause reheat cracks in the thick wall pipes.However,the micro defects in the weld will expand and develop into reheat cracks.Then,the article,in which the weld of austenitic stainless steel thick tube was simulated by ANSYS,introduces the processes using the APDL language for the simulations of circumferential pipe weld and equivalent stress.temperature field and equivalent stress field of weld were acquired,and the obtained results were analyzed.The results showed that,the temperature field on the pipe is axially symmetry of circumferential weld,and the welding residual stress is higher near weld bond of circumferential weld,which easily contributes to stress concentration.Finally,a series of basic tests were carried out on the welded joints of the failure parts,such as chemical composition,mechanical properties,impact test,electron microscope test and energy spectrum analysis.The results show that the welding jointof the failed specimen has a severe brittleness tendency,and there are low melting point metal compounds and metal oxide inclusions in the welded joint.These are the reasons for the failure of the heat treatment of the thick wall pipeline.In this paper,the studies show that the welding defects in high pressure thick wall pipes can be expanded to microcracks by stabilizing heat treatment.It is suggested that the heat treatment should be cancelled.