Microstructure And Mechanical Properties Of Heat-affected Zone(HAZ) Of Repeatedly Welded AISI 304N Austenitic Stainless Steel By Gleeble Simulator

During the 40-60 years of service life,some equipments of nuclear power plants need to be repaired and maintained in situ for many times due to the stress crossion cracking,aging of materials or other forms of failure.The repeated welding,that is,multiple in-situ welding has impacts on the microstructure and mechanical properties of heat-affected zone(HAZ).It is difficult to predict the microstructure evolution of the heat-affected zone of the repeated welding with multiple passes on austenitic stainless steel because of non-equilibrium phase transition.Moreover,it was almost impossible to characterize microstructure and mechanical properties accurately using the general methods because the width of the repeatedly welded HAZ was just one to three millimeters.The Gleeble weld-simulator is successfully employed to accurately control thermal cycles based on the heat input to easily generate a large volume heat-affected zone.This method provides a possibility for the further study of microstructure and mechanical properties on the heat-affected zone of the repeated welding.In this study,AISI 304N stainless steel was prepared as-welded and one to five repeatedly welding using automatic gas tungsten arc welding(GTAW)and Gleeble weld-simulator respectively,and then examined the effect of the repeated welding on the the microstructure and mechanical properties of heat-affected zone.It has become important to study the microstructure and mechanical properties of the HAZ of the repeated welding for solving the engineering and technical problems.Using the automatic GTAW with rigidly constrained test plates,the microstructure and mechanical properties of the heat-affected zone of the repeatedly welded joints with multiple passes were studied.Microstructure analysis showed that,with the repeatedly welded increasing,the width of the HAZ increased from about one to three millimeters.The microstructure of the HAZ was mainly composed of austenite matrix and 8-ferrite,and the austenitic grain size of 18.9 ?m-23.7 ?m with the 5-ferrite contents of 0.36 vol.%-1.9 vol.%.With the effect of welding heat input and welding deformation,the local misorientation of the HAZ increased from 2.05°of as-welded to 4.75° of repeatedly welding five times,and the preferred orientation changed from<001>to<111>.The microhardness of the welded metal,heat-affected zone and base metal showed that the microhardness near the fusion line of the HAZ was the lowest.With the increasing of the repeated welding,the elongations of the HAZ decresed,while the yiled strength(YS)increased.The ultimate tensile strength(UTS)increased firstly from the 555 MPa for as-welded joint to the maximum value of 582 MPa for the repeatedly welding one time,and then decreased to the 535 MPa for the repeatedly welding five times.With the increasing of the repeated welding,the yield strength of the HAZ increaseed,while the ultimate strength and elongation decreased.The highest yield strength of 340 MPa were obtained for repeatedly welding five times.With the increasing of the repeated welding,the room impact energy decreased.Using the above automatic welding process,the heat cycles in the HAZ of the welded joint root pass were measured in detail by using the 60 Hz high-frequency acquisition and high-precise thermocouples temperature measurement system.The peak temperatures of 13 140C?1290 0C?945 0C?843 0C?768 ? were recorded for one to five passes,respectively.Based on the heat cycles,Gleeble weld-simulator was used to prepare the HAZ samples of as-welded and one to five times repeated welding,and the microstructure and mechanical properties of the HAZ of repeated welding were studied in-suit.Microstructure analysis showed that,the the austenite matrix grain size had a slight variation of 41.4 ?m-47.3?m.The ?-ferrite inclusions had a lathy structure located in the interface of austenite,its content increased firstly,then decreased.The variation of the 8-ferrite content was caused by the changes of Cr/Ni.The frequencies of the coincident site lattice boundary and high angle boundary changed with the increasing of the repeated welding.The mechanical property tests showed that,with the increasing of the repeated welding,a slight variation in the microhardness of 160 HV3N-170 HV3N was observed for the as-welded and repeated welding.The room tensile strength containing ultimate tensile strength and yield strength firstly increased to a maximum value of 670 MPa and 245 MPa for the repeatedly welded 2 times,and then decreased to a minimum valu of 525 MPa and 190 MPa for the repeatedly welded 4 times.At 260 ?,the ultimate tensile strength and yield strength are 460 MPa-475 MPa and 200 MPa-215 MPa,respectively.The room and 260 ? temperature tensile strength mainly depended on the contents of 8-ferrite and high angle boundary.The tensile strength decreased with the increasing of ?-ferrite contents,and increased with the increasing of the high angle boundaries.The impact energy of the HAZ of the repeated welding is 225 J-272 J at room temperature,which firstly decreased with the increasing of the repeated welding,and then increased.The impact energy depended on the grain size and the ?-ferrite contents.As the grain size increased and the ?-ferrite contents decreased,the impact energy increased.The Gleeble mechanical property tests showed that AISI 304N austenitic stainless steel had the highest tensile strength and impact energy at room temperature in the heat-affected zone after two times of the repeated welding.After three times of the repeated welding,the tensile strength and impact energy can still meet the requirements of ASME standard.