Microstructure Evolution And Properties Analysis Of S32101 Duplex Stainless Steel Underwater Flux-cored Arc Welded Joints

Underwater welding is an important joining technology for the construction and maintenance of marine engineering,nuclear power engineering and ship engineering structures.Because flux-cored wires have the characteristics of high cladding efficiency and high quality of welded joints,underwater flux-cored wire arc welding（FCAW）is a promising underwater welding method.Meanwhile,duplex stainless steels are increasingly used as materials for structures such as oil and gas transportation,shipbuilding and nuclear power construction due to their excellent mechanical properties and corrosion resistance.In particular,in recent years,the development of economical duplex stainless steel has successfully improved the cost performance of duplex stainless steel and greatly promoted the application of duplex stainless steel.The complexity of underwater environment will inevitably increases the difficulty of underwater welding of duplex stainless steel.The welding process,microstructure and performance of the welded joints will be affected.However,detailed research on the microstructure and properties of duplex stainless steel underwater welded joints is still far away from meeting the needs of its application.In this context,this study uses the economical duplex stainless steel S32101 as tested material,and uses the high pressure chamber to simulate different water depths,and carries out dry and local dry underwater welding experiments using FCAW.This work has studied the welding parameters,microstructure evolution,mechanical properties and local corrosion resistance of S32101 duplex stainless steel dry multi-pass and local dry single-pass underwater welded joints in the range of 0.75 MPa（equivalent to 75 m water depth）.The main contributions are as follows:1)Study on the welding parameters,microstructure and mechanical properties of dry multi-pass welded joints.The increase in pressure reduced the heat input required for the root bead and increased the total number of welds required to fill the groove.The austenite content in the weld metal（WM）at 0.45 MPa was the highest,the austenite content in the WM at 0.75MPa was the second,and the WMs at 0.15 MPa and atmospheric pressure had the lowest austenite contents and the two were almost the same,which was mainly due to the different heat inputs.The austenite contents in the heat affected zones（HAZs）at each pressure were not much different,but smaller than the base metal（BM）.The tensile tests of the joints were all broken at BM,which was caused by the strengthening of WM and HAZ.The strength of WM was the highest at 0.45 MPa,followed by 0.75 MPa,and the lowest at atmospheric pressure and 0.15 MPa.The change law of hardness was the same as the change law of strength,and the change law of elongation was just opposite to the change law of strength.The impact toughness of WM at each pressure was less than BM.The absorbed energys of the WMs at 0.45 MPa and 0.75 MPa were higher,and the maximum was the WM at 0.75 MPa.The difference of impact toughness depended not only on the austenite content,but also on the N content.2)Study on the evolution behavior of secondary phase Cr₂N and secondary austeniteγ₂in various regions of dry multi-pass welded joints（including root WM,center WM,surface WM,HAZ and BM）and the responding local corrosion resistance.In the multi-pass welding process,the formation ofγ₂ in the reheating zone of WM and HAZ was not uniform,and the closer the reheating zone was to the following welding heat source,the moreγ₂ was formed.The more the austenite content was,the more balanced the two-phase structure was,the more it inhibited the formation of Cr₂N precipitation,which greatly reduced the formation of intragranularγ₂ during reheating.The localized corrosion was caused by the selective corrosion ofγ₂ and Cr₂N.3)Study on the influence of flux cored wires on the welding parameters,microstructure and mechanical properties of welded joints.At a pressure of 0.45 MPa,different flux-cored wires were used for welding,and the microstructure and mechanical properties of the joints were investigated.For welded joints with a large number of weld passes,the total heat input was higher and the inclusion content in the WM was also higher.The higher heat input and more inclusions promoted the formation of austenite in the WM.However,more inclusions not only damaged the plasticity and toughness of the WM,but also promoted more fine IGA and more ferrite-austenite special phase boundaries in the WM,which further reduced the plasticity and toughness of the WM.On the other hand,the alloying elements of the two WMs differed greatly in contents,which was the main reason for the large difference in their strength and hardness.4)Study on the welding parameters,weld formation,microstructure and mechanical properties of the local dry single-pass welded WMs.As the water depth increased from 0.1 m to 75 m,the austenite content in the WM decreased first and then increased and then decreased.The content ofΣ3 austenite twin grain boundary and the proportion of recrystallized ferrite and austenite grains also showed similar trends;however,the trend of K-S ferrite-austenite phase boundaries in the WMs was opposite.As the water depth increased,the elongation and impact energy of the WM decreased first,then rose and then fell,but the change in the strength and hardness was just the opposite.The difference in the microstructure and mechanical properties of the WMs when the water depth increasing from 15 m to 45 m was greater than that between 45 m and 75 m water depths.The WM at 45 m water depth had better plasticity and toughness properties.The results of this paper indicate that the influence of pressure（water depth）on the welding process,microstructure evolution and mechanical performance and local corrosion resistance of S32101 dry and local dry underwater welded joints can not be ignored.Meanwhile,during high pressure dry welding environment,the chemical composition of the flux-cored wire also affects the welding process,microstructure and mechanical properties.These results promote the application of new economical duplex stainless steel in the ocean,nuclear power,etc.,and also provide accumulation for the development of deep water welding power sources,farther developing the DSS underwater welding technology.