Microstructure And Properties Of LBW And GF-PAW Welded Joints Of Super Duplex Stainless Steel 2507

Super duplex stainless steel 2507 is a kind of stainless steel with excellent mechanical properties and corrosion resistance.And therefore,super duplex stainless steel 2507 has a wide range of applications in marine engineering,chemical industry,energy and other fields.A reasonable ratio of ferrite content to austenite content determines the performance of super duplex stainless steel 2507.Welding manufacturing is an important manufacturing process of super duplex stainless steel 2507.However,welding will lead to an imbalance in the ratio of the content of two phases in the welded joint,which will affect the performance of super duplex stainless steel 2507 welded joints.Regulating the ratio of the content of two phases in the welded joint is the key to obtaining a high-quality welded joint of super duplex stainless steel 2507.For the welding of super duplex stainless steel 2507,domestic and foreign welding researchers have carried out some research and achieved some results.However,during high energy density welding(laser welding,plasma arc welding)on thin plate super duplex stainless steel 2507,how to improve the nitrogen content and austenite content in the weld metal effectively? What is the transition mechanism of nitrogen to the weld metal? These key issues need to be explored urgently.The research on high energy density welding of super duplex stainless steel 2507 will enrich the welding theory of super duplex stainless steel 2507 and guide the welding of super duplex stainless steel 2507,which has important theoretical significance and engineering value.In this study,laser beam welding and gas focusing plasma arc welding are used to weld super duplex stainless steel 2507 to obtain the welded joint of super duplex stainless steel2507.The metallographic structure analysis,EBSD analysis and TEM analysis are carried out on welded joints.EPMA spot scanning is carried out on the micro-area of the welded joints.The nitrogen content of the weld metal is analyzed by oxygen-nitrogen-hydrogen analyzer.The microhardness test and the electrochemical corrosion test are carried out on the welded joints,and the pits after electrochemical corrosion test are observed by SEM.In this paper,the microstructure and properties of laser beam welded joints shielded by pure argon and shielded by combined argon-nitrogen are comparatively studied,while the effect of nitrogen proportion change on the microstructure and properties of laser beam welded joints is analyzed.The microstructure and properties of welded joints of conventional plasma arc welding without nitrogen addition and gas focusing plasma arc welding with nitrogen addition through focusing gas channel are compared and studied,while the effect of nitrogen flow rate on the microstructure and properties of welded joints of gas focusing plasma arc welding is analyzed.The morphology of welded joints and nitrogen content in weld metal of laser beam welding and gas focusing plasma arc welding are compared and analyzed.And the transition mechanism of nitrogen to weld metal in laser beam welding and gas focusing plasma arc welding is preliminarily explored.The main conclusions of this study are as follows.(1)Compared with laser beam welding without nitrogen addition,laser beam welding with 25% nitrogen addition(nitrogen flow rate of 5 L/min),the austenite content in the weld metal increases by 10.7%,and the nitrogen content in the weld metal increases by 0.012wt%,and the orientation of two phases in the weld metal,microhardness of welded joints,and corrosion resistance of welded joints do not change significantly.(2)During laser beam welding,in the range of nitrogen flow rate of 5-20 L/min,with the increase of nitrogen proportion,the content of austenite in the weld metal increases firstly and then decreases,and the orientation of two phases in the weld metal remains unchanged,and there is no significant change in nitrogen content of weld metal,microhardness of welded joints,and corrosion resistance of welded joints.When the nitrogen proportion is 75%(nitrogen flow rate of 15 L/min),the austenite content of the weld metal is the highest,reaching 40.8%.(3)Compared with conventional plasma arc welding without nitrogen addition,gas focusing plasma arc welding with 0.11 L/min nitrogen addition,the austenite content in the weld metal increases by 23.0%,and the morphology of austenite changes,and Widmanst(?)tten austenite appears,while the grain boundary austenite and intragranular austenite also exist,and the orientation of two phases in the weld metal remains unchanged,and the nitrogen content of the weld metal increases by 0.033 wt%,and the microhardness of austenite in weld metal increases,and the microhardness of ferrite in weld metal reduces,and the corrosion resistance of welded joint improves.(4)During gas focusing plasma arc welding,within the range of nitrogen flow rate of0.07-0.18 L/min,with the increase of nitrogen flow rate,the austenite content in the weld metal increases,and the orientation of two phases in the weld metal remains unchanged,and the nitrogen content of austenite in the weld metal increases,and the microhardness of austenite in the weld metal increases,and the microhardness of ferrite in the weld metal decreases firstly and then increases,and the corrosion resistance of the welded joint increases.When the nitrogen flow rate is 0.18 L/min,the austenite content of the weld is 57.6%.(5)During laser beam welding,nitrogen has limited effects on the increase of austenite content in weld metal and the improvement of performance of welded joint.Even shielded by 100% nitrogen(nitrogen flow rate of 20 L / min),the effect of nitrogen in laser beam welding is still not significant.During gas focusing plasma arc welding,adding a small amount of nitrogen(nitrogen flow rate of 0.11 L/min)through focusing gas channel can significantly increase the content of austenite in the upper surface of the weld metal and improve the performance of the welded joint.(6)Compared with laser beam welding,during gas focusing plasma arc welding,the dissolution rate of nitrogen in the molten pool is larger,and the width of the rising zone of nitrogen content in the molten pool is wider,while the escape rate of nitrogen in the molten pool,the width of the falling zone of nitrogen content at the melting front,and the width of the falling zone of nitrogen content at the solidification front are basically unchanged.It is easier for gas focusing plasma arc welding to increase the content of austenite in the weld metal,the nitrogen content of the weld metal and the nitrogen content of austenite in the weld metal.