The Research Of Gas Metal Arc Welding Of High Nitrogen Austenitic Steel And High Strength Martensitic Steel

The welding of austenitic stainless steels and martensitic high-strength steels is often used for the manufacture of many large and complex structures.The main body is austenitic stainless steel with good toughness.The complex structural parts,connecting parts or parts with strength requirements are made of martensitic high strength.Due to the large differences in the physical and chemical properties such as the thermal expansion coefficient and thermal conductivity of these two kinds of steels,the welding stress easily occurs during welding,in addition,the martensitic steel has a higher crack sensitivity so that the welding materials often choose austenitic stainless steel welding wire whose has better toughness.The high-nitrogen austenitic stainless steel used in this project is a new type of steel that has been rapidly developed in recent years.Its strength,corrosion resistance,biocompatibility,and impact resistance exceed the traditional austenitic stainless steel that use nickel as the main austenite element.However,high nitrogen steels are prone to accumulation and escaping of nitrogen during the welding process.It is necessary to properly add nitrogen to the welds.This is the main research content of this paper.Firstly,three flux cored wires with different nitrogen contents were developed on the basis of austenite wire matched with the nitrogen-oxygen-argon protective gas and the effects of nitrogen addition by these two nitrogen addition methods were studied.Focus on the analysis of the effect of nitrogen addition on the microstructure and mechanical properties of welded joints.It was found that the increase of nitrogen content in the weld can improve the tensile strength of the welded joints,but meanwhile the content of pores in the welds increases significantly so that toughness and ductility decreased,and it has also been proved that the nitrogen-containing protective gas is not suitable for the welding of the high-nitrogen steel and high-strength steel dissimilar materials and that a large number of welding spatter and other problems and development in the welding process when using the flux-cored wire whose composition is relatively rough.On the basis of previous research on nitrogen addition in welds,three different nitrogen content solid wires were developed designing by thermodynamic calculation software and the thermodynamic calculation results of the parent metal and welding wire were analyzed to obtain the sensitive temperature range during welding: 550°C-900 °C,so the welding process must strictly control the inter-layer temperature.In order to facilitate the comparison with high-nitrogen welding wire and from different welding consumables to characterize the weldability of high-nitrogen steel-high-strength steel dissimilar materials,also selected martensitic wire,dual-phase steel welding wire and non-nitrogen containing austenitic steel welding wire and characterized the dissimilar joints from several aspects such as the use performance of welding wire,welding porosity,welding cracks,microstructure and mechanical properties.A kind of welding wire with nitrogen with better performance is also selected to explored the asymmetric angle groove.It is suggested that the groove angle of high nitrogen steel is slightly smaller than that of high strength steel in future groove design.Analysis of weldability of different kinds of welding wire,nitrogen-free welding wire has problems of nitrogen element escaping,nitrogen-containing welding wire has a tendency of large blowholes,so the two kinds of welding wires are combined and welded by single-arc/double-wire method.When the composition of the welding wire is greatly different,a lump-like structure is formed in the weld.Compared with the joint with the same amount of nitrogen,the porosity completely disappears,the impact energy of the welded joint increases by 62.7%,and the elongation after fracture increases by 81.1%.However,due to the presence of magnetic difference,there was a phenomenon of poor fusion on the high nitrogen steel side of the backing weld and the problem was hoped to be improved by using a protective gas containing 40% helium,but no obvious effect was obtained.In the future,this problem will be solved by using asymmetrical shaped groove or magnetron arc.