The Research Of Adding Nitrogen To Welded Metal And Microstructure &properties Of High Nitrogen Steel MIG Welded Joint

As for the solid solution of nitrogen element, high nitrogen austenitic stainless steel(HNS) has outstanding mechanical properties such as high tensile strength, good corrosion resistance and work hardening ability. At the same time, nitrogen can be used as an austenitizing elements, replacing Ni element to obtain austenitic steel. High nitrogen steel would be widely applied in many fields if it has well weldability. However, current studies state that the welded metal and heat-affected zone(HAZ) have problems of N element losing, which would damage its mechanical properties.So solving these problems and increasing welded joint mechanical properties is the key to widespread application of HNS.Aimed at solving the above problems, this paper utilize Mn N powder and optimize shielding gas to add N element in MIG welded metal, and investigate the microstructure as well as the mechanical properties in different conditions. one of the base metals is 3mm thickness and the other is 16 mm thickness, the former has been solution heat treated and the later has been rolled. The welding wire used in this paper is called HCr20Ni10Mn7 Mo.The results show that the welding wire in as-welded(AW) station possesses of organization consisting of austenite and ferrite, under the normal process. Adding appropriate amount of Mn N can obtain single austenitic organization, and the weight percent of N can reach 0.3%, the XRD test shows that no nitride is generated and the N exists as solid solution station. Use this MIG method to conduct the weld progress of 3mm thickness high nitrogen steel, which had 30°groove and 1mm blunt edge, the results show that the tensile strength of welded joint had a promotion of 80 MPa, and mean while kept good plastic toughness.The results of optimize the sheilding gas suggests that adding a small amount of oxidizing gas in protective gas, having an important influence on the wt(N)%, the content of nitrogen element in the deposited metal is obvious increase, but the mechanism of why oxidizing gas could increase N still needs further research; For 3mm thickness high nitrogen steel, the result shows that the nitrogen weight percent is more in the welded matal, with the nitrogen partial pressure of shielding gas higher, and the same as tensile strength of welded joint; For determination of nitrogen content of heat affected zone, show that the heat affected zone exists the phenomenon of nitrogen loss, but not serious, its mechanical performance organization was the main cause of coarsening, at the same time shows that adding nitrogen gas protection cannot stop the loss of heat affected zone and nitrogen.For those 16 mm rolling state HNS with double "V" groove, researched on heat affected zone after welding, the results showed that the sensitive temperature range for the nitrides precipitation residence time is shorter, does not appear Cr2 N, etc., but reply recrystallization, the disappear of rolling banded texture, form a hexagonal austenitic organization, at the same time a large number of annealing twin organizations; Impact test was carried out on the joint, the results show that the fusion zone for the joint mechanical properties of the weak link, the fracture analysis shows that existing in the fusion zone of nitrides precipitation.