Research On High Nitrogen Austenitic Stainless Steel By DSAW Welding Robot

Comparing to the nickel chromium stainless steel, high nitrogen austenitic stainless steel not only saves plenty of nickel, but also has more excellent mechanical properties and corrosion resistance. As an important means of machining, The welding of high nitrogen austenitic stainless steel has important effects on the application of high nitrogen steel. But research shows that during the welding process, nitrogen loss and porosity in the joint and HAZ are great difficulties. Therefore, how to solve these problems has been the key to welding the materials.The purpose of this paper is to research the weldability of high nitrogen austenitic stainless steel by DSAW. By adjusting the welding current and the composition of Ar+N2 protection gas, the influence of the nitrogen partial pressure on the welding process, welding appearance and the nitrogen content in the weld are studied. In this paper, the methods to increase the nitrogen content of the weld metal are found, and research of the best welding process parameters has been completed.As the N2 ratio increases, the arc stability is reduced and splash begins to increase. After the N2 proportion reaches 40%, defects appears, and weld deformation is poor. At the same time, with the increase of N2 ratio in protection gas, the actual welding current decreases, and the width of seam grows.The higher of the N2 ratio in protection gas is, the more nitrogen pore will appear. When the N2 ratio is lower than 20%, there is no nitrogen pore in the weld. When N2 ratio reaches higher than 40%, more and more nitrogen pores can be found in the weld. The number of pores achieves the highest when the protection gas is pure N2.The N-content of the weld metal increases as the nitrogen in the shielding gas containing Ar and N2 increases. Nitrogen content in the weld is 0.25% when pure Ar is used; The highest content of N in the weld can be 0.75% when the protection gas is pure N2. The N-content in base metal is 0.76%.The nitrogen content in the weld has significant effect on the weld solidification mode and microstructure. The solidification mode is A and the microstructure is single austenite. The austenite dendrites become progressive coarse while there is a higher nitrogen level. No nitrides and hot cracks can be found in the weld joints.The weld has a best tensile strength, about 857MPa, when Nitrogen content in the weld is 0.62%, it is 83.2% of the level of base metal; Hardness test shows that the weld has higher hardness with the higher nitrogen level, and the micro-hardness of the weld increases with the N-content, and values between those of the HAZ and the base metal. Weld impact absorbed energy increases at first, but then it decreases. Because the more nitrogen content in the weld, the greater the tendency to appear holes.The optimal welding technological parameters:(1) 10%N2+90%Ar, double sides DC 120 A; (2) 20%N2+80%Ar, one side DC 115 A, the other side DC pulse. The peak of DC pulse is 191A, the base value is 20% of the peak value, duty cycle is 50% and the frequency is 5Hz. Shielding gas flow is 13 L/min, and the welding speed is 25 cm/min. In these conditions, weld deformation are good, and the X-ray detection shows I level; the microstructure are single austenite, and the grains have tiny sizes with different growth directions; tensile strength reaches more than 80% of the parent metal, and the analysis of the morphology of the toughening nest in tensile fracture show that the plastic are best at the time; Micro-hardness in these two welding zones value higher than others; Impact toughness test also shows that toughness of these two kinds of joint are the best; the two processes realize the purpose of nitrogen fixation, and the nitrogen content in the weld are 0.48% and 0.62%.