Plasma Arc Welding And Postweld Heat Treatment Of 15-5PH Stainless Steel

In recent years, the martensitic precipitation hardening stainless steel has been widely used in different areas such as aerospace, marine and ordnance industry because of its good obdurability after heat treatment. Better quality of the welding joints is required for the poor service environment of areas above. When using traditional fusion welding, segregation, softening and lower strength are often found.While solid-state welding and high energy beam welding can effectively reduce these problems. In this paper, plasma arc welding was conducted on 15-5PH stainless steel of 6 mm thick and heat treatment was used after welding. The influence of welding current, gas flow rate and welding speed on the mechanical behavior of weld were studied. And effects of different heat treatment methods on the structure and properties of the joint were learned as well.The results show that the increase of ion gas flow rate and welding current can improve the penetration of plasma arc and the mechanical properties of the weld. And incomplete penetration and pores can be avoided too. With the increase of welding speed and the decrease of welding linear energy, the width of weld decreases and the surplus height of seam increases. The amount of the pores, especially the through-holes, can be reduced by changing the cooling conditions(preheating and reducing the welding speed).The best welding parameters are obtained after the orthogonal experiment with no welding defect found. The best welding current of plasma arc, gas flow rate and the welding speed are 220 A, 3.0L/min and 400mm/min, respectively. The main factor is gas flow rate. The weld joint can be divided into four parts: weld zone, coarse grain zone, two phase zone and the base metal. Compared with TIG, the joint property of PAW is better than TIG. During TIG welding, the higher welding linear energy together with multipass welding leads to a bigger weld heat affected zone than PAW, a sound joint is achieved with good appearance and tensile strength of 1100 MPa, as well as impact energy of 20.5J, ductility of 8%.Parent metal organization is quenching martensite after solution treatment at1050℃ for 1 h. And reverted austenite and precipitated phase appears after aging treatment at 550℃ for 4h. With the increase of aging temperature, more revertedaustenite are observed. And the tensile strength of the material is reduced and the impact toughness of the material is improved because of the coarsening of the precipitated phase.After solution treatment and solution-aging treatment, two-phase region of the joint disappeares. The joint after solution-aging treatment shows the best property, and the microstructure are tempered martensite and reverted austenite. However,two-phase region still exists on the joint after single aging treatment, which leads to the lower hardness of two-phase region. According to the results of microhardness test,the joint after solution-aging treatment has the highest microhardness, and from the HAZ to parent metal, the microhardness of joint has no great change. But for the joint without treatment and with single-aging treatment, the distribution of microhardness presents as “V” shape.After solution treatment, the tensile fracture mode of joint changes from quasi-cleavage fracture with crystalloid and dimple to crystalloid brittle fracture.Meanwhile, after single-aging treatment or solution-aging treatment, the tensile fracture mode of joint is microvoid accumulation fracture. In summary, the ductility and toughness of joint is better after solution-aging treatment.