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The Investigation of Welding Procedure and Weld Microstructures and Properties of Nickel Free High Nitrogen Austenitic Stainless Steel

Author: SunHui
Tutor: WangHuai
School: Changchun University of
Course: Materials Processing Engineering
Keywords: High nitrogen austenitic stainless steel Welding joint Alloy welding wire Nitrogen hole Organization morphology Mechanics performance
CLC: TG457.11
Type: Master's thesis
Year: 2011
Downloads: 176
Quote: 2
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Abstract

The welding is the important processing means for austenitic stainless steel plates and templates. Because Ni-Cr austenitic stainless steel contains lots of valuable Ni elements, the costs of making Ni-Cr stainless steel have increased. The nickel free high nitrogen austenitic stainless steel which adopts nitrogen instead of nickel can save lots of Ni. The mechanical properties and corrosion resistant properties is better than Ni-Cr austenitic stainless steel. So it has the remarkable social benefits and economic benefits. But the research shows that, the welding seam and heat affected zone in this kind of steel prone to loss nitrogen which produces unfavorable effects on the mechanical properties of the joints. How to solve the problems that the lossing of N in the welding joints and the degrading of performance of heat affected zones in order to maintain the mechanics performance of this stainless steel is an important topic.In this paper, the designed solid wires and cored welding wires were used to undertake TIG welding test. The base materials were 0Cr18Mn18N0.7 nickel free high nitrogen austenitic stainless steel plates which were treated by hot rolling, solid solution treatment, cold rolling deformation and reply recrystallization. The influences of wire compositions and welding process on nitrogen losses and organizations and performances of the joints were studied. The joints were analyzed by metallographic microscope, scanning electron microscopy (SEM)、XRD etc. And the micro hardness and tensile strength was tested.Experimental results show that, all the organization structures of joints were mainly austenitic, a few ofδferritic,Cr2N compounds and other compounds. The grains in fusion area is small branches crystals. The equiaxial crystals distributed from fusion area to mother material and a large of twin crystals distributed in the grains. The nitrogen holes were existed in the weld and fusion area of the joints welded by the wires made by mother material. But the nitrogen holes disappeared when welded by solid wires and cored welding wires. The hot rolled plates were welded by TIG welding with current between 120A to 150A. There were no significant heat affected zones in the joints. When the current increased to 180A-200A, the small heat affected zones in which the grains grown appeared in the joint. Cold rolling deformation plates and reply recrystallization plates were welded by TIG welding with current 120A. An obvious heat affected zone appeared in the joints. And a wider transition zone formed between the mother materials. The grain size reduce gradually along the mother materials direction in the transition zone. But the biggest grain size still obviously less than that of in hot-rolled mother materials. Mechanical performance tests show that, the micro hardness of joints welded by alloy wires fluctuated small. The value average of the hardness was HV370 which was higher than mother materials. The tensile strength test shows that happened at the mother materials which explained that the strength of joints was higher than the mother materials. Polarization curve tests shows that, the corrosion resistance of joints welded by alloy wires was not less than that of mother materials. Research shows that the designed alloy wires better met the requirements of TIG welding of nickel free high nitrogen austenitic stainless steel. Small splash phenomenon appeared in the welding, but it had good joint forming.Studies suggest that. the Mo、Nb added to the alloy wires and Cr、Mn in the wires and mother materials can form strong coordination nitrogen-fixing role. Significantly inhibited the precipitation of nitrogen in the pool and fusion area when solidification. Lattice distortions produced by solid solution nitrogen and the pinning nitrides separated out from grain boundary can effectively prevent the movement of grain boundary. This made it hard for grains in the heat affected zones to grow up. Mo、Nd added to the alloy wires can enhance the solid solution strengthening, fine-grain strengthening of fusion area and weld, the resistance of pitting and intergranular corrosion. And made the strength and hardness and corrosion resistance of joints better than mother materials.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Welding, metal cutting and metal bonding > Welding process > Welding of a variety of metal materials and components > Welding of metallic materials > Steel
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