1, welding crack
(1) The thermal conductivity of 316L stainless steel wire is only about half of that of low carbon steel, and the coefficient of linear expansion is much larger, so there is a large internal welding stress in the joint after welding.
(2) The range of liquid and solid phase lines of 316L stainless steel wire is large, the crystallization time is long, and the austenite crystal has strong dendritic orientation, so the impurity segregation phenomenon is serious.
In summary, 316L stainless steel wire stainless steel is more prone to weld hot cracks during welding, including longitudinal and transverse cracks in the weld, cracks in the fire, root cracks in the bottom weld, and interlaminar cracks in the multilayer weld.
2. Intergranular corrosion of welded joints Intergranular corrosion occurs at grain boundaries, so it is called intergranular corrosion. It is one of the most dangerous forms of destruction of austenitic metals. The reason for the intergranular corrosion of austenitic stainless steel is currently recognized as the theory of chromium deficiency in the grain boundary region. At room temperature, the solubility of carbon in austenite is very small, about 0.02% to 0.03%. When the carbon content exceeds this range, carbon continuously diffuses into the austenite grain boundary, and it is easy to chrome. The formation of chromium compounds results in a lack of chromium content at the austenite boundary. When the chromium content is lower than a certain concentration, the corrosion resistance is lost. When used in a corrosive medium, intergranular corrosion occurs.
Therefore, welding intergranular corrosion and welding hot cracking are the key factors affecting the welding quality of 316L austenitic stainless steel. During the welding process, care should be taken to prevent the formation of chromium-depleted layers and hot cracks.
316L austenitic stainless steel welding process
1 Welding method
Welding of 316L austenitic stainless steel can be carried out by electrode arc welding, or by welding methods such as argon arc welding, submerged arc automatic welding and gas welding. The 316L stainless steel used in the irrigation zone of the third natural gas processing plant is argon-arc welded and the welding method of the manual arc welding cover. Here we explore the welding process of 316L austenitic stainless steel using tungsten argon arc welding and electrode arc welding.
2 welding material selection
For special properties of 316L austenitic stainless steel, in order to meet the same performance of welded joints, the welding material should be selected according to the principle of “equal composition”, and at the same time, the joints are resistant to weld hot cracks and intergranular corrosion, so that a small amount appears in the joint. For ferrite, H00Cr19Ni2Mo2 welding wire for argon arc welding and welding rod CHS022 for hand arc welding should be used as the filling material. The carbon content is less than 0.03%, and carbon is a major element causing intergranular corrosion. When the carbon content is less than 0.03%, the amount of carbon can be precipitated, and the carbon content is less. When it is 0.03% or more, the amount of carbon which can be precipitated increases rapidly. At the same time, carbon is also one of the main elements causing hot cracking in welding. Therefore, this kind of stainless steel welding consumables with low carbon content should be selected. Second, the electrode contains elemental bismuth. The affinity of bismuth with carbon is stronger than that of chromium, and it can combine with carbon to form stable carbides, thus avoiding chromium deficiency in austenite grain boundaries and playing a good role in improving the resistance to intergranular corrosion. 5重量。 The third, the amount of chromium containing the material is about 8. 5 %. Chromium forms ferrite in the weld. Because chromium diffuses faster in ferrite than in austenite, chromium diffuses faster into the grain boundary in ferrite, which reduces the chromium deficiency in austenite grain boundaries and prevents intergranular corrosion. . At the same time, the ferrite and austenite two-phase structure formed in the weld is an important measure to prevent welding hot cracks. When there are about 5% of ferrite in the weld, the grain growth of the austenite is hindered, the grains are refined, the impurities of the weld are uniformly dispersed, and the accumulation of impurities is prevented. Fourth, the electrode is a low-hydrogen type coating of a basic electrode. The low-hydrogen stainless steel electrode has high thermal crack resistance, which is beneficial to prevent the occurrence of welding hot cracks.
3 welding process
3.1 Preparation before welding The machining and plasma cutting methods are used to cut and machine the groove. The groove type adopts the V-shaped groove. The blunt edge of the groove is smaller than the carbon steel, and the groove angle is larger than that of the carbon steel. In order to avoid mixing of carbon and impurities into the weld during welding, the sides of the weld should be cleaned with acetone from 20 mm to 30 mm on both sides of the weld. At the same time, the surface should be brushed with stainless steel wire and wiped with acetone. .
3.2 Welding process The welding process mainly pays attention to the following points:
(1) The outstanding feature of austenitic stainless steel is that it is sensitive to overheating. Therefore, it adopts small current and fast welding. The welding current should be about 20% lower than that of low carbon steel to prevent intergranular corrosion, hot crack and welding deformation.
(2) In order to ensure stable arc combustion, the manual arc welding machine adopts DC reverse connection method; the argon arc welding adopts DC positive connection.
(3) When argon arc welding is used, the thickness of the weld is as thin as possible, and the root is fused well. When the arc is closed, it should be a gentle slope. If there is an arc shrinkage hole, it should be worn away by a sander. The inside of the pipe must be filled with argon gas. To ensure the bottom forming; hand arc welding uses short arc welding, the arc is slower, fills the arc pit, and prevents the arc pit crack.
(4) Forced cooling can be applied after welding.
(5) Do not ignite the arc outside the groove, and the ground wire should be connected.
(6) Post-weld deformation can only be corrected by cold working.
3.3 Post-weld treatment
(1) Surface treatment, such as nicks, dents, rough spots and stains on the surface of stainless steel weldments, will accelerate corrosion. If the surface of the stainless steel is polished, it can improve its corrosion resistance. The finer the surface roughness, the better the corrosion resistance.
(2) Passivation treatment is to form an oxide film artificially on the surface of stainless steel to increase its corrosion resistance. The passivated stainless steel has a silver-white appearance and high corrosion resistance.
3. Conclusion Welding practice shows that 316L austenitic stainless steel can avoid weld hot cracks and intergranular corrosion defects during welding by adopting reasonable welding methods, welding materials and welding processes, thus obtaining good welding quality. .