The welding characteristics of austenitic stainless steel: the amount of elastic and plastic stress and strain during welding is large, but cold cracks rarely occur. There is no quench hardening zone and grain coarsening in the welded joint, so the tensile strength of the welded joint is high.
The main problems of austenitic stainless steel welding: welding deformation is large; because of its grain boundary characteristics and sensitivity to certain trace impurities (S, P), it is easy to produce hot cracks.
Five major welding problems of austenitic stainless steel and their solutions
01The formation of chromium carbide reduces the resistance to intergranular corrosion of welded joints.
Intergranular corrosion: According to the theory of poor chromium, when the weld and the heat-affected zone are heated to the 450-850 ℃ sensitization temperature zone, chromium carbide will precipitate on the grain boundary, resulting in the grain boundary of poor chromium, which is not enough to resist corrosion.
(1) For the intergranular corrosion of the weld and the corrosion of the sensitized temperature zone on the target material, the following measures can be used to limit:
a. Reduce the carbon content of the base metal and the weld, and add stabilizing elements Ti, Nb and other elements to the base metal to preferentially form MC to avoid the formation of Cr23C6.
b. Make the weld form a dual-phase structure of austenite and a small amount of ferrite. When there is a certain amount of ferrite in the weld, the grains can be refined, the area of the grains can be increased, and the amount of chromium carbide precipitation per unit area of the grain boundary can be reduced. Chromium has a large solubility in ferrite, and Cr23C6 is preferentially formed in ferrite, without causing the austenite grain boundary to be depleted in chromium; the ferrite scattered between austenite can prevent corrosion along the grain boundary to the interior diffusion.
c. Control the residence time in the sensitization temperature range. Adjust the welding thermal cycle to shorten the residence time at 600-1000 °C as much as possible, choose a welding method with high energy density (such as plasma argon arc welding), choose a smaller welding line energy, and pass argon gas on the back of the weld or use a copper pad Increase the cooling rate of the welded joint, reduce the number of arcing and arcing to avoid repeated heating, and apply the last welding on the contact surface with the corrosive medium during multi-layer welding.
d. After welding, carry out solution treatment or stabilization annealing (850-900 ℃) and air-cooling after heat preservation, so that the carbides can be fully precipitated and the diffusion of chromium can be accelerated).
(2) Knife-like corrosion of welded joints, for this reason, the following preventive measures can be taken:
Due to the strong diffusivity of carbon, it will segregate at the grain boundary to form a supersaturated state during the cooling process, while Ti and Nb remain in the crystal due to their low diffusivity. When the welded joint is heated again in the sensitization temperature range, the supersaturated carbon will precipitate in the form of Cr23C6 in the intergranular.
a. Reduce carbon content. For stainless steel containing stabilizing elements, the carbon content should not exceed 0.06%.
b. Use a reasonable welding process. Select a smaller welding line energy to reduce the residence time of the overheated area at high temperature, and pay attention to avoid the effect of "medium temperature sensitization" during the welding process. When double-sided welding, the welding seam in contact with the corrosive medium should be welded last (this is the reason why the inner welding of the large-diameter thick-walled welded pipe is carried out after the outer welding). The overheated area in contact with the corrosive medium is again heated by sensitization.
c. Post-weld heat treatment. Solution or stabilization treatment is carried out after welding.
02. Stress corrosion cracking
The following measures can be taken to prevent the occurrence of stress corrosion cracking:
a. Correct selection of materials and reasonable adjustment of weld composition. High-purity chromium-nickel austenitic stainless steel, high-silicon chromium-nickel austenitic stainless steel, ferritic-austenitic stainless steel, high-chromium ferritic stainless steel, etc. have good stress corrosion resistance, and the weld metal is austenitic stainless steel. Stress-corrosion resistance is good when the structure of the dual-phase steel is ferritic and ferritic.
b. Eliminate or reduce residual stress. Post-weld stress relief heat treatment is carried out, and mechanical methods such as polishing, shot peening and hammering are used to reduce surface residual stress.
c. Reasonable structural design. to avoid large stress concentrations.
03Welding hot cracks (weld crystallization cracks, heat affected zone liquefaction cracks)
The thermal crack susceptibility mainly depends on the chemical composition, structure and properties of the material. Ni is easy to form low melting point compounds or eutectic with impurities such as S and P, and the segregation of boron and silicon will cause thermal cracking. The weld is easy to form a coarse columnar grain structure with strong directionality, which is conducive to the segregation of harmful impurities and elements. Thereby promoting the formation of a continuous intercrystalline liquid film and improving the sensitivity of thermal cracking. If the welding is heated unevenly, it is easy to form a large tensile stress and promote the generation of welding hot cracks.
Preventive measures:
a. Strictly control the content of harmful impurities S and P.
b. Adjust the texture of the weld metal. The dual-phase structure weld has good crack resistance. The delta phase in the weld can refine the grains, eliminate the directionality of single-phase austenite, reduce the segregation of harmful impurities at the grain boundary, and the delta phase can dissolve more S, P, and can reduce the interfacial energy and organize the formation of intercrystalline liquid film.
c. Adjust the weld metal alloy composition. Appropriately increase the content of Mn, C, and N in single-phase austenitic steel, and add a small amount of trace elements such as cerium, pickax, and tantalum (which can refine the weld structure and purify grain boundaries) to reduce hot crack sensitivity.
d. process measures. Minimize the overheating of the molten pool to prevent the formation of coarse columnar crystals, and use small line energy and small cross-section weld beads.
For example, type 25-20 austenitic steels are prone to liquefaction cracks. By strictly limiting the impurity content and grain size of the base metal, adopting high energy density welding methods, small line energy and increasing the cooling rate of the joints and other measures.
04Embrittlement of welded joints
The hot-strength steel should ensure the plasticity of the welded joint and prevent high temperature embrittlement; the low temperature steel is required to have good low temperature toughness to prevent the low temperature embrittlement of the welded joint.
05Welding deformation is large
Due to the low thermal conductivity and large expansion coefficient, the welding deformation is large, and a fixture can be used to prevent deformation. Welding methods and selection of welding materials for austenitic stainless steels:
Austenitic stainless steel can be welded by argon tungsten arc welding (TIG), melting electrode argon arc welding (MIG), plasma argon arc welding (PAW) and submerged arc welding (SAW). Austenitic stainless steel has low welding current due to its low melting point, low thermal conductivity and high resistivity. Narrow welds and beads should be used to reduce high temperature residence time, prevent carbide precipitation, reduce weld shrinkage stress, and reduce thermal crack sensitivity.
The composition of welding consumables, especially the alloying elements of Cr and Ni, is higher than that of the base metal. Welding consumables containing a small amount (4-12%) of ferrite are used to ensure good crack resistance (cold cracking, hot cracking, stress corrosion cracking) of the weld. When the ferrite phase is not allowed or impossible in the weld, the welding consumables containing Mo, Mn and other alloying elements should be selected.
The C, S, P, Si, and Nb in the welding consumables should be as low as possible. Nb will cause solidification cracks in pure austenite welds, but a small amount of ferrite in the welds can be effectively avoided. For welded structures that need to be stabilized or stress relieved after welding, Nb-containing welding materials are usually selected. Submerged arc welding is used for welding medium plates, and the burning loss of Cr and Ni can be supplemented by the transition of alloy elements in the flux and welding wire; due to the large penetration depth, care should be taken to prevent the generation of hot cracks in the center of the weld and the corrosion resistance of the heat-affected zone. Sexual reduction. Attention should be paid to the selection of thinner welding wire and smaller welding line energy, and the welding wire should be low in Si, S, and P. The ferrite content in the heat-resistant stainless steel weld should not exceed 5%. For austenitic stainless steel with Cr and Ni content greater than 20%, high Mn (6-8%) welding wire should be used, and alkaline or neutral flux should be used as the flux to prevent the addition of Si to the weld and improve its crack resistance. The special flux for austenitic stainless steel has very little Si addition, which can transfer alloy to the weld to compensate for the burning loss of alloy elements to meet the requirements of weld performance and chemical composition.
