Welding characteristics of austenitic stainless steel: the elastic and plastic stress and strain in the welding process are very large, but cold cracks rarely appear. There is no quench hardening zone and grain coarsening in the welded joint, so the tensile strength of the weld is relatively high.
The main problem of austenitic stainless steel welding: is large welding deformation; because of its grain boundary characteristics and sensitivity to certain trace impurities (S, P), hot cracks are easy to occur.
Five Welding Problems and Treatment Measures of Austenitic Stainless Steel
The formation of chromium carbide reduces the ability of welded joints to resist intergranular corrosion.
Intergranular corrosion: According to the chromium-depleted theory, when the weld and heat-affected zone are heated to the sensitization temperature zone of 450-850°C, chromium carbide precipitates on the grain boundary, resulting in a chromium-depleted grain boundary, which is not enough to resist corrosion.
(1) For the intergranular corrosion of the weld and the corrosion in the sensitization temperature zone on the target material, the following measures can be adopted to limit:
a. Reduce the carbon content of the base metal and weld, and add stabilizing elements such as Ti and Nb to the base metal to give priority to the formation of MC to avoid the formation of Cr23C6.
b. Make the weld form a duplex structure of austenite plus a small amount of ferrite. When there is a certain amount of ferrite in the weld, the grains can be refined, the grain area can be increased, and the precipitation of chromium carbide per unit area of the grain boundary can be reduced.
Chromium has a high solubility in ferrite, and Cr23C6 is preferentially formed in ferrite without causing chromium-deficient austenite grain boundaries; ferrite scattered between austenites can prevent corrosion along the grain boundaries to the inside diffusion.
c. Control the residence time in the sensitization temperature range. Adjust the welding thermal cycle, shorten the residence time at 600-1000°C as much as possible, and choose a welding method with high energy density (such as plasma argon arc welding),
Choose a smaller welding line energy, pass argon gas on the back of the weld or use a copper pad to increase the cooling speed of the welded joint, reduce the number of arcs and arcs to avoid repeated heating, and the contact surface with the corrosive medium should be as last as possible during multi-layer welding Welding, etc.
d. After welding, carry out solution treatment or stabilization annealing (850-900 ℃) and air cooling after heat preservation, so as to fully precipitate carbides and accelerate the diffusion of chromium).
(2) Knife-like corrosion of welded joints, for this reason, the following preventive measures can be taken:
Due to the strong diffusion ability 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 diffusion ability. When the welded joint is reheated in the sensitization temperature range, supersaturated carbon will be precipitated in the form of Cr23C6 in the intergranular.
a. Reduce carbon content. For stainless steels containing stabilizing elements, the carbon content should not exceed 0.06%.
b. Use a reasonable welding process. Choose a smaller welding line energy to reduce the residence time of the overheated area at high temperatures, and pay attention to avoid the “medium temperature sensitization” effect during the welding process.
During double-sided welding, the weld seam in contact with the corrosive medium should be welded last (this is the reason why the internal welding of large-diameter thick-walled welded pipes is performed after the external welding). If it cannot be implemented, the welding specification and weld shape should be adjusted to avoid The superheated area in contact with the corrosive medium again subjected to sensitization heating.
c. Post-weld heat treatment. Solution or stabilization treatment after welding.
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. Nitenite-ferritic dual-phase steel has good stress corrosion resistance.
b. Eliminate or reduce residual stress. Perform post-weld stress relief heat treatment, and use mechanical methods such as polishing, shot peening, and hammering to reduce surface residual stress.
c. Reasonable structural design. In order to avoid a large stress concentration.
Welding thermal cracks (weld crystallization cracks, heat-affected zone liquefaction cracks)
The thermal crack sensitivity mainly depends on the chemical composition, structure, and performance of the material. Ni is easy to form low-melting point compounds or eutectics with impurities such as S and P, and the segregation of boron and silicon will promote hot cracks.
The weld easily forms a coarse columnar grain structure with a strong direction, which is conducive to the segregation of harmful impurities and elements. This promotes the formation of a continuous intercrystalline liquid film and increases the sensitivity to thermal cracks. If the welding is heated unevenly, it is easy to form a large tensile stress and promote the generation of welding thermal cracks.
a. Strictly control the content of harmful impurities S and P.
b. Adjust the structure of the weld metal. The weld with an adual-phase structure has good crack resistance. The δ phase in the weld can refine the grain, eliminating the directionality of single-phase austenite, reduce the segregation of harmful impurities at the grain boundary, and the δ phase can dissolve the S,
P, and can reduce the interfacial energy, and organize the formation of the intercrystalline liquid film.
c. Adjust the weld metal alloy composition. Appropriately increasing the content of Mn, C, and N in single-phase austenitic steel, and adding a small number of trace elements such as cerium, pickaxe, and tantalum (which can refine the weld structure and purify the grain boundary) can 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, 25-20 type austenitic steel is prone to liquefaction cracks. Measures such as 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 joint can be adopted.
Embrittlement of welded joints
Hot-strength steel should ensure the plasticity of welded joints to prevent high-temperature embrittlement; low-temperature steel requires good low-temperature toughness to prevent low-temperature brittle fracture of welded joints.
Large welding deformation
Due to the low thermal conductivity and large expansion coefficient, the welding deformation is relatively large, and clamps can be used to prevent deformation. Welding methods of austenitic stainless steel and selection of welding materials:
Austenitic stainless steel can be welded by tungsten argon arc welding (TIG), molten argon arc welding (MIG), plasma argon arc welding (PAW), and submerged arc welding (SAW).
Austenitic stainless steel has a low welding current due to its low melting point, small thermal conductivity, and high resistivity. Narrow weld seam and narrow weld bead should be used to reduce high-temperature residence time, prevent carbide precipitation, reduce weld shrinkage stress, and reduce thermal crack sensitivity.
The composition of the welding material, especially the Cr and Ni alloy elements, 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 to exist in the weld, welding consumables containing alloying elements such as Mo and Mn 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 the pure austenitic weld, but a small amount of ferrite in the weld can effectively avoid it.
For welded structures that need to be stabilized or stress-relieved after welding, welding materials containing Nb are usually selected. Submerged arc welding is used to weld the middle plate, 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 occurrence of thermal cracks in the central area of the weld and the reduction of corrosion resistance in the heat-affected zone. Attention should be paid to the selection of thinner welding wire and smaller welding energy, and the welding wire needs to be low in Si, S, and P.
The ferrite content in heat-resistant stainless steel welds should not exceed 5%. For austenitic stainless steel with Cr and Ni content greater than 20%, high Mn (6-8%) welding wire should be selected, and alkaline or neutral flux should be selected as the flux to prevent Si addition to the weld and improve its crack resistance.
The special flux for austenitic stainless steel has very little Si increase, which can transition alloy to the weld, compensate for the burning loss of alloy elements, and meet the requirements of weld performance and chemical composition.