Welding characteristics of austenitic stainless steel: the elastic, plastic stress and strain variables in the welding process are large, but cold cracks rarely appear. There is no quenching hardening area and coarsening of grains in welded joints, so the tensile strength of the weld is high.
The main problem of austenitic stainless steel welding: large welding deformation; Because of its grain boundary characteristics and sensitivity to certain trace impurities (S, P), it is easy to produce thermal cracks.
5 major welding problems and treatment measures of austenitic stainless steel
01
The formation of chromium carbide reduces the resistance of welded joints to intergranular corrosion.
Intergranular corrosion: According to the chromium-poor theory, chromium carbide precipitates on the grain boundary when the weld and the heat-affected zone are heated to the sensitization temperature zone of 450-850 °C, resulting in the chromium-poor grain boundary, which is not enough to resist the degree of corrosion.
(1) For intergranular corrosion of welds and corrosion in the sensitization temperature zone of the mesh, the following measures can be used to limit:
a. Reduce the carbon content of the base metal and weld, and add stabilized 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 duplex structure of austenite plus a small amount of ferrite. When a certain amount of ferrite exists in the weld, the grain can be refined to increase the grain area, so that the amount of chromium carbide precipitation per unit area of grain boundary can be reduced. Chromium has a large solubility in ferrite, and Cr23C6 is preferentially formed in ferrite without causing austenite grain boundary poor chromium; Ferrite that walks between austenitic bodies prevents corrosion from spreading inward along grain boundaries.
c. Control the residence time in the sensitization temperature range. Adjust the welding thermal cycle, shorten the residence time of 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, pass argon gas on the back of the weld or use copper pads to increase the cooling rate of the welding joint, reduce the number of arc initiation and arc retraction to avoid repeated heating, and the contact surface with the corrosive medium during multi-layer welding is welded as much as possible.
d. After welding, solution treatment or stabilization annealing (850~900 °C) is air cooled after heat preservation to make carbide charge and analyze 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 be concentrated at the grain boundary to form a supersaturated state during the cooling process, while Ti and Nb will remain in the crystal due to low diffusion ability. When the welded joint is heated again in the sensitization temperature range, supersaturated carbon will precipitate in the form of Cr23C6 between the grains.
a. Reduce carbon content. For stainless steels containing stabilized elements, the carbon content should not exceed 0.06%.
b. Adopt reasonable welding process. Choose a smaller welding line energy to reduce the residence time of the overheated zone at high temperature, and pay attention to avoid the "medium temperature sensitization" effect during the welding process. When double-sided welding, the weld in contact with the corrosive medium should be welded last (this is the reason why the internal welding of the large-diameter thick-walled welded pipe is carried out after external welding), if it cannot be implemented, the welding specification and weld shape should be adjusted to try to avoid the overheating area in contact with the corrosive medium being sensitized again.
c. Post-weld heat treatment. Solution or stabilization treatment after welding.
02
Stress corrosion cracking
The following measures can be taken to prevent 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 has good stress corrosion resistance when the structure of austenitic-ferritic duplex steel.
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. This avoids large stress concentrations.
03
Welding hot cracks (weld crystalline cracks, heat-affected zone liquefaction cracks)
Thermal crack sensitivity depends primarily on the chemical composition, structure, and properties of the material. Ni is easy to form low melting point compounds or eutectic compounds with impurities such as S and P, and the segregation of boron, silicon, etc. will promote the generation of thermal cracks. The weld is easy to form a coarse columnar crystal structure with strong directionality, which is conducive to the segregation of harmful impurities and elements. This promotes the formation of a continuous intergranular liquid film and improves the sensitivity of thermal cracks. If the welding is not evenly heated, 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 structure of the weld metal. Duplex structure weld has good crack resistance, the δ phase in the weld can refine the grain, eliminate the directionality of single-phase austenite, reduce the segregation of harmful impurities in the grain boundary, and the δ phase can dissolve more S and P, and can reduce the interface energy, the formation of tissue intergranular liquid film.
c. Adjust the composition of the weld metal alloy. In single-phase austenitic steel, appropriately increase the content of Mn, C and N, and add a small amount of cerium, pickaxe, tantalum and other trace elements (can refine the weld structure and purify the grain boundary), which can reduce the sensitivity to hot cracks.
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. Measures such as high energy density welding methods, small wire energy and increased cooling rates of joints can be adopted by strictly limiting the impurity content and grain size of the base metal.
04
Embrittlement of welded joints
Hot strength steel should ensure the plasticity of the welded joint and prevent high temperature embrittlement; Low-temperature steel requires good low-temperature toughness to prevent low-temperature brittle fracture of welded joints.
05
The welding deformation is large
Due to the low thermal conductivity and large expansion coefficient, the welding deformation is large, and the fixture can be used to prevent deformation. Welding method and selection of welding material for austenitic stainless steel:
Austenitic stainless steels can be welded by tungsten argon arc welding (TIG), MIG, plasma TIW and submerged arc welding (SAW). Austenitic stainless steel has a small welding current because of its low melting point, small thermal conductivity and large resistivity. Narrow welds and narrow welds should be used to reduce the high temperature residence time, prevent carbide precipitation, reduce the shrinkage stress of the weld, and reduce the sensitivity of hot cracks.
The composition of welding consumables, especially Cr and Ni alloying elements, is higher than that of the base metal. Welding materials containing a small amount (4~12%) ferrite are used to ensure good crack resistance (cold cracking, hot cracking, stress corrosion cracking) performance of the weld. When the ferrite phase is not allowed or may not exist in the weld, the welding material containing alloying elements such as Mo and Mn should be selected.
C, S, P, SI, NB in the welding consumables should be as low as possible, Nb will cause solidification cracks in pure austenitic welds, but a small amount of ferrite in the weld can be effectively avoided. Welded structures that need to be stabilized or stress relieved after welding, usually using welding materials containing Nb. Submerged arc welding is used to weld the middle plate, and the burn loss of Cr and Ni can be supplemented by the transition of alloying elements in the flux and welding wire; Due to the large penetration depth, attention should be paid to preventing the generation of thermal cracks in the center 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 line energy, and the welding wire needs to be low Si, S, P. The ferrite content in the heat-resistant stainless steel weld should not be more than 5%. Austenitic stainless steel with Cr and Ni content greater than 20% needs to use high Mn (6~8%) welding wire, and alkaline or neutral flux is used for flux to prevent Si from being added to the weld to improve its crack resistance. The special flux for austenitic stainless steel has very little Si addition, which can transition the alloy to the weld and compensate for the burn loss of the alloying elements to meet the requirements of weld performance and chemical composition.