Austenitic stainless steel in the welding characteristics: the welding process of elastic, plastic stress and stress variables are very large, but rarely appear cold cracks. There is no hardening zone and coarse grain in the welded joint, so the tensile strength of the weld is high.
Main problems of austenitic stainless steel welding: large welding deformation; Because of its grain boundary characteristics and sensitivity to some trace impurities (S, P), it is easy to produce hot cracks.
Five welding problems and treatment measures of austenitic stainless steel
01 The formation of chromium carbide reduces the intergranular corrosion resistance of welded joints
Intergranular corrosion: According to the chrome-poor theory, chromium carbide precipitates on the grain boundaries when the weld and heat-affected zone are heated to the sensitization temperature zone of 450-850 ° C, resulting in chrome-poor grain boundaries that are not enough to resist the degree of corrosion.
(1) For weld intergranular corrosion and corrosion in the sensitized temperature zone on the mesh, the following measures can be adopted to limit:
a. Reduce the carbon content of the base metal and 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. The weld is formed into a biphase structure of austenite and a small amount of ferrite. When there is a certain amount of ferrite in the weld, the grain size can be refined, the grain area can be increased, and the amount of chromium carbide precipitation on the grain boundary unit area can be reduced.
Chromium has high solubility in ferrite, Cr23C6 preferentially forms in ferrite, and does not lead to chromium deficiency in austenite grain boundary. The ferrite, which walks between the austenites, prevents corrosion from spreading inwards along the grain boundaries.
c. Control the residence time in the sensitized temperature range. Adjust the welding thermal cycle, shorten the residence time of 600 ~ 1000℃ as much as possible, and choose a welding method with high energy density (such as plasma argon arc welding).
Select smaller welding line energy, argon gas on the back of the weld or 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 of the multi-layer welding with the corrosive medium is welded as far as possible.
d. After welding, solid solution treatment or stabilization annealing (850 ~ 900℃) after heat preservation and air cooling, so that the carbide is fully precipitated, and the chromium is accelerated diffusion).
(2) Knife 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 polarized in the grain boundary to form a supersaturated state during the cooling process, while Ti and Nb remain in the crystal due to the low diffusion ability. When the welded joint is reheated in the sensitization temperature range, the susaturated carbon will precipitate in the form of Cr23C6 between the crystals.
a. Reduce carbon content. For stainless steel containing stabilizing elements, the carbon content should not exceed 0.06%.
b. Adopt 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 "medium temperature sensitization" effect during the welding process.
When welding on both sides, the weld in contact with the corrosive medium should be finally welded (this is the reason why the welding inside and outside of the large-diameter thick wall welding is carried out), if it cannot be implemented, the welding specification and weld shape should be adjusted, and the overheated area in contact with the corrosive medium should be sensitized again as far as possible.
c. Heat treatment after welding. Solid solution or stabilization 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 resistance to stress corrosion, and good resistance to stress corrosion when the weld metal is austenite-ferrite duplex steel structure.
b. Eliminate or reduce residual stress. The residual surface stress is reduced by mechanical methods such as polishing, shot blasting and hammering.
c. Reasonable structural design. To avoid large stress concentration.
03 Welding hot crack (weld crystallization crack, heat affected zone liquefaction crack)
The thermal crack sensitivity mainly depends on the chemical composition, structure and properties of the material. Ni is easy to form a low melting point compound or eutectic with impurities such as S and P, and the segregation of boron and silicon will promote the production of hot cracks.
The weld is easy to form a strong directional coarse columnar crystal structure, which is conducive to the segregation of harmful impurities and elements. Thus, a continuous intergranular liquid film is formed and the sensitivity of thermal crack is increased. If the welding is not uniformly 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. The weld has good crack resistance. The δ phase in the weld can refine the grain, eliminate the directivity of single-phase austenite, reduce the segregation of harmful impurities at the grain boundary, and the δ phase can dissolve more S and P, reduce the interface energy, and organize the formation of intergranular liquid film.
c. Adjust weld metal alloy composition. The thermal crack sensitivity can be reduced by appropriately increasing the content of Mn, C and N in single-phase austenitic steel and adding a small amount of trace elements such as cerium, iron, tantalum (which can refine the weld structure and purify the grain boundary).
d. Process measures. Minimize the overheating of the molten pool to prevent the formation of thick columnar crystals, and adopt small line energy and small cross-section weld. For example, 25-20 austenitic steel is prone to liquefaction cracks. By strictly limiting the impurity content and grain size of the base material, high energy density welding method, small line energy and improving the cooling speed of the joint can be adopted
04 Embrittlement of welded joints
Thermal 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.
05 Large welding deformation
Due to low thermal conductivity and large expansion coefficient, the welding deformation is large, and the fixture can be used to prevent deformation. Selection of welding methods and welding materials for austenitic stainless steel:
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).
Because of its low melting point, small thermal conductivity and large resistivity, the welding current of austenitic stainless steel is small. Narrow weld and narrow weld pass should be used to reduce high temperature residence time, prevent carbide precipitation, reduce weld contraction stress, and reduce thermal crack sensitivity.
Welding material composition, especially Cr, Ni alloy elements are higher than the base material. Welding materials containing a small amount (4 ~ 12%) of ferrite are used to ensure good cracking 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, the welding material should be selected with Mo, Mn and other alloying elements.
C, S, P, Si, Nb in the welding material should be as low as possible, Nb in the pure austenite weld will cause solidification cracks, but a small amount of ferrite in the weld can be effectively avoided.
Welding structures that need to be stabilized or stress-relieved after welding are usually welded materials containing Nb. Submerged arc welding is used to weld medium plate. The burning loss of Cr and Ni can be supplemented by the transition of alloying elements in flux and wire.
Due to the large penetration depth, attention should be paid to preventing the occurrence of hot cracks in the center of the weld and reducing the corrosion resistance of the heat affected zone. Attention should be paid to choosing a thinner wire and a smaller welding line energy, and the wire needs to be low Si, S, and P.
The ferrite content in the heat-resistant stainless steel weld should not be greater than 5%. For austenitic stainless steel with Cr and Ni content greater than 20%, high Mn (6-8%) wire should be selected, and alkaline or neutral flux should be selected to prevent Si from being added to the weld to improve its cracking resistance.
The special flux for austenitic stainless steel increases Si very little, which can transition alloy to the weld and compensate for the burning loss of alloying elements to meet the requirements of the weld performance and chemical composition.