Various "Problems" and "Prescriptions" in the Process of Stainless Steel Pipes Welding (1)
The welding characteristics of austenitic stainless steel: the elastic and plastic stress and strain during the welding process are very large, but cold cracks are rarely seen. There is no quenching hardening zone and grain coarsening in the weld, so the tensile strength of the weld is relatively high.
The main problems of austenitic stainless steel welding: large welding deformation; due to its grain boundary characteristics and sensitivity to certain trace impurities (S, P), it is easy to produce hot cracks.
Five major welding problems and treatment measures of austenitic stainless steel
01 The formation of chromium carbide reduces the ability of the weld to resist intergranular corrosion.
Intergranular corrosion: According to the theory of chromium depletion, chromium carbide precipitates on the grain boundaries when the weld and heat-affected zone are heated to the sensitization temperature zone of 450-850℃, resulting in chromium-depleted grain boundaries, which are insufficient to resist corrosion.
(1) The following measures can be used to limit the corrosion between the weld seam and the corrosion in the sensitization temperature zone on the target material:
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 give priority to the formation of 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 grain area can be increased, and the precipitation of chromium carbide per unit area of the grain boundary can be reduced.
Chromium is highly soluble in ferrite. Cr23C6 is preferentially formed in ferrite without causing austenite grain boundaries to be depleted in chromium; ferrite spreading between the austenites can prevent corrosion along the grain boundary to the inside diffusion.
c. Control the residence time in the sensitization temperature range. Adjust the welding thermal cycle, shorten the residence time of 600～1000℃ as much as possible, choose a welding method with high energy density (such as plasma argon arc welding), select a smaller welding heat input, and use argon gas on the back of the weld or use a copper pad Increase the cooling rate of the welded joint, reduce the arc starting and ending times to avoid repeated heating, and the contact surface with the corrosive medium during multilayer welding should be welded as last as possible.
d. After welding, carry out solution treatment or stabilization annealing (850～900℃) and air cooling to make the carbide charge out and accelerate the diffusion of chromium).
(2) Knife-shaped corrosion of welds. For this reason, the following preventive measures can be taken:
Due to the strong diffusion ability of carbon, it will segregate in the grain boundary to form a supersaturated state during the cooling process, while Ti and Nb remain in the crystal due to low diffusion ability. When the weld is heated again in the sensitization temperature range, the supersaturated carbon will be precipitated 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. Use a reasonable welding process. Choose a smaller welding heat input to reduce the residence time of the overheated zone at high temperature, and pay attention to avoiding the effect of "medium temperature sensitization" 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 large-diameter thick-wall welded pipes are carried out after the external welding). If it cannot be implemented, the welding specification and weld shape should be adjusted, and try to avoid The overheated area in contact with the corrosive medium is again sensitized and heated.
c. Post-weld heat treatment. Carry out solution or stabilization treatment after welding.
02 Stress corrosion cracking
The following measures can be used to prevent the occurrence of stress corrosion cracking:
a. Correctly select materials and reasonably adjust the 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 It has good stress corrosion resistance in the structure of the dual-phase steel of stenite and ferrite.
b. Eliminate or reduce residual stress. It can be used for post-welding stress-relieving heat treatment, such as a protective atmosphere on-line bright heat treatment induction annealing furnacethat adopts the principle of induction heating. The bright annealing furnace of Hangao Tech (SEKO Machinery) does not require preheating, it only takes 15 seconds to quickly reach the ideal annealing temperature. At the same time, it has superior air tightness, which can effectively prevent air backflow during annealing. The annealed welded pipe has a uniform metal structure and the intergranular stress becomes smaller. In addition, mechanical methods such as polishing, shot peening and hammering can also reduce surface residual stress.
c. Reasonable structure design. To avoid large stress concentration.
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