Why Does Austenitic Welded Pipe Have Hot Cracks And Deformation?
- By:The welding characteristics of austenitic stainless steel: The elastic and plastic stress and strain during the welding process are very large, but ...
The welding characteristics of austenitic stainless steel: The elastic and plastic stress and strain during 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 higher.
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
1. The formation of chromium carbide reduces the ability of welded joints 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 sensitization temperature zone on the target material:
a. Reduce the carbon content of the base metal and welds, 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 pass argon 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 carbides charge out and accelerate the diffusion of chromium).
(2) Knife-shaped 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 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 welded joint is heated again in the sensitization temperature range, supersaturated 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. 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 "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 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 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.
2. Stress corrosion cracking
The following measures can be used to prevent stress corrosion cracking from occurring:
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. Carry out post-weld stress relief heat treatment, and use mechanical methods such as polishing, shot peening and hammering to reduce surface residual stress.
c. Reasonable structure design. To avoid large stress concentration.
3. Welding hot cracks (crystallization cracks in welds, liquefaction cracks in the heat-affected zone)
The sensitivity of thermal cracking mainly depends on the chemical composition, organization and performance of the material. Ni is easy to form low melting point compounds or eutectic with impurities such as S and P. The segregation of boron and silicon will promote thermal cracking. 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 cracking. If the welding is not uniformly heated, it is easy to form a larger tensile stress and promote the generation of welding hot cracks.
a. Strictly control the content of harmful impurities S and P.
b. Adjust the structure 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 in the grain boundary, and the delta phase can dissolve more The S and P can reduce the interface energy and organize the formation of intergranular liquid film.
c. Adjust the weld metal alloy composition. Appropriately increase the content of Mn, C, and N in the single-phase austenitic steel, and add a small amount of trace elements such as cerium, pickaxe, and tantalum (which can refine the weld structure and purify the grain boundary), which can reduce the sensitivity of thermal cracking.
d. Process measures. Minimize the overheating of the molten pool to prevent the formation of thick columnar crystals. Use small heat input and small cross-section weld beads.
For example, 25-20 austenitic steel is prone to liquefaction cracks. It is possible to strictly limit the impurity content and grain size of the base material, adopt high energy density welding methods, small heat input and increase the cooling rate of the joints.
4. Embrittlement of welded joints
Heat-strength steel should ensure the plasticity of welded joints to prevent high-temperature embrittlement; low-temperature steels are required to have good low-temperature toughness to prevent low-temperature brittle fracture of welded joints.
5. Large welding distortion
Due to low thermal conductivity and large expansion coefficient, welding deformation is large, and clamps can be used to prevent deformation.
If you also have the above-mentioned troubles, you can analyze the phenomenon by observing the pipe welding process. Then take corresponding measures to solve the problem according to the corresponding situation. In addition to the element content of the material itself, the mold, the distribution of the arches, the welding current, and the welding speed of the welded pipe unit will have a certain impact on the quality of the welded pipe. As a professional manufacturer of industrial welded pipe production equipment, welcome to communicate with Hangao Tech about the problems you encounter in the production of austenitic welded pipes, and look forward to making progress with you.
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