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Why Does Austenitic Welded Pipe Has Hot Cracks and Deformation?

Source:Iris Liang Time:2020-10-12 17:28:06

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 problem of austenitic stainless steel welding: large welding deformation; because of its grain boundary characteristics and sensitivity to certain trace impurities, 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) In view of the corrosion between the weld seam and the sensitization temperature zone on the target material, the following measures can be used to improve:

a. Reduce the carbon content of the base metal and welds, and add stabilizing elements such as Ti and Nb to the base metal.

b. Make the weld form a dual phase structure of austenite and a small amount of ferrite.

c. Control the residence time in the sensitization temperature range.

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-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 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.

c. Post-weld heat treatment. Carry out solution or stabilization treatment after welding.

2. Cracking caused by stress corrosion

The following measures can be used to prevent stress corrosion cracking from occurring:

a. Correctly select materials and reasonably adjust the weld composition.

b. Eliminate or reduce 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 easily forms low melting point compounds or segregation with impurities, which promotes thermal cracking. The weld is easy to form a 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.

Preventive measures:
a. Strictly control the content of harmful impurities.

b. Adjust the structure of the weld metal.

c. Adjust the weld metal alloy composition.

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 deformation during welding

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 stainless steel welded pipe tube mill machine, welcome to communicate with SEKO Machinery about the problems you encounter in the production of austenitic welded pipes, and look forward to making progress with you.



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Iris Liang
SEKO Machinery & Technology Co., Ltd
www.gdseko.com
E-mail: sales3@gdseko.com
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Tel: 0757-29390988
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