1. Strictly control the carbon content
Carbon is the main element that causes intergranular corrosion. When the carbon content is below 0.08%, the amount of carbon that can be precipitated is small. Above 0.08%, the amount of precipitated carbon increases rapidly, so the carbon content of the base metal and welding rod is often controlled below 0.08%. In addition, when the carbon content of the austenitic steel is less than 0.02%-0.03%, all the carbon is dissolved in the austenite and no intergranular corrosion occurs.
2. Add stabilizer
The addition of titanium, niobium and other elements that have a stronger affinity with carbon than chromium in the welding material can improve the resistance to intergranular corrosion.
3. Carry out solution treatment
After welding, heat the joint to 1050-1150°C, hold for 1 hour and then water quench. At this time, the grain boundary chromium carbide is completely dissolved, so that the carbon is re-dissolved into the austenite, and then rapidly cooled, so that the carbon is too late to precipitate and form a stable uniform The austenite structure eliminates the chromium-depleted layer at the grain boundary and avoids intergranular corrosion.
4. Perform homogenization treatment
The austenitic stainless steel is heated to 850-900℃ for 2h, so that the chromium inside the grains will also diffuse to the grain boundaries, so that the mass fraction of grain boundary chromium is restored to more than 12%, thereby eliminating the grain boundary chromium-depleted layer.
5. Reduce welding heat input
In terms of the welding process, small current, high welding speed, short arc and straight welding are adopted. In multi-layer and multi-pass welding, the temperature between layers should not be too high, and the next layer should be welded after the first layer is completely cooled (<60℃), and the joints between layers should be staggered.
6. Speed up the cooling of the joint
Weldments should not be preheated before welding. After welding, the cooling of the joints should be accelerated as much as possible, such as using copper backing plates, or even water cooling and other compulsory measures when conditions permit, to reduce the residence time of the joints at dangerous temperatures and prevent intergranular corrosion.
7. Arrange the welding sequence reasonably
In multi-layer welding and double-sided welding, the heat effect of the next weld may have a sensitizing temperature heating effect on the overheated area of the first weld. Therefore, the side of the double-sided weld that is in contact with the corrosive medium should be welded last as far as possible.
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