Analysis on heat treatment and rewelding

Although heat treatment may also cause damage to material properties (such as reheat cracks, etc.), the damage caused by welding (such as residual stress, etc.) is greater. For materials that are prone to cold cracks under the action of residual stress, and for vessels that may produce stress corrosion, the welding residual stress should be strictly controlled.

However, it needs to be pointed out that during heat treatment, reheat cracks may also occur. Therefore, the material performance should be avoided during heat treatment, such as controlling heating and cooling rates, that is, the correct heat treatment process needs to be formulated and strictly implemented.

After this heat treatment, welding can no longer be carried out.

2. After PWHT, if welding is required, if some requirements are met and the owner agrees, the heat treatment may not be performed again.

This should be considered from two aspects: 1. In some cases, the harm caused by welding is small, or acceptable, and no heat treatment is required, such as a general carbon steel container; 2. In other cases, compared to welding, heat treatment For example, the heat treatment process is complicated, the implementation is difficult, and the hazards (such as heat deformation, cracks, etc.) are not easy to control, especially for materials with a serious tendency to reheat cracks, or composite plate materials.

When welding:

a) The waist height of the fillet weld should be less than 1/3 of the shell thickness and no more than 13mm; (control the heat input from the total amount)

b) Surface non-destructive testing of welded joints should be carried out; (limiting cracks)

c) Use low-hydrogen type coated electrode; (avoid hydrogen-induced cracks and control cracks from the source)

d) The preheating temperature is higher than or equal to 100℃; (to avoid a sharp rise in temperature during welding, control welding thermal cracks, etc.)