1. The formation of chromium carbide reduces the resistance to intergranular corrosion of welded joints.
Intergranular corrosion: According to the theory of poor chromium, welds and heat affected zone in the heating to 450-850 ℃ sensitized temperature zone in the grain boundary precipitation of chromium carbide, resulting in poor chromium grain boundaries, not enough to resist the degree of corrosion.
(1) For intergranular corrosion of welds and corrosion of sensitized temperature zones on the material, the following measures can be used to limit:
A. Reduce the base metal and weld carbon content, the base material to add stabilizing elements Ti, Nb and other elements to give priority to the formation of MC, to avoid the formation of Cr23C6.
B. So that the weld to form austenite plus a small amount of ferrite biphase structure. When there is a certain amount of ferrite in the weld, the grain can be refined and the grain area is increased to reduce the amount of chromium carbide in the grain boundary per unit area. Chromium in the ferrite in the larger solubility, Cr23C6 priority in the formation of ferrite, without causing austenite grain boundary poor chromium; walking in the austenite between the ferrite, to prevent corrosion along the grain boundary to the internal diffusion.
C. Control the residence time in the sensitized temperature range. Adjust the welding heat cycle, as short as possible to shorten the residence time of 600 ~ 1000 ℃, you can choose high energy density welding methods (such as plasma argon arc welding), the choice of smaller welding line energy, the back of the weld with argon or copper Increase the cooling speed of the welded joints, reduce the number of arcs, the number of arcs to avoid repeated heating, multi-layer welding and corrosive media contact surface as much as possible the final welding and so on.
D. After the solution for solid solution or stabilized annealing (850 ~ 900 ℃) after the air-cooled insulation, so that the carbide fully precipitated, and accelerated diffusion of chromium).