Intergranular corrosion of stainless steel is a phenomenon of corrosion damage, which is manifested by the loss of bonding force between grains, so that the strength of the material deteriorates. There are many different theories for the cause of intergranular corrosion, such as chromium-depleted theory, grain boundary adsorption theory, precipitation phase metastable theory, metastable phase dissolution theory, stress theory, precipitation phase morphology theory and corrosion electrochemical theory.
Among them, the theory of chromium deficiency is the earliest proposed and widely accepted theory. For 18-8 austenitic stainless steel, the lattice at the grain boundary is incomplete, which is beneficial to the diffusion of metal atoms; Cr in the grain boundary and its adjacent region will occur due to the precipitation of carbide Cr23C6 at the grain boundary. Poor phenomenon, causing a chromium-depleted area around the grain boundary. When the mass fraction of Cr is reduced to about 12%, corrosion occurs along the grain boundary of the material in some corrosive media, causing loss of bonding between the grains, ie, grain boundary corrosion. phenomenon.
TP321 stainless steel (UNSS32168) is based on the addition of Ti element to TP304 stainless steel to enhance its resistance to intergranular corrosion and high temperature resistance. The principle is to form a stable MC type carbide TiC, which is measured by the free enthalpy change of carbide formation. TiC is much more stable than chromium carbide and reduces the formation of chromium carbide.
In developed markets such as Europe and the United States, TP321 stainless steel seamless pipe has been gradually replaced by low carbon and ultra low carbon stainless steel seamless pipes such as TP304L and TP316L; however, in China, the demand for TP321 stainless steel seamless pipe is still large, according to international stainless steel. According to the data released by the Forum (ISSF), in 2012, the apparent consumption of China’s TP321 stainless steel seamless pipe was around 100,000 tons. Due to the limitation of production process and inspection conditions, the pass rate of intergranular corrosion resistance of TP321 stainless steel seamless pipe produced in China is not high, and the pass rate of one test is about 80%. Since Shanxi Taigang, has been put into production in 2009, the output of TP321 stainless steel seamless pipe accounts for about 35% of the total output. The pass rate of intergranular corrosion test is about 70%, which is far lower than the level of other stainless steel products (≥95%). ). In order to improve the yield and better meet the needs of users, the company’s technicians have carried out technical research on the intergranular corrosion resistance of TP321 stainless steel seamless pipe.
At present, the production of TP321 stainless steel seamless pipes in China mostly adopts the production methods of perforation→cold rolling (pulling)→heat treatment→straightening→acid washing→inspection→packaging. After the cold deformation of the steel pipe, the solution heat treatment is used to remove the deformation stress and improve the structure, that is, the steel pipe is heated to a temperature above the austenite carbon saturation curve, so that the carbide is sufficiently dissolved into the solid solution and then rapidly cooled, and the high temperature structure is at room temperature. It is fixed to obtain a supersaturated solid solution of carbon. Through the benchmarking activities, the analysis of the production process, starting from the aspects of chemical composition control, heat treatment system adjustment, degreasing process optimization, etc., the pass rate of intergranular corrosion of TP321 stainless steel seamless pipe is stable at 95% or more, achieving the goal of tackling the target. . Specifically in:
- (1) Through the chemical composition design, adjusting the ratio of elements such as C, Cr, Ni, Ti, etc., the intergranular corrosion resistance of TP321 stainless steel seamless pipe can be optimized;
- (2) When TP321 stainless steel seamless seam steel pipe is subjected to solution heat treatment, the reducing atmosphere in the furnace tends to cause carbonation on the surface of the steel pipe; the weak oxidizing atmosphere has better surface quality of the steel pipe, eliminates carbonation factors, and saves energy;
- (3) TP321 stainless steel seamless pipe is not degreased after cold rolling, which has a great influence on the surface quality of the steel pipe after heat treatment, and has adverse effects on corrosion resistance; by improving the degreasing method, the intergranular corrosion resistance of the steel pipe can be effectively improved;
- (4) The solution heat treatment temperature is set to 1050 ° C, which is favorable for the intergranular corrosion resistance of TP321 stainless steel seamless pipe;
- (5) If the TP321 stainless steel seamless pipe is used in the sensitive temperature range (450 ~ 900 °C) and the environment has strong corrosive medium, the steel pipe should be stabilized.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)