Effect of Heat Treatment on Intergranular Corrosion of Low Nickel Chromium Manganese Nitrogen Austenitic Stainless Steel
Austenitic stainless steel is used in various production and processing fields because of its own austenitic single-phase, stable structure and performance, and good corrosion resistance. In recent years, the international nickel prices have soared and reserves have fallen sharply, causing the research upsurge of low-nickel-chromium-manganese-nitrogen austenitic stainless steel to be set off once again. Compared with traditional stainless steel, low-nickel-chromium-manganese-nitrogen austenitic stainless steel not only saves the amount of nickel added, reduces the cost, but also has excellent corrosion resistance, which is one of the current development directions of stainless steel. However, under certain special processes, such as welding, when a certain sensitization temperature is reached, the material has a tendency to undergo intergranular corrosion.
For this reason, the EPR of low-nickel-chromium-manganese-nitrogen austenitic stainless steel in the sample sensitized state after solution treatment in (eg 0.5mol/L H2SO4+0.01mol/LKSCN) solution at different sensitization times and temperatures was studied. And EIS characteristics, some information about intergranular corrosion can be obtained by analyzing the test data. The electrochemical test can qualitatively and quantitatively study the above-mentioned intergranular corrosion and can have a deeper understanding of corrosion research.
The chemical composition (mass fraction, %) of the low nickel-chromium manganese nitrogen austenitic stainless steel is 0.093C, 0.18N, 11.05Mn, 0.98Ni, 15.52Cr, and the balance Fe. The material was kept at 1100°C for 2h, and then water was quenched so that a small number of carbides and compounds in the test steel were dissolved and solid-dissolved in the austenite matrix. The intergranular corrosion sensitization temperature of austenitic stainless steel is above 400℃. The sensitization heat treatment system measured in this test is shown in Table 1. AC impedance spectroscopy and electrochemical potential reactivation were used to study the effect of heat treatment on the intergranular corrosion susceptibility of low-nickel-chromium-manganese-nitrogen austenitic stainless steel.
The results showed that:
(1) In the range of 550 to 750°C, the reactivation rate of sensitization treatment at 750°C reaches 0.5481, which is the highest in the temperature range. It shows that the degree of intergranular corrosion in this temperature range increases with the increase of temperature. Intergranular corrosion occurs in this temperature range, so this temperature range should be avoided during the production and use of austenitic stainless steel.
(2) The reactivation rate of the test after heat treatment at 950℃ for 2h is 0.0385, and the sample has no tendency of intergranular corrosion. It means that 950℃ is not within the sensitization temperature range of low nickel-chromium-manganese-nitrogen austenitic stainless steel, and 950℃ is the safe use temperature.
(3) The degree of intergranular corrosion at 650℃ for 0.5～12h increases with the extension of heat treatment holding time. Therefore, if production or use at 650°C cannot be avoided, the heating time must be shortened as much as possible to avoid intergranular corrosion.