The types of metal corrosion are: continuous corrosion, intergranular corrosion, pitting corrosion, stress corrosion, etc. Defects that need to be prevented during the heat treatment of 18-8 type austenitic stainless steel products are continuous corrosion, pitting corrosion, and intergranular corrosion.
1) Causes of corrosion:
The corrosion of stainless steel is caused by the existence or precipitation of carbides and intergranular chromium deficiency. The theory of chromium deficiency in the grain boundary area believes that the intergranular corrosion of austenitic stainless steel is due to the precipitation of Cr23C6 along the grain boundary during the aging process. , causing the austenite near the grain boundary to be deficient in Cr, and reducing the chromium content in the solid solution to below the limit required for passivation.
2) Preventive measures:
(1) Solid solution treatment is adopted, so that the carbides in the matrix are not precipitated or less precipitated;
(2) After the solution treatment, sensitization treatment is adopted, the heating temperature is 400-800 °C, and the treatment time is from several tens of hours to more than 1000 hours. The purpose of extending the sensitization treatment time is to make the chromium-depleted area caused by the precipitated carbide by diffusion. The effect is to obtain the compensation of chromium from the chromium-rich area and restore the corrosion resistance.
(3) For 18-8 type Ti-containing or Nb-containing austenitic stainless steel, stabilization treatment is added after solution treatment. Stabilization process: 850～900℃, heat preservation for 2～4h, air cooling.
