The commonly used heat treatment for stainless steel flange is stress removal treatment, solution treatment, sensitization treatment, stabilization treatment, and heat treatment to eliminate σ phase.

1. Stainless steel flange stress removal annealing

In order to eliminate cold working stress, it can be heated to 300 ~ 350℃, insulated for 1 ~ 2h, and air cooling. To eliminate welding stress, the general use of 850 ~ 950℃ heating, insulation 1 ~ 3h, air cooling, or water cooling.

2. Stainless steel flange solution treatment

The solution treatment process is similar to the quenching process, except that no phase transition occurs in the steel, so the treated room temperature microstructure is the saturated γ-Fe solid solution instead of the saturated α-Fe solid solution. The main purpose of solution treatment is to make the Austenitic stainless steel have excellent corrosion resistance.

The heating temperature of solution treatment is usually 1050 ~ 1100℃. The upper limit temperature is taken when the carbon content is high, and the lower limit temperature is taken when the carbon content is low. See Table 8-1 for heat treatment heating and insulation coefficient in an air furnace. After treatment should be quickly cooled, generally using water cooling, thin wall parts can use air cooling.

This kind of steel should be heated in a neutral or weak oxidizing atmosphere, so an air furnace is often used as heating equipment and an ammonia decomposition atmosphere as the heating medium. A salt bath is not suitable for heating because salt chloride will corrode steel. In order to ensure the heating quality, the surface of the parts must be cleaned before processing.

3. Sensitization of stainless steel flange

It is heated in the temperature range of 400 ~ 800℃ to test the resistance of steel to intergranular corrosion, which is called sensitization. This temperature range is called the sensitization temperature.

Heating steel in the sensitized temperature range should be avoided as far as possible except in special circumstances. Solid solution treatment is to make the precipitated chromium carbide re solid soluble in austenite, which can be used to eliminate the effect of sensitization treatment.

4. Stainless steel flange stabilization treatment

Metals suffer from corrosion in many ways. One type of corrosion is along the grain boundaries of the metal surface, called intergranular corrosion. In order to prevent intergranular corrosion, alloy elements such as titanium and niobium are added to Austenitic stainless steel. Stabilization treatment is only applied to chromium-nickel austenitic stainless steels containing titanium or niobium.

After solution treatment, the intergranular corrosion tendency of the steel increases due to chromium carbide precipitation along the grain boundary. Therefore, solid solution treatment should be followed by a second stabilization treatment in order to transfer the carbon atoms from chromium carbide to titanium carbide or niobium carbide, thereby improving the resistance of steel to intergranular corrosion. The process of stabilization treatment is: heating to 850 ~ 900℃, holding for 2 ~ 6h, air cooling, or water cooling.

5. Stainless steel flange to eliminate the heat treatment of σ phase

The presence of the σ phase, a hard and brittle FeCr intermetallic compound, reduces the toughness, corrosion resistance, and oxidation resistance of steel. The σ phase is most likely to appear in high chromium ferrites. It may also occur in austenitic-ferritic steel and austenitic steel.

The σ phase can be dissolved in austenite at high temperatures, and it can exist in steel at 820℃. Heat treatment to remove the σ phase is to heat at a temperature above the upper limit for its existence. For 1Crl8Ni9Nb, the σ phase will disappear when heated at 850℃. The upper limit temperature of the σ phase varies with steel composition, so the specific heating temperature should be determined by test.

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