YUHONG GROUP - Three types of heat treatment for austenitic stainless steels

Austenitic stainless steel Cr, Ni and other alloying elements in the role of the results of Ms point down to below room temperature (-30 to -70 ℃). Ensure that the austenitic organisation is stable, so that no phase change occurs above room temperature when heating and cooling. Therefore, the main purpose of heat treatment of austenitic stainless steel is not to change the mechanical properties, but to improve corrosion resistance.

1 Solid solution treatment of austenitic stainless steel
Process:
In the GB1200 standard, the recommended heating temperature range is wide: 1000 ~ 1150 ℃, usually using 1020-1080 ℃. Considering the composition of specific grades, whether castings or forgings, etc., within the allowable range, the heating temperature is appropriately adjusted. Heating temperature is low, C-Cr carbide can not be fully dissolved, the temperature is too high, there are also grain growth, reduce corrosion resistance problems.
Cooling mode: should be cooled at a faster rate to prevent carbide re-precipitation. In our country and some other national standards, marked solid solution after ‘fast cooling’, a combination of different literature and practical experience, ‘fast’ scale can be mastered as follows:
- C content ≥ 0.08%; Cr content > 22%, Ni amount of higher; C content < 0.08%, but the effective size of > 3mm, should be water-cooled.
- C content <0.08%, size <3mm, air-cooled.
- Effective size ≤ 0.5mm can be air-cooled.

2 austenitic stainless steel stabilisation heat treatment
Stabilisation heat treatment is limited to austenitic stainless steel containing stabilisation elements Ti or Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Nb and so on.

Process:
Heating temperature: this temperature should be higher than the dissolution temperature of Cr23C6 (400-825°C), lower or slightly higher than the beginning of the dissolution temperature of TiC or NbC (e.g., the dissolution temperature range of TiC is 750-1120°C), the stabilisation heating temperature is generally selected at 850-930°C, which will cause Cr23C6 to dissolve sufficiently, so that Ti or Nb will then be combined with C therein, and Cr will continue to be retained in the austenite.
Cooling method: air cooling is generally used, water cooling or furnace cooling can also be used, which should be determined according to the specific conditions of the parts. The cooling rate has no significant effect on the stabilisation effect. From the results of our experimental studies, from the stabilisation temperature of 900 ℃ cooling to 200 ℃, the cooling rate of 0.9 ℃ / min and 15.6 ℃ / min, compared to the metallurgical organization, hardness, resistance to intergranular corrosion is basically equivalent.

3 Austenitic stainless steel stress relief treatment
Process:
Under the conditions, the use of solid solution treatment, stabilisation treatment can be a better stress relief (solid solution water cooling also produces a certain amount of stress), but, sometimes do not allow the use of this method, such as pipe fittings in the circuit, there is no margin of completion pieces, the shape of the particularly complex deformation-prone parts, etc., which can be used in the following 450 ° C temperature heating of the de-stressing method can be used to eliminate some of the stress. If the workpiece is used in a strong stress corrosion environment, the stress must be completely eliminated, then in the selection of materials, it should be considered, such as the use of steel containing stabilising elements, or the use of ultra-low carbon austenitic stainless steel.