Heat treatment of high-performance austenitic stainless steels – stress relief

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Heat treatment of high-performance austenitic stainless steels – Solution annealing: The temperature used to relieve stress is lower than the annealing temperature, and it is not sufficient to dissolve precipitates. Stress relief cannot eliminate all residual stresses; it can only remove some of them, and its impact on the hardness of the material is minimal. Due to the lower temperature, the stress-relief process is slow, measured in hours, whereas the solution annealing process is measured in minutes. The temperature for stress relief usually drops to the lower limit of the secondary phase precipitation temperature. At these temperatures, the annealing time must be carefully selected to prevent the formation of precipitates that reduce corrosion resistance. The annealing time is related to the amount of stress eliminated. When deciding whether to use low-temperature stress relief, medium-temperature stress relief, or full solubilization annealing, it is necessary to carefully weigh the potential advantages of stress relief against the risks of sensitization. Many applications that use a combination of stainless steel, carbon steel, or alloy steel require stress relief of the carbon steel. In this case, the annealing time and temperature specified in the carbon steel standards shall be followed. Engineering and technical personnel must determine whether the heat cycles required for carbon steel will have a negative impact on stainless steel. If negative effects occur, other stainless steel grades may need to be used. Stress relief at low temperatures is usually carried out within a \"safe\" temperature range, with an upper limit of no more than 540°C (1000°F). The figure below shows that, as a result of this treatment, the original stress can be reduced by nearly 40%. The greatest advantage of low-temperature treatment is the absence of a sensitization risk. The graph also shows that medium-temperature treatment at 540–900° (1000–1650°F) can eliminate more stress. However, this temperature range is the range at which carbides and secondary phases precipitate. Therefore, the insulation time must be strictly controlled. Fortunately, the vast majority of the stress is eliminated within the first half hour of reaching moderate temperatures. When performing stress relief at moderate temperatures, it is necessary to consider the cumulative effects of all thermal cycles on the workpiece. The stress relief cycle itself includes heating, holding, and cooling. Before stress relief, the heat-affected zone (HAZ) of the multi-pass weld may have been exposed to this temperature for a considerable length of time. Stress relief in the critical TTS region increases the heat exposure time, leading to the unexpected precipitation of secondary phases and reducing the corrosion resistance and mechanical properties of the heat-affected zone. The vast majority of high-chromium or high-molybdenum high-performance austenitic stainless steels form intermetallic phases in a relatively short period of time, so they can only be subjected to low-temperature stress relief treatment. It is worth considering what kind of cooling to use for the components after stress relief. Due to differences in thermal contraction, high cooling rates, such as water cooling, may re-create stress. It is very difficult to cool large, complex components with different cross-sections without generating new stresses. For treatments carried out within specific sensitivity time and temperature ranges, it is a good approach to force air cooling down to the lower limit of the precipitation temperature, followed by natural air cooling. Under normal circumstances, natural air cooling after low-temperature stress relief treatment is sufficient. Before stress relief treatment, the surface treatment of stainless steel requires the same as that for solution annealing: contaminants must be completely removed. Low-temperature stress relief treatment does not produce a large amount of oxide scale like solution annealing, but it does cause surface oxidation. If the surface turns golden or blue after treatment, pickling is required to restore its corrosion resistance.