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These precautions are necessary because in the as-welded condition the joint is vulnerable to stress-corrosion cracking.
Post-weld heat treatment: Following completion of welding, the temperature of the component should be reduced below 375°F (190°C) in cases where the Ni plus Mn content of the weld filler metal is less than 1.2 percent. In cases where the Ni plus Mn content is greater than 1.2 percent, the temperature of the component should be reduced below 200°F (95°C). This, of course, should be the temperature at the center of the component wall to ensure complete austenite transformation to martensite. The component should then be post-weld heat-treated within 8 hours of the completion of welding. If this is not possible, either:
- The component should be maintained at a minimum temperature of 175°F.
- The humidity of the environment in which the weld is stored should be controlled to guarantee that no condensation can occur at any time (e.g., due to changes in
temperature) on either the OD or ID surfaces of the joint until the post-weld heat treatment can be initiated.Advertisement
An agreement between client and contractor should be established on the best practice to minimize the risk of stress-corrosion cracking occurring on any Grade 91 components. The post-weld heat treatment should be performed within the range of 1,350°F to 1,420°F (730°C to 770°C). The maximum temperature at any point in the PWHT process should not exceed 1,420°F (770°C). The temperature and time at temperature for the post-weld heat treatment should be selected to ensure that the hardness at all locations in the area heated is within the specified range.
No additional limits on the rate of heat-up or cool-down are specified for PWHT. However, for thick-walled components, or for assemblies of complex shape, an appropriate rate of heat-up or cool-down, as determined by experienced engineering judgment, should be adopted to minimize distortion and residual stresses.
It has been shown that significant recovery of the dislocation substructure occurs during creep of Grade 91 steel. Indeed, it is apparent that this recovery is greatly enhanced by creep strain effects since, for the same steels, research has shown that holding for long periods at high temperature in the absence of stress, i.e. aging, has minimal effect on the dislocations. In contrast, significant increases were observed in the subgrain size during creep.
