Various methods are available to reduce the level of residual stresses in welded joints. Heat treatment, overloading, and vibratory treatment can all be used, but the most common method is based on a controlled heating-and-cooling cycle, i. e., thermal stress relief. This technique makes use of the fact that the yield stress of a metal decreases as the temperature is raised. If a welded joint is heated to, say, 600°C, the residual tensile stress, which was equivalent to the yield stress at room temperature, is in excess of the yield stress of the metal at 600°C. Local­ised plastic deformation occurs, and the tensile stresses are reduced. At the same time, the compressive stresses which were in equilibrium with the tensile stresses are also reduced, to restore the equilibrium.

In stress-relieving practice, the temperature is raised until the yield stress has fallen to a low value at which residual stresses can no longer be supported. This clearly depends on the metal being treated, since the relationship between yield stress and temperature is critically influenced by alloy content, and this is reflected in the temperatures recommended in BS 5500: 1976 for the stress-relieving of fusion-welded pressure vessels (Table 5.3).

If thermal treatment is to give a satifactory reduction of residual-stress levels, it is important that differential expansion and contraction must not occur, otherwise new residual stresses will be included. The heating and cooling must be carefully controlled so that the temperature is uniform throughout the component, and special furnaces equipped with com­prehensive temperature-control systems have been designed for this purpose. In these fur­naces the whole of the component of fabrication is heated, thus easing the problem of avoiding temperature gradients. Localised heating for stress relief is usually not recommended, espe­cially with joints in flat plates, since there is always the risk of creating further stresses. In this connection, pipe welding poses particular problems. Stress relieving might often be desir­able to reduce corrosion problems, but it would be impracticable to heat-treat a complete pipework installation. Local stress relief of pipe joints in situ is, therefore, allowed by some authorities, provided that the temperature distribution is controlled. This is usually achieved by specifying the minimum temperature at the joint line and at some specific point remote from the weld a typical example is shown in Fig. 5.23.