The thermal cycle induced by welding also causes mechanical effects; in fact, the metal parts that are heated are not completely free to expand or contract, because they are surrounded by other material at different temperature. Consequently this effect causes localized plastic deformation, as a result of internal stresses which are proportional to the self-constraints and the external constraint of the weld.
In practice, an after welding heat treatment has the main function of relaxing the residual stresses within the joint (stress relieving). At the treatment temperature, the material resistance in the vicinity of the fusion zone is not sufficient to resist the internal stresses, which are therefore stretched through mechanisms of plastic deformation, creep and diffusion.
For a given material, the effective stretching is correlated with the PWHT parameters, and in particular with the soak temperature and time. These variables and the methods of treatment carrying out are defined by national and international standards and are generally correlated with very harsh service conditions, in which the designer considers the critical interactions between external stresses and welding residual stresses.
Metalprove has considerable experience in this field: the variety of electrical resistances in terms of shape and size, the use of calibrated thermocouples and efficient insulation materials assure the fulfillment of the requirements.