The post-welding introduction of heat to the weldment, to remove or improve conditions brought about by the heat of welding. Reduction in grain size, surface hardening, annealing or normalizing, or stress relief are all within the capability of correct heat treatment.
In most shops, post weld heat treat (PWHT) is accomplished in a heat treat furnace with controlled temperature modes allowing for temperature increase, hold-at-soaking temperature, and controlled cooling rate. For small weldment made by oxyacetylene welding, the torch flame can be used for heat treating. For field projects, two heating processes are available: exothermic and electrical resistance. Each method has advantages as well as limitations for use as a heat treating process.
Exothermic. Exothermic materials are special combustible materials which burn under controlled conditions. They are commercially available in molded shapes and flexible lengths that can be stored and cut to fit as needed. The exothermic material is wrapped around the weldment, such as a pipe joint, and wired in place. Then a flame is applied to the material and it burns rapidly, giving off large quantities of heat. When the temperature reaches a predetermined point, determined by experimentation, the exothermic material is completely consumed. Cooling of the joint is controlled by the thickness of the insulation backing on the form. After cooling, the wrapping wires are cut and the material is removed from the joint. In recent years, however, the use of exothermics for PWHT has fallen off sharply due to environmental and thermal control considerations.
Electrical Resistance. Post weld heat treatment using resistance heating involves wrapping the joint with a number of resistance heaters. Advanced electrical resistance systems with automatic controllers make it possible to heat treat several weldments simultaneously.