Preheating, concurrent heating and postweld heating are important steps in the welding of pipings; and their successful performance in service often depends upon correct heat treatment. The heat treatment procedure includes consideration of the maximum temperature to be attained, time at maximum temperature, rates of heating and cooling, and the width of the heating band. The usual methods of heat treatment are : (a) oxyfuel, (b) electric resistance heating, (c) induction heating and (d) heating in furnace.
In the oxyfuel method, a simple gas torch is adequate for small diameter pipes. For larger pipes and connections, ring burners are more effective. For temperature control, temperature indicating crayons are used. In this method, surface thermometers or electrically operated pyrometers are used to control automatically the current flow to the heating units. Thermocouples are usually attached to the metal to be heated by induction heating. The thermocouple wires are then connected to control equipment, which may automatically control the time-temperature cycle and even program the heating and cooling rates of the metal.
During postweld heat treatment, it sometimes becomes necessary to support the welded pipe sections suitably, to prevent deformation and distortion. This is accomplished in the shop by placing adjustable roller-type supports under the parts being welded as near to the joint as possible, allowing sufficient space for the placement of the heating apparatus over the joint. In field work, where the welds are made in position, chain falls or other suitable rigging secured to the building or other supporting structures are used to accomplish the same objectives.