Also commonly referred to as stress relieving, PWHT is the application of controlled heat applied after the welding is complete. During the welding of metals, the high temperatures and rapid cool down creates stresses in the molecular structure of the parts being welded. When the weld has a PWHT carried out the metallurgical structures and molecules of the weld and parent metal blend together during the heat up, soak period, and cool down to create a finished product that meets the intended design criteria of the parts being heated.
This is achieved by the application of thermocouples, electrical resistance heating elements or induction coils, and insulation being placed around the welded area to achieve a controlled heating temperature, soak period and cooling period. The heating elements and thermocouples are connected to cabling (triple cable sets) that are connected to and controlled by a control console, most control console requires a primary power supply of 480 volts 90 amps 3 phase.
The control consoles on the secondary side have outputs/zones that reduce the voltage output from the primary side through a series of coils/lumps down to 80 volt 45 amps 3.6 Kilowatt per heater being utilized. Making the post weld heat treat operation lower voltage and a safer application for field heat treatment. Each control point/zone can utilize 1,2 or 3 heating elements dependent on the configuration of the parts being post weld heat treated or the code stipulations and control thermocouples that have to be adhered to.
Some of the most prevalent codes that drive post weld heat treatment requirements are:
Some key elements that drive the need for PWHT or Stress relieve are the process the parts being treated will be used in, the metal thickness, and the pressure retaining welds and corrosion resistance in the heat affected zone. All are reasons to carry out post weld heat treatment on a new weld as well as weld repairs on components that have previously been heat treated.
Industry codes and standards for treating are becoming more stringent, Axiom’s knowledge and ability to meet or exceed these requirements are the reason our customers rely on us for a quality driven approach that delivers the maximum return on investment for their heat treatment spend.