PWHT – Post Weld Heat Treatment
The Post Weld Heat Treatment service reduces internal stresses that can form during welding, and restores the micro-structure of the welded metal.
PWHT is typically specified for critical, high-pressure welds performed in manufacturing vessels and welded piping systems. The PWHT is performed as a safety procedure to prevent hazardous, catastrophic component failures from anomalies in metal stress and welded metal properties.
When a weld is performed, the parent material will be at ambient temperatures. When the extremely high temperature of the molten weld puddle cools down, thermal expansion and contraction can result in variable physical stress accumulation in the surrounding metal. Along with stress, the fast cooling of the weld could also change the molecular characteristics of the metal that can make it weaker and brittle. This occurs not only in the immediate weld track, but also in the adjacent heat affected zones (HAZ).
The original cooling rates that are utilized when making steel pipes and plate contribute greatly to their increased strength. This applies to specialty steels and other exotic materials as well. Temperatures introduced during the welding process can affect the temper and strength of the metal to a point where it can become a major safety concern.
Welding service technicians perform PWHT to counteract and reduce these undesirable and potentially hazardous stress effects by heating the area in and around the weld for a sufficiently long time, and then allowing the weld area to cool (in a strictly controlled manner) giving the micro-structure sufficient time to readjust to its original state, and removing metal stresses.
Tri Tool Service technicians perform PWHT utilizing electrical resistance heating where they wind ceramic beaded heating coils over the weld area. The placements and the temperature produced by these ceramic heaters is finely controlled to exacting standards.
Thermocouples on the heated metal surfaces measure the temperatures throughout the PWHT procedure. These temperatures are recorded as an important part of the service, as this will become part of the project documentation ensuring that the heat treatment service has been accurately and properly performed.
Note that Post Weld Heat Treatment is often referred to along with Pre Weld Heating (as in “Pre and Post Weld Heat Treatment”) but these heating processes are much different. Weld pre heating is the practice of heating up the weld area with electrical resistance heaters or flame torch, in advance of performing the weld. This preventive measure is performed to reduce the large differences in temperature between the weld puddle and the metal in the weld area.
Contact Tri Tool for assistance with any heat treatment services you may require. Our service technicians can combine heat treatment along with our specialty and code welding to comply with any of your project specifications and safety procedures.