Post-weld heat treatment (pwht) is a procedure designed to enhance the mechanical strength and toughness of the weld. It also reduces the amount of tensile welding residual stresses. The procedure requires a number of precautions. One important consideration is the type of equipment used for the process. An automatic temperature recorder is required to ensure proper
calibration.
Calibration should be done at least once a year. A potentiometer is also necessary to control the level of power applied to the coils. A post-heating process is necessary to prevent hydrogen from causing cold cracking on the surface of the weld.
The need for the pwht is driven by either fabrication code or by service environment concerns. The requirement and specification of pwht operation is governed by the objectives as stated bellow, not all the pwht operations are same.
As specified by fabrication code, which generally aimed at tempering and relaxation of residual stress, so as to reduce the susceptibility to brittle failure, and is triggered by material type and thickness. As needed for service requirements, which commonly aimed at reduction of hardness and stress relaxation. The following information is gathered from aws a 10. 10, it condenses the pwht requirements of several codes such as asme b 31.
The following safety precautions shall be provided during pwht: equipment and panels shall be proper ly earthed. Electrical technicians shall work with proper safety wears such as rubber gloves, shoes, etc. Only a certified electrician will work. Joints under pwht shall be well cordoned with red tape/red light and danger display to avoid unknown persons coming in contact with high voltage electrical connections. An adequate platform shall be made for in situ joints to avoid fall of a person. Based on the above, pwht is the critical process needed for welded material, which should be paid enough attention to during production.
if pwht is not performed correctly, or not done at all, residual stresses can exceed a material’s limitations. What happens when design limitations have been exceeded? this can lead to failures, higher cracking potential, and increased susceptibility to brittle fracture. Due to the nature of the industry that mostly uses pwht – the petroleum and chemical processing industry – the designs are usually very large in stature. The manufacturing, welding, and pwht need to happen at one site to ensure the safety of the product and the manufacturing site. Transportation of such large items can pose a dangerous hazard – one that can be avoided by mobilizing units to apply pwht on site.