Local Post Weld Heat Treatment (PWHT) for Steel Bridge Members

Local post weld heat treatment (PWHT) can be an economical and less-invasive way of mitigating weld residual stresses in large pressure vessels and pipes, helping reduce equipment costs by shrinking its size and decreasing equipment needs.

Local PWHT processes typically consist of three separate zones for soak, heated and gradient control – though there is no standard measurement on their widths.

Cost

In this study, local postweld heat treatment (PWHT) for steel bridge members was examined from both economic and mechanical standpoints. Cost estimates and comparisons for furnace PWHT versus local PWHT treatments were made.

Local PWHT costs were found to be significantly less than furnace PWHT costs; however, its duration varied based on the number of heater units being utilized; for instance if five welded parts around stiffeners are heated using five heater units at one time then it could take five hours of PWHT.

This study demonstrated that local PWHT is more cost-effective than the conventional method, offering significant savings in overall project budget. Furthermore, local PWHT may reduce rework and repair work.

Time

Local PWHT requires less time and cost compared to furnace PWHT, making it the superior option in terms of cost/time considerations. When selecting between these options, however, careful thought must be given as to the number of heater units used as this will influence how long the PWHT process lasts – temperature gradient control bands, soak bands and transfer areas all depend on how many heaters are utilized during PWHT.

Seguridad

Post Weld Heat Treatment (PWHT) is an essential element in mitigating residual stress in pressure vessels and piping equipment, particularly carbon steels exceeding a certain thickness threshold, lower-alloy materials, or any material susceptible to crack initiation and brittle fracture. PWHT may be performed locally within a vessel rather than treating all pressure boundary zones at once; however, improper engineering of local PWHT may lead to distortion and cracking between adjacent welds.

To avoid such problems, advanced computational simulations can be conducted to study the thermal and mechanical response of local spot PWHT configurations. Such models will help ensure that PWHT doesn’t cause distortion, cracking of adjacent welds, or severe residual stresses; additionally, results of these simulations can guide selection of an optimal local PWHT setup.

Local spot PWHT is often done using sheet-type ceramic heaters that can be tailored specifically for each vessel being heated, consisting of electrically heated wires and ceramic pieces encased by thermal insulation materials. Temperature history for each welded region is set up using thermocouples attached to target members – while both heaters and thermal insulation materials are controlled in order to achieve the appropriate heating range – providing more economic and safer results than traditional furnace PWHT methods.

Compliance

Local post-weld heat treatment (PWHT) is a technique for relieving residual stresses in welded structures by heating areas around their weld seam. PWHT may be used to repair cracks in pressure vessels or prevent future structural failure, though its initial cost may be higher compared with production without PWHT; any additional benefits should be carefully evaluated against its associated costs, particularly regarding long-term service life.

The local PWHT method involves placing heat bands either side of a welding seam. Their width can be tailored according to steel’s unique metallurgical properties and modified further for improved weld quality.

Local PWHT differs from conventional welding techniques in that it doesn’t require postweld tempering and can be completed faster, making it ideal for dissimilar thickness structures and successfully relieving stress accumulation during welds. Furthermore, an investigation demonstrated its success at mitigating both hoop and axial stresses through this technique.

This research investigated both the economical and mechanical implications of local PWHT for stiffened plate members in steel bridges. The results demonstrated that local PWHT requires less money and time than furnace PWHT; however, its cost depends on how many heater units are utilized; when one heater unit alone is utilized it could take 25 h to complete PWHT.