Local Post Weld Heat Treatment (PWHT)

Local PWHT can be an effective means of mitigating welding residual stresses in ultra-large pressure vessels when furnace-based PWHT is impractical, but finding a universal calculation criterion for its auxiliary heat band width in current local PWHT standards can be challenging.

After local PWHT treatment is applied to an ultra-large atmospheric tower, residual stresses on its axial and circumferential structures before and after PWHT are compared and the effects of an additional HB width simulation simulated.

Cost

Local post weld heat treatment (PWHT) is an effective means of mitigating welding-induced residual stresses in large and ultra-large pressure vessels. It may be employed when furnace-based PWHT becomes impractical for large components like weld seams in piping systems or those installed on site, although careful planning must take place to ensure an even distribution of heated band width and heat treatment rate; uneven heating or too rapid treatment rates could produce harmful temperature gradients which increase residual stresses when the component cools.

Due to their large diameters and lengths, current local PWHT procedures prescribed in codes and standards can be challenging to apply in the weld area of ultra-large vessels. This paper introduces a primary-secondary heating-based local PWHT method which has proven successful at reducing welding-induced residual stress reduction and suppressing Risk of Stress Corrosion Cracking within the weld zone. Both experimental and finite element analyses were carried out on an AISI 316L stainless steel weld to evaluate this technique’s success.

Apparatus

Local post-weld heat treatment (PWHT) is used to reduce residual stress in welded structures. This process is particularly helpful when placing all components at once in a furnace is impractical, such as for large pressure vessels or ultra-long pipes. PWHT may be more expensive than fabrication without heat treatment but contributes to longer service lives for structures. To be effective, however, heating rates must be managed carefully so as not to exceed yield strength, leading to new residual stresses forming in welded assemblies.

The most efficient means of performing local PWHT involves flexible sheet-type ceramic heaters that can be arranged flexibly, covered by thermal insulation to avoid transmission of heat to adjacent surfaces and connected to an electric power supply that has been programmed to adjust their temperatures according to predefined histories – these heaters can then be monitored via thermocouples on target members and controlled automatically.

As part of the heat treatment process, components require adequate support from their surroundings. To do so, trestles shaped to suit them should be strategically arranged; their spacing will depend on their size, shape, and thickness – this ensures there will be no distortion during treatment.

Time

Time required to complete local post weld heat treatment (PWHT) depends on the number of heater units used. For instance, five welded parts requiring PWHT may require up to 25 hours with one heater unit as they need multiple cycles heated during heating cycles. But by increasing or decreasing the number of heater units used during treatment time can be reduced significantly.

Local PWHTs are used for the tempering and relaxation of residual welding stress, however their effectiveness depends on initial welding residual stress levels. Therefore, before performing one it’s essential to consider initial stress distribution prior to performing local PWHTs; the width of local PWHT auxiliary heating sources can significantly change this distribution of stresses.

A suitable auxiliary heating width for local PWHT should equal or slightly less than the maximum tensile stress in the base plate, and temperature gradient should be limited accordingly; this value can be established by comparing maximum tensile strength of welded part to temperature gradient in local PWHT; 3Rt would be an appropriate limit value in such cases where part thickness exceeds 30mm.

Temperature

Local post-weld heat treatment should only be utilized when placing all components within a furnace is impractical, such as when treating large pressure vessels or ultra-long pipes. Care must be taken to ensure the heating and cooling rates remain constant and accurate, and an even temperature is reached across the thickness of the component. Uneven heating can result in dangerous temperature gradients which create residual stresses which exceed yield stress, leading to failure, mechanical integrity loss or breakage of operating equipment.

By employing local PWHT, hoop and axial stresses of unequal-thickness plates can be significantly reduced, yet there is no uniform calculation criterion to establish the width of an auxiliary heating zone – leading to differing results of residual deformation elimination after PWHT treatment – making determining an ideal width necessary. In order to do this effectively on unequal-thickness plates it is vital that an appropriate width for PWHT be established on unequal thickness plates with dissimilar wall thicknesses using large diameter butt-welded cylinders with dissimilar wall thicknesses. In this study analysis conducted on effects of various criteria on residual deformation reduction results as well as elimination results of residual axial stresses through local PWHT of butted weld joints made up from large diameter butt-welded cylinders joined together using PWHT on large diameter butt-welded cylinders joined together using local PWHT on large diameter butt-welded cylinders with dissimilar wall thicknesses. This research studied effects of different criteria on residual deformation and stress elimination results after local PWHT on large diameter butt welded cylinders joined together, using PWHT of butted weld joints of large diameter butt welded cylinders with dissimilar wall thicknesses of large diameter butt-welded cylinders with dissimilar wall thicknesses via local PWHT when used on large diameter butt welded cylinders with dissimilar wall thicknessed joints, butted weld joints from large diameter butt-welded butt-welded butted weld joints; results were then assessed when applied using local PWHT applied local PWHT on large diameter butted-welded cylinders with dissimilar wall thicknesseds combined PWHT treatment when local PWHT on butted weld joint local PWHT treatments on large diameter butt-welded cylinders with dissimilar wall thicknesses with dissimilar wall thicknesses with dissimilar wall thicknesses with local PWHTs with dissimilar wall thicknesseds butt-welded cylinders using butted weld joints with dissimilar wall thicknesss was reduced via butt welding heat welding through PWHT when butt-welded butt-welded butt-welded butt-welded welded butt weld joints using local PWHT of butted butt-welded butt-welded butted butt-welded butt-welded butt-welded butted butt-welded butt-welded butt weld joints were butt-welded butt-welded butt welding after local PWHT treatments of large diameter butt weld joints of large diameter butt-welded cylinders with different wall thicknesseds with local PWHT using butt weldings with dissimilar wall thickness cylinders of butted butt-welded butted-welded butt-welded cylinders using PWHT when butt-welded cylinders using dissimilar wall thicknesss butt-welded with butted weld joints using butt weld weld welding technique local PW welded welded butt-welded cylinders butt welding on butt-welded butt-weld joints butt-welded joints to butted weld weld joint butt welding technique when butt welding in butt welding using butted butt-welded butt-welded large diameter butt-welded butt welding, with butt welding process using local PWHT treatment due local PW