Current fabrication codes for P-4 and P-5A materials vary considerably in their thickness criteria for exemption from PWHT; further variations also exist.<\/p>\n
Generaly speaking, structural steels used for bridges and buildings may be exempt from PWHT provided their thickness limits meet fracture toughness requirements. Pressure vessel and piping codes often specify different requirements when it comes to PWHT exemption.<\/p>\n
Post-weld heat treatment (PWHT) of thick sections of weldments depends on their material and welding procedure used. PWHT addresses residual stresses created by uneven cooling rates that lead to distortions and cracking in weldments; PWHT reduces these stresses to levels that meet toughness requirements, improving both dimensional stability and performance of these weldments.<\/p>\n
Current design codes for piping and pressure vessels often specify PWHT for weldments that exceed a specified thickness, often related to reference toughness values based on Charpy energy absorption. While PWHT requirements cannot always be reduced rationally, one approach might be to make sure WPS and PQR adequately qualify welding techniques used.<\/p>\n
PWHT processes differ depending on material and weld configuration; generally speaking they involve heating weldments to a soak temperature set by project specifications or welding method parameters, then holding at this level for an agreed-upon duration (typically one hour per inch of weld thickness). Accurate monitoring is key and all weldments must be meticulously inspected following PWHT.<\/p>\n
Preheat temperature is an integral component in controlling the amount of hydrogen introduced into newly deposited weld metal, and increasing resistance against cracking caused by residual stresses.<\/p>\n
Variables related to both material and welding consumable variables affect preheat requirements, such as carbon content. Weldments with higher carbon contents require additional preheating than lower-carbon content weldments; its temperature must also be closely managed to avoid overheating that could cause distortions or defects in the weld joint.<\/p>\n
There have been various methods developed to calculate the appropriate preheat temperature. One such approach, known as hydrogen control technique, uses a carbon equivalent composition parameter and an equation based on diffusible hydrogen levels in filler metals to derive susceptibility index values that allow determination of minimum preheat temperate requirements. Unfortunately, these techniques may not always work in practice; for instance when used for welding butt splices in large sections using AWS D1.1-96 requirements may not provide enough preheating to prevent cracking.<\/p>\n
Post-weld heat treatment (PWHT), commonly known as stress relief, involves heating parts of a welded component above its yield strength for an extended period. This alters its microstructure while also decreasing levels of residual stresses that could potentially cause damage under load.<\/p>\n
PWHT does not effectively reduce residual stresses to zero; even with careful thermal cycling, residual stresses may still exceed 30% of yield strength of material being treated.<\/p>\n
Tensile stresses still have the ability to affect fatigue crack growth rates and the path and shape of crack formation. Their exact magnitudes depend on material characteristics, the temperature of PWHT, soaking time duration, controlled heating as well as support trestles that accommodate their dimensions in order to ensure consistent heating gradients across welded components and eliminate thermal shocks.<\/p>\n
Steels used in power\/process piping applications would benefit from being exempted from PWHT over a wider thickness range, such as aligning limiting thickness requirements in various codes that cover them (e.g. ASME VIII Div 1 and PD 5500). Unfortunately, this task can be daunting given that variations in chemical composition and toughness levels among different grades exist as well as no set absolute value of Charpy impact energy absorption limit thicknesses.<\/p>\n
Current exemptions from mandatory PWHT are determined by factors including carbon content, weldability and operating temperature. Based on research done over time it has become clear that these requirements were originally established based on good engineering practice but are no longer suitable. It is suggested that minimum wall thickness be set at 0.625 inches without regard to carbon content or weld diameter – providing a more appropriate foundation for applying the Master Curve approach to fracture toughness.<\/p>","protected":false},"excerpt":{"rendered":"
Current fabrication codes for P-4 and P-5A materials vary considerably in their thickness criteria for exemption from PWHT; further variations also exist. Generaly speaking, structural steels used for bridges and buildings may be exempt from PWHT provided their thickness limits meet fracture toughness requirements. Pressure vessel and piping codes often specify different requirements when it … <\/p>\n