ASME B31 3 Eliminated PWHT Requirements

The 2014 edition of ASME B31.3 Code for Process Piping removed PWHT requirements on carbon steel weld materials provided that multi-pass welding techniques and temperatures of at least 95oC (200oF) were employed, as determined through fracture mechanics tests and analyses. These changes reflect fracture mechanics analysis findings.

Preheat Temperature

Preheat temperature is the keystone to successfully controlling cracking, and must be sufficient to avoid oxidation, delay cooling and provide adequate weld metal toughness. There are various approaches to setting it depending on welding code and section thickness requirements.

Welding codes often outline preheat temperature specifications in their welding procedure specifications (WPS) document, so it is crucial that these guidelines are observed. Preheat temperature should be measured directly surrounding a weld joint and through its thickness if possible. Preheating can be accomplished through gas burners, oxy-gas flames, electric blankets or electronic induction heating methods and must be managed carefully to ensure uniform results.

Temperature-indicating crayons like the Tempilstik should be used to keep an eye on preheat temperatures, with verification using thermocouples or contact thermometers to make sure preheating has occurred as intended. Considerations must also be given to the size and availability of heating equipment when selecting a preheat method. Large-production weldments often necessitate banks of heating torches or electric strip heaters, which must also be taken into account. Smaller weldments may be heated using a furnace or resistance heating systems. Preheat temperature should be constantly monitored throughout the weld sequence and specified in your Work Process Specification document to minimize cracking in both the heat-affected zone and base material. To do so, set it lower than minimum interpass temperature to minimize cracking effects on adjacent materials.

PWHT Temperature

Many codes and standards mandate Postweld Heat Treatment (PWHT) of specific weld structures such as steel pipe girth welds. PWHT helps reduce tensile residual stresses in the weld area, temper microstructure, improve ductility and yield strength as well as improve ductility and yield strength. Requirements vary based on material type and thickness – with carbon-manganese steels typically having higher PWHT temperatures compared to lower-carbon weld materials.

PWHT is an integral step in fabricating a piping system, but can be both costly and time consuming to complete. To minimize costs associated with PWHT, optimizing temperature settings is key – this includes selecting appropriate heating/cooling rates as well as using appropriate equipment and facilities for PWHT.

Current design codes in the pressure vessel and piping industries mandate PWHT welds when their thickness exceeds a set threshold, determined based on Charpy energy absorption test properties of base metals as well as service temperature requirements. This practice has been prevalent for decades and can be justified from both an economic and technical perspective.

There are various arguments in support of lowering the PWHT temperature, and in this paper we will examine those arguments and demonstrate why they are flawed. This white paper will be submitted as part of an initiative for changes to B31.1 and B31.3 code sections; however it could be used similarly with other Boiler and Pressure Vessel Code Sections as well.

PWHT Time

ASME B31.3 provides guidelines for the design, fabrication, assembly, and erection of process piping systems. It serves to guide various parties involved – designers, owner’s inspectors, fabricators, erectors and component manufacturers among them – in this process. Furthermore, this standard covers topics like material selection, allowable stress limits and examination requirements among many others.

As part of the welding process, residual stresses are created in the weld area that may approach yield strength of weld material, increasing chances for environmentally assisted cracking (EAC). PWHT reduces these residual stresses to help avoid EAC in welded steel structures.

PWHT time is an integral component of successful weld heat treatments. The longer that PWHTs are held, the better their results are likely to be; furthermore, using higher temperatures increases effectiveness of PWHT.

EPRI’s Repair and Replacement Applications Center recently issued a recommendation that PWHT temperature requirements be reduced in B31.1 and B31.3 code sections in order to align more closely with requirements in other code sections. This paper will explore why such changes are necessary and their possible benefits, as well as outline methods by which this change can be pushed through committee. Phil Flenner of Flenner Engineering Services Kalamazoo Michigan for helping push these changes through.

PWHT Thickness

Codes that regulate the fabrication, assembly, erection, examination, inspection and testing of process piping contain diverse PWHT requirements that vary significantly in terms of minor variations and potential safety concerns. Significant discrepancies could compromise a project’s ability to meet service conditions as well as cost and performance goals; the specifications in each code tend to depend on material specifications as well as welding parameters as well as any likely failure mechanisms in its structure.

PWHT for carbon steels has been stipulated in several standards and codes, such as BS 1113 [22], BS 2633 [24], PD 5500 [25] and Pr EN 13445 [27]. Recently, B31.3 Code added Table 331.1 that exempted certain P No 1 carbon steels from mandatory post weld heat treatment requirements provided preheat temperatures of 95degC were utilized and multipass welding techniques used during fabrication.

Tomerlin et al conducted a recent study which demonstrated that for high-quality welded joints, PWHT is often unnecessary, even for thicker walled pipes. Care must be taken when designing, fabricating, erection and operating structures as appropriate PWHT temperatures must be selected for maximum risk reduction and endurance resistance. Achieve optimal temperatures will minimize any risk of degrading material properties and degrading its toughness or strength as well as fatigue resistance issues.