Post Weld Heat Treatment (PWHT) Requirements of ASME B31.3

Current design codes in the piping and pressure vessel industries differ considerably in their post weld heat treatment (PWHT) requirements, with some appearing justifiable while others seeming marginal from a technical viewpoint.

EPRI-sponsored tests suggest that the PWHT requirement for P No 4 materials should be reduced, and this paper seeks to support that change within B31.1, B31.3 and other Code Sections.

Preheat Temperature

Preheat temperature is the minimum temperature in the base metal of a weldment or, for multi-pass welding operations, between each pass of welding. It must remain constant during all of the operation for at least an equal distance equal to its thickest part thickness; its specifications are listed in WPS (see 3.6 for prequalified WPS limitations and Table 4.5 for essential variable restrictions).

Preheating plain carbon or low alloy steels requires careful temperature regulation to avoid overheating, since an increase in preheating temperature could reduce strength and hardness while too low a temperature could decrease toughness and ductility compared to its original state.

For complex alloy steels, it is crucial that the preheat temperature be set so as to achieve transformation and crystallographic orientation within the base metal. To do this correctly requires accurate calculations of temperature gradients within the weld zone.

Post weld heat treatment (PWHT) is essential to the design and construction of low alloy steel pressure vessels and pipes welded together during assembly processes, in order to soften their metal, reduce residual stresses, increase ductility, and prevent hydrogen cracking. This heat treatment also has additional advantages: making the metal soft while simultaneously decreasing residual stresses and increasing ductility as well as helping avoid hydrogen cracking altogether.

Thickness Limits

The base Code provides a broad range of allowable longitudinal stresses for pipework, as well as an option to increase them for occasional loads. Time-dependent creep properties of material serve as the limiting factor. A material quality factor must also be applied in the case of cast iron and nonductile materials.

In cases of occasional loads, SL is limited to 90% of yield strength at temperature plus a strength reduction factor. This allows designers to employ higher stress levels than would otherwise be permitted under base codes while still guaranteeing safety from sudden load increases due to earthquakes.

On the other hand, the base Code stipulates that stress intensification factors be limited to 1.33 times of basic allowable stress for sustained and occasional loads; however, this leaves certain issues such as the application of these factors up for debate among engineers and programmers of stress analysis programs.

For elevated temperature piping systems, span length is determined by factors including primary longitudinal stress, bending moment and shear forces. An equation that includes these components allows us to calculate these limits for simply supported as well as fixed supports.

Exemptions

As in any industry, adhering to standards and documentation can be challenging, yet compliance and thorough documentation are critical in protecting both employees and customers alike. This is especially relevant in highly regulated industries like power plants, chemical manufacturing or mining where strict compliance standards must be maintained for safe working environments.

ASME B31.3 was recently revised to provide more accurate descriptions of examination, inspection, and testing procedures for process piping systems. Revisions included updating definitions of construction, designated lot, combination welded (COW) pipe post-weld hydrogen bakeout pressure set pressure; progressive sampling requirements were clarified while basis for allowable stresses was increased from 0.67 times yield strength up to as much as 0.80 times yield strength.

Table 331.1.3 has also been modified to exempt some piping from mandatory PWHT requirements based on material and group number, control thickness, weld type and preheat temperature.

Example: Carbon steel pipe that has been welded using the P-No 1 group welding procedure and material may be exempted from postweld heat treatment if subjected to a preheat temperature of 95degC or above; this exception also applies for weld material classified as Elevated Temperature Fluid Service.

Preheat Time

ASME B31.3 is a set of comprehensive standards covering various aspects of process piping systems. This document includes information regarding materials, fittings, joint construction techniques and testing. ASME B31.3 has become an industry standard used to ensure the safety and integrity of piping systems across many different industries.

The 2014 edition of ASME B31.3 featured several significant updates, such as revised preheat requirements, an increase in maximum design pressure for power piping systems and more information regarding inspection and testing procedures. Furthermore, national or international central certification programs may now be used in lieu of ASNT’s own central certification program; while specific acceptance criteria were added for magnetic particle and liquid penetrant examinations with reduced hydrotest pressure while checking for leaks as well as simplified calculation of hydrotest pressure calculations.

This code section mandates minimum wall thickness requirements for pressure piping used in elevated temperature fluid service and specifies its maximum allowable stress, which is determined by multiplying allowable stress by yield strength of material. Furthermore, rules are provided for calculating longitudinal stresses and maximum permitted stress variations, with provisions to determine fatigue properties using either S/N curve analysis or more thorough fracture mechanics analysis methods. Finally, after weld completion and assembly of piping system it must undergo pressure testing using either hydrostatic or penumatical testing techniques – further details in Section 8.4.