PWHT Requirements for Materials Used in Equipment Designed to ASME Section VIII Div 1

UCS-56-1 to UCS-56-11 of Table UCS-561 provide the requirements and exemptions for materials used in equipment designed according to ASME Section VIII Div 1. These tables also list exemptions.

PWHT (Powder Waste Heat Treatment) is a controlled process in which materials are heated above their lower critical transformation temperature for an extended period, to alleviate residual stresses and enhance weld metal toughness.

PWHT Requirements

PWHT requirements for pipe materials vary across codes, with specific exemptions likely based on traditional practices in specific industries rather than any specific metallurgical or structural considerations.

Even though PWHT requirements vary between various codes, it’s likely that a unified requirement could be established for steel compositions – creating more uniform production across industries. Current codes list temperature ranges based on group numbers and P-numbers of each steel composition as governing criteria for PWHT requirements.

Current PWHT requirements are based on an assumption that preheat temperatures will temper weldment microstructure and diffuse hydrogen (11). However, tests conducted by EPRI have demonstrated that PWHT temperature used in petrochemical applications does not accomplish these goals, as higher preheat temperatures cause undesirable changes to impact strength and ductility; additionally the current code limitations preclude using low-alloy steels for power generation applications.

Exemptions

The ASME code offers certain exemptions to its impact test requirements for carbon and low alloy steels. By taking advantage of these exemptions, you can avoid incurring the expense for this testing.

To determine whether you qualify for an exemption from ASME VIII Div 1’s UCS-66 chart, consult Figure UCS-66 and locate where your MOC falls within its curve and meets up with -20 degree F temperature – if this cross point falls above curve D then an exemption can be granted from impact testing.

As pressure vessel and boiler fabrication processes use arc welding processes that expose carbon and low alloy materials to subzero temperatures, carbon-low alloy combinations may experience losses in ductility caused by weld residual stress that result in brittle fracture. To mitigate these failures, the code offers exemptions for impact testing based on colder MDMT-thickness combinations that are less likely to experience such losses in ductility due to weld residual stress.

Preheat Temperatures

Preheat temperature requirements depend on the material being welded, welding codes and its desired application. It’s important to understand what this means for your project: Preheat temperatures provide ideal thermal conditions for welding by decreasing shrinkage stresses in weld and heat affected zones (HAZs), decreasing hydrogen generation/dissipation cycles, increasing weld metal toughness and decreasing distortion caused by thermal shock.

Welding codes specify minimum interpass temperatures to control weld metal microstructural development and guarantee that resulting welds match those created during procedure qualification.

Local post-weld heat treatments may not be necessary if welds are performed according to their pre-heat temperature and the welding process continues uninterrupted, such as groove and fillet welds less than 1/2 in width and 11/4 in thickness respectively. Furthermore, multi-pass welding techniques may make local postweld heat treatments unnecessary.

Preheat Time

If you’re planning on cooking something that requires an extended cook time, such as lasagna or turkey, it is essential that you preheat your oven. Doing this will ensure that the food cooks evenly without overdoing it. Some recipes may not require preheating; always consult your recipe beforehand!

The ASME Section VIII Division 1 Code provides a set of formulas and rules derived from industry experience that provide guidance for designing thickensses of pressure vessels with extreme caution and an emphasis on safety. This results in highly conservative designs with superior safety levels.

Preheat temperature and hold time (PWHT) requirements vary slightly between codes, yet remain similar in essence. For instance, most low alloy steels need PWHT treatment, though some used in pipe applications do not need it. Also, different requirements exist for weldments to non-pressure parts as well as for connections or weldments with non-pressurised parts; hence the importance of understanding each code’s PWHT requirements and hold times.