Standard Heat Treatments for Fabrication Processes
|Publication Date:||1 January 2017|
This Standard provides requirements for heat treatment of piping assemblies that meet the requirements of ASME B31 Code Sections. These requirements apply to
(b) postweld heat treatment (PWHT)
(c) postforming heat treatment (PFHT) required by the ASME B31 Code Sections for other fabrication processes, including forming processes such as bending
(d) heat treatments required by contract specifications
The heat treatments addressed in this Standard include weld preheating, PWHT, PFHT, and heat treatment in order to enhance material properties resulting from fabrication. Weld preheating is generally required on hardenable steels in order to reduce local hardness or to reduce the occurrence of hydrogen cracking. Preheat is typically required based on both hardenability (as indicated by the P-Number) and thickness at the weld. A minimum temperature for all materials prior to initiating a weld is expected to be needed to reduce the possibility of condensation. It should be noted that weld preheat is not usually considered a heat treatment, but it is a requirement of the ASME B31 Code Sections.
PWHT is generally required on hardenable steels in order to temper any hardened areas or to reduce residual stress in the weldment. PFHT may be used to address reduction of the properties within the bend after hot bending or to address the effects of strain after cold bending (the additional strain may cause a degradation of creep rupture properties when the operation is at high temperatures). In some cases of PWHT, PFHT, or material heat treatments, non-standard heat treatment may be required by the contract specification, e.g., an austenitizing heat treatment or a solution heat treatment.
This Standard is intended to provide consistent code heat treatment rules. It may be incorporated by reference in a code, or it can become a basis for code or contract requirements.
Appendices provide more-specific controls that may be needed or desired for specific applications. These specific controls are not mandatory unless specified.
Appendices are also included that outline alternative methods to exempt PWHT of welds when PWHT is difficult or impossible. The alternatives are required to be addressed within Welding Procedure Specifications (WPS) qualified in accordance with ASME Boiler and Pressure Vessel Code (BPVC), Section IX.
The requirements for the heat treatments done in accordance with material or product specifications are not addressed within this Standard. Such heat treatments are governed by those specifications.