Controlled process of reheating materials
post weld heat treatment (pwht) is a controlled process in which a material that has been welded is reheated to a temperature below its lower critical transformation temperature, and then it is held at that temperature for a specified amount of time.
It is often referred to as being any heat treatment performed after welding; however, within the oil, gas, petrochemical and nuclear industries, it has a specific meaning. Industry codes, such as the asme pressure vessel and piping codes, often require mandatory performance of pwht on certain materials to ensure a safe design with optimal mechanical and metallurgical properties.
By samudrapom dam aug 10 2022 this article discusses the post weld heat treatment (pwht) process and its effectiveness in reducing the residual stresses and improving the mechanical properties of weldments. Image credit: funtay/shutterstock. Com.
Comment 5 | share | tweet | share | pin it | print | email have you heard of post weld heat treatment (pwht)? of course you have. If you are involved in welding you have definitely heard this term. But what is phwt? when is pwht required? what temperature should pwht be done at? heat treating of materials can be a very complicated subject, but when it comes to heat treatment after welding it boils down to a simple concept: pwht is done in order to maintain or improve material strength and mechanical properties and to relieve residual stresses.
Holding temperature and time [ edit ]
Post weld heat treatment (pwht), or also called stress relief (sr) is a method for reducing- relieving, and redistributing the residual stresses in the material that have been introduced by welding. Post weld heat treatment is a controlled heat treatment method in that welded components (welded material) are reheated in a furnace or localized heating arrangement to a temperature that is lower than it’s critical transformation temperature (known as ac1).
This is followed by keeping the part at this pre-determined temperature for certain time duration known as holding time. This is followed by slow & controlled cooling in the furnace to allow uniform-controlled expansion & contraction.
The fabrication standards, design details and construction drawings to which the piping , vessel or other structural parts are constructed contains the requirements for post weld heat treatment. The relevant construction codes may refer the requirements stated to aws d1. 1 for structural parts, asme section viii for pressure vessels or asme b31. 3 for piping, as applicable. Considering the process asme b31. 3 clause 331. 1. 1, table 331. 1. 1 and table 331. 1. 2 (requirements for piping typically found in petroleum refineries) an alloy steel piping, p no. 3 group 1 and temperature rage 595 – 650⁰c is 1 hr/25mm (15 min.
After reaching the target temperature, the weldment is held at that temperature for a specified period of time. This is known as the “soaking” or “holding” time. The duration varies based on factors like:
funderburk, r. Scott, 1998. "key concepts: postweld heat treatment. " welding innovation vol xv, no. 2. Ahmed khaleel, krishan j. , 2002. "post-weld heat treatment – case studies," barc newsletter, founder's day special issue, pp 111-115.
The full form of pwht is post-weld heat treatment. Pwht is a controlled process that involves reheating the metal below its lower critical transformation temperature, following a welding process. The material is then held at the elevated temperature for a predetermined period of time to alleviate residual stresses, increase the strength, increase or decrease the hardness, and reduce the risk of cracking by microstructural changes. An array of heating processes can be used to carry out post-weld heat treatment. This article shall cover pwht of welded butt joints, branch joints in carbon steel and low alloy steel materials, as well as all types of welded joints in killed carbon steel materials for low-temperature service to guide technicians, supervisors, and other persons involved in the safe execution of pwht activity so that final result is acceptable.
Date 10-23-2009 16:28 my personal opinions on the subjects are; if the post weld heat treatment is required, it belongs in the wps. Whether it needs to be qualified is a subject that should be addressed by the applicable code. Aws d1. 1 is intended for structural applications where both the carbon content and the carbon equivalencies of the steels are limited. The post weld heat treatments most often encountered are for the purpose of stress relief, but even for that purpose, stress relief is not a frequent operation for structural applications. Even the hydrogen bake-out is not a normal production operation in structural applications.