Overview of Post Weld Heat Treatment (PWHT)
Welding is an essential part of operating and maintaining assets in the petroleum (upstream, midstream, downstream) and chemical processing industries. While it has many useful applications, the welding process can inadvertently weaken equipment by imparting residual stresses into a material, leading to reduced material properties.
In order to ensure the material strength of a part is retained after welding, a process known as Post Weld Heat Treatment (PWHT) is regularly performed. PWHT can be used to reduce residual stresses, as a method of hardness control, or even to enhance material strength.
If PWHT is performed incorrectly, or neglected altogether, residual stresses can combine with load stresses to exceed a material’s design limitations. This can lead to weld failures, higher cracking potential, and increased susceptibility to brittle fracture.
PWHT encompasses many different types of potential treatments; two of the most common types are post heating and stress relieving:
- Post Heating:
- Hydrogen induced cracking (HIC) often occurs when high levels of ambient hydrogen permeate into a material during welding. By heating the material after welding, it is possible to diffuse hydrogen from the welded area, thus preventing HIC. This process is known as post heating and should begin immediately after the weld is completed. Rather than being allowed to cool, the material needs to be heated to a certain temperature depending on the type and thickness of the material. It should be held at this temperature for a number of hours dependent on the thickness of the material.
- Stress Relieving:
- By the time it’s complete, the welding process can leave a large number of residual stresses in a material, which can lead to an increased potential for stress corrosion and hydrogen induced cracking. PWHT can be used to release these residual stresses and reduce this potential. This process involves heating the material to a specific temperature and then gradually cooling it.
Whether or not a material should undergo PWHT depends on a number of factors, including things like its alloying system or whether it’s been subject to heat treatment previously. Certain materials can actually be damaged by PWHT, while others almost always require it.
In general, the higher the carbon content of a material, the more likely it needs PWHT after welding activities have been conducted. Similarly, the higher the alloy content and cross-sectional thickness, the more likely the material is to need PWHT.
References
- 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.
Is this definition incomplete? You can help by contributing to it.
Share this Topic
Related Topics
Heat Affected Zone (HAZ) Repair Welds Weld Overlay-
January/February 2019 Inspectioneering JournalBy Marc McConnell, P.E. at Pro-Surve Technical Services, and Andrew Bohm at Fulcrum Reliability
Understanding how steel can change and be manipulated is crucial to our ability to spot potentially harmful situations in the field. Whether it is a blade made too brittle or a weld that needs post-weld heat treat, knowledge is power.
-
May/June 2018 Inspectioneering JournalBy Samer E. Ibrahim at Robert Gordon University
HTHA of hydrogen-containing equipment can be prevented with appropriate material selection and fabrication, appropriate welding procedures, regular inspection of equipment using proven, effective technologies and equipment operated by qualified...
-
January/February 2018 Inspectioneering JournalBy Phillip E. Prueter at The Equity Engineering Group, Inc.
Given the concern throughout industry regarding the potential for brittle fracture failures, PWHT guidance to address potential issues arising from the recent changes in PWHT code requirements for carbon steel is examined in this article, and...
-
November/December 2017 Inspectioneering JournalBy Marc McConnell, P.E. at Pro-Surve Technical Services, and David A. Hansen, PhD, PE at Metallurgical Consulting, Inc.
Hydrogen is a common culprit of equipment damage in the process industries. As hydrogen-induced damage can occur in multiple forms, it’s critical to identify the specific damage mechanism you’re dealing with before undertaking measures to...
-
July/August 2016 Inspectioneering JournalBy Mark Geisenhoff at Flint Hills Resources, Jonathan D. Dobis at The Equity Engineering Group, Inc., Phillip E. Prueter at The Equity Engineering Group, Inc., and Dr. Michael S. Cayard at Flint Hills Resources
This article summarizes a recent finite element analysis (FEA)-based study that employs creep simulation techniques to investigate the elevated temperature response of piping with peaked longitudinal weld seams.
-
To Test or Not to Test: A Comparison of the Pressure Testing Requirements between ASME B31.3 and ASME Section VIII, Division 1January/February 2015 Inspectioneering JournalBy Hugo Julien, P.E. at GCM Consultants, and Serge Bisson at GCM Consultants
Are you still hitting the welded joints of pressure vessels with a hammer during hydrostatic testing? If yes, then you’re due for a refresher on the pressure testing requirements of ASME Section VIII Division 1 since this requirement was for...
-
Abnormal Cracks Led to Premature Decommissioning of Boiler Feed Water Exchanger – What Happened and Why?
-
January/February 2014 Inspectioneering JournalBy Hugo Julien, P.E. at GCM Consultants, Serge Bisson at GCM Consultants, and Guy St-Arneault, P.E. at GCM Consultants
Inspections, repairs, modifications, or Fitness-For-Service (FFS) assessments on an old, unfired ASME Section VIII (Div. 1) pressure vessel - Which ASME Section VIII (Div. 1) Code Edition should you use?
-
November/December 2010 Inspectioneering JournalBy John Reynolds at Intertek
High Temperature Hydrogen Attack (HTHA) is a long known and still occurring degradation issue for fixed equipment construction materials in the hydrocarbon process industry where hydroprocess plants (hydrogen plus hydrocarbons) are in service....
-
March/April 2007 Inspectioneering JournalBy John Reynolds at Intertek
When we talk about welding QA/QC we typically focus on the technical requirements and what QA/QC is needed to assure that the technical requirements are met. Examples include the preheat, interpass, and PWHT temperatures and how to assure that the...
-
May/June 2005 Inspectioneering JournalBy Julian Speck at TWI Ltd., and Amin Muhammed at TWI
Current BSI and ASME codes for the construction of pressure vessels, boilers and piping specify that post-weld heat treatment is required if the thickness of the components being welded exceeds a specified value. This value depends on the type of...
-
January/February 2005 Inspectioneering JournalBy John Reynolds at Intertek
Amine cracking is a form of stress corrosion cracking, which is related to alkaline and carbonate stress corrosion cracking. Amine cracking is often intertwined with wet H2S and carbonate cracking, as amines, carbonates and wet sulfides often exist...
-
January/February 2004 Inspectioneering JournalBy John Reynolds at Intertek
Inadequate PWHT is one of our pressure equipment nemeses. We normally specify PWHT for a variety of pressure equipment integrity reasons including when we need to lower residual stresses, increase resistance to cracking or soften weld hardness. All...
-
November/December 2003 Inspectioneering JournalBy John Reynolds at Intertek
There are a variety of forms of wet H2S cracking. In this short article I will focus on two of the most common forms: hydrogen induced cracking and stress-oriented hydrogen induced cracking (HIC/ SOHIC). HIC is often fairly innocuous (but not...
-
November/December 2003 Inspectioneering JournalBy John Reynolds at Intertek
Carbonate cracking (CC) of carbon steel has seen an increase recently in frequency and severity in some refinery cat crackers, especially in fractionator and gas processing overheads. Some gas scrubbing units are also susceptible. CC is a form of...
-
November/December 2003 Inspectioneering JournalBy John Reynolds at Intertek
Ammonia stress corrosion cracking (SCC) has been around a long time. Most everyone has experienced it from time to time. It's not uncommon in brass tubes in cooling water service that is contaminated with ammonia due to biological growths or other...
-
January/February 2003 Inspectioneering JournalBy John Reynolds at Intertek
I already mentioned this common affliction in the introduction. Caustic cracking was long called caustic embrittlement, but since no embrittlement actually occurs that name is fading away.
-
July/August 1998 Inspectioneering JournalBy Constance Reichert at Edison Welding Institute
Visual inspection is the most common nondestructive testing method. For critical applications, machine vision technology provides advantages over visual inspection.
Inspectioneering Journal
Explore over 20 years of articles written by our team of subject matter experts.
Company Directory
Find relevant products, services, and technologies.
Job Postings
Discover job opportunities that match your skillset.
Case Studies
Learn from the experience of others in the industry.
Event Calendar
Find upcoming conferences, training sessions, online events, and more.
Industry News
Stay up-to-date with the latest inspection and asset integrity management news.
Blog
Read short articles and insights authored by industry experts.
Acronyms
Commonly used asset integrity management and inspection acronyms.
Downloads
Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.
Videos
Watch educational and informative videos directly related to your profession.