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:
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.
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January/February 2019 Inspectioneering Journal
By 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. |
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May/June 2018 Inspectioneering Journal
By Samer E. Ibrahim at wood
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 technicians, well monitored and controlled temperatures and hydrogen partial pressure. |
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January/February 2018 Inspectioneering Journal
By 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 commentary on the potential reduction in fracture toughness due to PWHT is provided based on a review of published literature. |
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November/December 2017 Inspectioneering Journal
By 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 prevent, mitigate, or repair. |
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July/August 2016 Inspectioneering Journal
By 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. |
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January/February 2015 Inspectioneering Journal
By 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 pressure vessels back in the mid 1940’s. This article will help you by highlighting the main requirements of, and differences between, the hydrostatic test for new pressure vessels fabricated according to ASME Section VIII, Division 1 and the hydrostatic leak test for new piping systems made under ASME B31.3. |
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January/February 2014 Inspectioneering Journal
By 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? |
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November/December 2010 Inspectioneering Journal
By 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. Though HTHA failures in these units are the focus of this article, it is important to recognize that HTHA damage can also occur in high pressure boiler tubes, hydrogen producing units, synthetic gas units, ammonia plants and other equipment where hydrocarbons may not be involved but high temperatures are involved. |
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March/April 2007 Inspectioneering Journal
By 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 correct temperatures and hold times are used. |
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May/June 2005 Inspectioneering Journal
By 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 material being used, and varies from code to code. An alternative procedure is available for deciding whether or not PWHT is necessary to avoid the risk of failure by fracture. This involves conducting a fracture mechanics assessment using procedures such as those in BSI 7910, or API 579. The use of these procedures is permitted in the British pressure vessel standard BS PD 5500:2003. |
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January/February 2005 Inspectioneering Journal
By 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 together in amine treating systems. |
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January/February 2004 Inspectioneering Journal
By 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 for the purpose of prolonging the service life of our equipment and preventing unexpected failures. But this issue is clearly where those old sayings of "Buyer beware" and "You don't get what you expect, you get what you inspect" apply quite often. |
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November/December 2003 Inspectioneering Journal
By 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 always), while SOHIC is a type of cracking that can easily lead to failure and needs to be mitigated. HIC is a form of tiny blistering damage that is mostly parallel to the surface and to the direction of hoop stress, hence is usually not damaging until it is extensive and affects material properties or gives rise to step-wise cracking that propagates into a weld or begins to go step-wise through the wall. |
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November/December 2003 Inspectioneering Journal
By 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 alkaline stress corrosion cracking that often occurs more aggressively at higher pH and higher concentrations of carbonate solutions. |
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November/December 2003 Inspectioneering Journal
By 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 contamination. Sometimes ammonia is added intentionally to process streams as a neutralizer by folks who do not know what it might do to brass tubes. Brass condenser tubes will fail brittlely when bent after they have significant ammonia stress corrosion cracking present. Eddy current inspection of brass tubulars is effective at finding ammonia cracking. Cupro-nickel alloys are usually not susceptible, and if necessary you can upgrade to austenitic stainless steels (which has it's own set of problems). |
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January/February 2003 Inspectioneering Journal
By 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. |
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July/August 1998 Inspectioneering Journal
By 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. |
