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Overview of Heat Affected Zone (HAZ)
The Heat-Affected Zone (HAZ) refers to a non-melted area of metal that has experienced changes in its material properties as a result of exposure to high temperatures. The alterations in material properties are usually a result of welding or high-heat cutting procedures. The HAZ is identified as the area between the weld or cut and the base metal. These areas can vary in size and severity depending on the properties of the materials involved, the intensity and concentration of heat, and the process employed.
What are the Causes of Heat-Affected Zones?
During welding operations, the HAZ may range from small to large depending on the rate of heat input. Welding processes with high rates of heat input (i.e. fast heating) have faster cooler rates compared to welding processes with low rates of heat input (i.e. slow heating) and thus, have smaller HAZs. Conversely, a process with low rates of heat input will result in a larger HAZ. The size of a HAZ also increases as the speed of the welding process decreases. HAZ problems can be mitigated by performing a pre- and/or post-weld heat treatment. Weld geometry also plays a role in the size of the HAZ.
During high-temperature cutting operations, the depth of the HAZ is associated with the cutting process, cutting speed, material properties, and material thickness. Similar to the results of welding processes, cutting processes that operate at high temperatures and slow speeds tend to lead to large HAZs. Furthermore, cutting processes that operate at high speeds tend to reduce the width of the HAZ.
What are the Effects of Heat-Affected Zones?
Because the HAZ experiences sufficient heat for a long enough period of time, the layer undergoes microstructure and property changes that differ from the parent metal. These property changes are usually undesirable and ultimately serve as the weakest part of the component. For example, the microstructural changes can lead to residual stresses, reduced material strength, increased brittleness, and decreased resistance to corrosion and/or cracking. As a result, many failures occur in the HAZ.
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Articles about Heat Affected Zone (HAZ)
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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.
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May/June 2018 Inspectioneering JournalBy 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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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.
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March/April 2007 Inspectioneering JournalBy John Reynolds at Intertek
In the welded condition many stainless steels are susceptible to rapid intergranular corrosion or stress corrosion cracking. This is because the heat from welding sensitizes the base metal heat affected zone (HAZ) and the weld.
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May/June 1995 Inspectioneering JournalBy Bob Browne at ERA Technology
For reason of economy, the hot reheat pipework in many US power plants is fabricated from seam-welded low chrome-moly carbon steel spools. Unlike girth butt welds, where the critical weldment microstructures can off-load stress to the stronger parent material, seam welds are subject to the full pressure hoop stress. A number of failures have occurred, some of which have been evidenced by fast fracture over extended lengths of the seam weld with a massive and violent steam release.
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