Heat-affected zone: meaning and how to check it
The heat-affected zone, or HAZ, is the band of base metal next to a weld or a thermal cut that never melted but got hot enough to change. Its grain structure, hardness or toughness can differ from the plate further away. It sits between the fusion line, where weld metal meets base metal, and the unaffected plate beyond.
Use this diagram
How to see and think about the HAZ
- Etch a cross-section. Cut through the weld, polish the face and etch it with a mild acid. The weld metal, the HAZ and the base metal show up as different bands. A bare weld face won't show the HAZ.
- Find the fusion line. The boundary where the weld metal ends is the fusion line. The HAZ starts there and runs outward.
- Think about the heat. More heat input and slower travel widen the HAZ. Preheat slows cooling, which matters most in hardenable steels where fast cooling can leave a hard, crack-prone HAZ.
- Think about the passes. In a multipass weld, each new pass reheats the HAZ of the passes before it. That's one reason interpass temperature and the order of passes are controlled.
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Definitions and Terminology
A fatigue crack in a cyclically loaded fillet-welded attachment started along the line where the surface of the weld blends into the flange plate. At which weld feature did the crack start?
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Common mix-ups
- HAZ versus weld metal. Weld metal melted and froze again. HAZ never melted. If a defect is in the HAZ, it's in base metal.
- HAZ versus fusion zone. The fusion zone is the melted area, weld metal included. The HAZ starts where melting stopped.
- Cutting makes a HAZ too. Oxyfuel, plasma and arc gouging all heat the edge they cut. Edges prepared this way have a thin HAZ before any welding starts.
- Wider isn't always worse. In some steels a slower cooling rate (and wider HAZ) is safer than a narrow, very hard one. The procedure, not a rule of thumb, sets the balance.
- Hydrogen cracking. Delayed cracks in steel often start in a hard HAZ. Low-hydrogen practice and preheat target exactly that.
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The HAZ on the CWI exam
The HAZ is a term in Definitions and Terminology, at least 12% of Part A, and the metallurgy behind it sits in Base Materials, Heat Treatments and Metallurgy, at least 6% of Part A. Part A is closed book.
On Part B, cross-section etches and destructive test results come up under Destructive Testing and Properties, at least 8% of the practical, and the HAZ is where you look when a test bar breaks beside the weld.
On a D1.1 Part C, preheat and interpass rules that protect the HAZ are in Clauses 5 and 7, and toughness testing, where a contract asks for it, is in Clause 6. Use your own copy for the requirements.
Practice it
The metallurgy cards in the CWI Part A flashcards drill the HAZ, interpass temperature and the heat treatment words around them. Then see how passes stack on the weld pass vs weld layer page, and every term in one table in the welding terms glossary.
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Sources
Show the list
- AWS bookstore, terminology and symbols listing, American Welding Society. Retrieved 20 Oct 2026.
- CWI Part A Fundamentals Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
- AWS B5.1:2025, Specification for the Qualification of Welding Inspectors, American Welding Society, Tables 5.1 and 8.1. Retrieved 20 Oct 2026.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
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