When you weld, the weld metal melts and the metal beside it gets hot, with the temperature falling off with distance. The part of that gradient that changes the metal is the HAZ. You cannot see its edge, but its effects are real.
What happens in the zone
In steel, the metal closest to the weld can be heated high enough to harden when it cools quickly, which makes it brittle. Farther out, heat can soften steel that was hardened or cold worked. In stainless steel, a long stay in a certain heat range can reduce corrosion resistance. In aluminum, heat can reduce the strength gained from heat treatment.
The width of the zone grows with the amount of heat put in, so a welder who controls travel speed and bead size controls the zone. Slow travel, big welds and thick, hot parts make a wider zone.
Why it matters on a repair
Cast iron is the clearest case. It cracks easily in and beside the HAZ, so castings are warmed first and cooled slowly. The same logic applies to hardened shafts and high-carbon steels. Read the entry on preheat and interpass temperature for the usual way of controlling it.
After a repair weld, the HAZ is also a part of the metal that will be machined or loaded, so it is considered when planning how much material to leave. See cast iron welding repair for an example.
Sources
- Heat-affected zone (overview) (2026-10-18)
