Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of heat-affected zone (haz). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The HAZ is not melted, but it is changed — the heat cycle alters grain structure and often hardens or embrittles the metal. Its extent depends on the process, the heat input and the material's thermal conductivity.
For machining, the HAZ is where the surprises are: it can be considerably harder than the base metal and it is not uniform, so a weld preparation that cuts cleanly at one point may chatter a few millimetres away.
Common misuse
Assuming a welded fabrication machines like the base metal. The HAZ is often harder and the workpiece carries residual stress that moves it during cutting.