Welding Inspection & Metallurgy API ICP Self Study Notes

The heat-affected zone (HAZ) is adjacent to the weld and is that portion of the
base metal that has not been melted, but whose mechanical properties or
microstructure have been altered by the preheating temperature and the heat
of welding. There will typically be a change in grain size or grain structure
and hardness in the HAZ of steel. The size or width of the HAZ is dependent
on the heat input used during welding. For carbon steels, the HAZ includes
those regions heated to greater than 1350°F (700°C). Each weld pass
applied will have its own HAZ and the overlapping heat affected zones will
extend through the full thickness of the plate or part welded.