The heat-affected zone is the region of a material whose microstructure or properties are changed by nearby welding or thermal cutting without itself melting. It lies outside the fusion region and can contain different subregions corresponding to different thermal histories.[ref]
What is the Heat Affected Zone? by TWI. Definition and thermal-cycle mechanism
In a steel, heating and cooling can change phases, grain size and hardness; in another alloy the thermal cycle may soften a previously strengthened region. Heat input, travel speed, geometry and thermal transport determine the affected region together. Measuring a weld therefore requires examining both the joint and adjacent material. Surface heat-tint colours alone do not uniquely identify the extent of the metallurgically altered region.[ref]
What is the Heat Affected Zone? by TWI. HAZ regions, property changes and distinction from heat tint
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The heat-affected zone (HAZ) is a region in the base material of a workpiece that has been thermally impacted by welding, cutting, or other processes involving heat. While the material in the HAZ has not melted, it has been subjected to temperatures high enough to alter its microstructure and mechanical properties.