A fusion weld changes material beyond the region that actually melts. The surrounding heat-affected zone (HAZ) experiences a thermal cycle that can alter microstructure and properties while remaining solid.

Peak temperature and cooling rate vary with distance from the fusion boundary. Consequently, the HAZ is not one uniform material.

Possible effects include:

• grain growth in regions heated strongly;
• phase transformations during heating or cooling;
• tempering or overaging of previously strengthened material;
• dissolution or coarsening of precipitates;
• local softening, hardening or loss of toughness.

For steels, a sufficiently rapid cooling cycle can create hard transformation products in some compositions; slower cooling can produce different phase mixtures. For precipitation-strengthened alloys, welding can locally erase the heat treatment that produced the original strength.

Heat input, plate thickness, joint restraint, preheat and surrounding temperature all affect the thermal cycle.

The practical lesson is that base-material properties cannot simply be assigned to the whole welded joint. Weld metal, HAZ subregions and unaffected base material can have different microstructures and failure behavior.

Welding procedure design therefore includes control of thermal history, not merely deposition of enough material to fill the joint.