Heat-affected zone (HAZ)

The heat-affected zone sits between two regions that get most of the inspection attention — the weld metal itself, and parent material far enough away to be unaffected — and it is often where a joint actually fails. Nothing in the HAZ melted, so it carries none of the porosity or fusion defects that visual and volumetric inspection look for in the weld metal. What it carries instead is a microstructure gradient: material closest to the fusion line saw the highest peak temperature and the fastest cooling, material furthest away saw the least of both, and the properties change continuously across a band that is rarely more than a few millimetres wide.

Two failure modes are specific to this zone. Grain coarsening and martensite formation occur when the cooling time from 800 to 500 °C is too short for the material’s hardenability, leaving a hard, brittle region with reduced toughness — the reason preheat temperature and carbon equivalent are assessed together before welding starts. Sensitization, specific to austenitic stainless steel, is the opposite mechanism: exposure in a mid-range temperature band precipitates chromium carbides at grain boundaries, depleting chromium locally and leaving the zone vulnerable to intergranular corrosion even though its hardness looks unremarkable. Both mechanisms are driven by the same underlying variable — how long the material spends at temperature — reached from opposite directions.

Because none of this is visible on the surface once the joint has cooled, HAZ condition is usually inferred rather than seen directly: from a hardness traverse and impact tests on a procedure qualification sample, from the welding parameters that produced it, or from a thermal record of the cycle the material actually experienced. A real-time thermal monitoring system captures that cycle on every production weld rather than only on the qualification coupon, which is the practical argument for reading hydrogen cracking risk from measured thermal history instead of from the welding parameters alone — a monitored cycle shows what the HAZ actually experienced, not what the procedure intended.