2.2 The Heat-Affected Zone (HAZ)

Defining the Heat-Affected Zone

The heat-affected zone (HAZ) is the portion of the base metal adjacent to the weld that was heated to a temperature high enough to alter its microstructure or mechanical properties but was not melted. Per AWS A3.0 terminology, the HAZ lies between the fusion line (the boundary of melting) and the unaffected base metal. Although it never melts, the HAZ is frequently the weakest and most crack-prone region of a welded joint, which makes it a primary focus of welding inspection. Many failures that look like "weld" failures actually initiate in the HAZ.

A complete welded joint is a thermal map: each point experienced a different peak temperature and a different cooling rate depending on its distance from the arc. Understanding these sub-zones lets an inspector predict where problems concentrate.

Zones of a Welded Joint

Notice the irony: the fine-grained HAZ is actually improved (refined grains, good toughness) because it reached the ideal austenitizing temperature and then transformed back to a fine structure, while the coarse-grained HAZ right next to the fusion line is degraded. The subcritical HAZ, which never exceeded A1, sees no transformation at all but may be tempered or softened — relevant when welding quenched-and-tempered base metals, where it forms a soft band that can lower local strength.

Why the Coarse-Grained HAZ Is the Danger Zone

The coarse-grained HAZ (CGHAZ) immediately adjacent to the fusion line is the most critical region in the entire joint for four compounding reasons:

1. Large grain size — the very high peak temperature caused austenite grains to grow; coarse grains lower toughness and raise hardenability.
2. Fastest cooling rate — being closest to the surrounding cool base metal, this band quenches quickly, favoring martensite.
3. Highest residual stress — thermal contraction concentrates tensile stress here.
4. Hydrogen migration — diffusible hydrogen released by the cooling weld metal migrates toward the CGHAZ.

The combination of coarse grains + martensite + tensile residual stress + hydrogen is exactly the recipe for hydrogen-induced (cold) cracking, the most common and dangerous cracking mechanism in structural steel welding (covered in Section 2.4). This is why the CGHAZ, though invisibly thin, drives so much of welding procedure design.

Factors That Control HAZ Size and Hardness

Heat input is calculated as HI = (Volts × Amps × 60) / Travel speed (in/min or mm/min), giving joules per inch (or per mm). Higher heat input deposits more energy, heats a wider band above the critical temperatures, and slows the cool-down — producing a wider but softer HAZ. This is a common exam relationship that students reverse, so anchor it firmly.

A practical corollary: a stringer bead run fast and cold cools quickly and hardens the HAZ, whereas a weave bead at the same current adds heat per unit length, slows cooling, and softens it — one reason WPSs control bead technique, travel speed, and amperage together rather than in isolation.

HAZ Width by Process

HAZ width scales roughly with heat input, so it varies enormously between processes. An inspector who knows the typical order of magnitude can quickly judge whether a reported HAZ width is plausible.

Electroslag welding (ESW) deposits so much heat that its HAZ can be inches wide with very coarse grains, which is why ESW joints in structural work historically required supplemental impact testing or grain-refining heat treatment. At the opposite extreme, electron-beam and laser welds put so little heat in that the HAZ is barely measurable — but their fast cooling can still harden susceptible steels.

For the Exam: The CGHAZ adjacent to the fusion line is the most susceptible region for hydrogen-induced cracking because it combines the largest grains, the highest hardness, and the highest residual stress. Remember the directional rule: higher heat input → wider, softer HAZ; lower heat input → narrower, harder HAZ. Preheat and low-hydrogen practice are the inspector's main defenses for a hardenable HAZ.