What is the heat-affected zone?

The heat-affected zone (HAZ) is the region of base metal adjacent to a weld that did not melt but was heated hot enough, and long enough, for the welding heat to change its microstructure. It sits between the fusion zone (the metal that actually melted and resolidified) and the unaffected base metal, and it is graded into sub-zones — coarse-grain, fine-grain, intercritical, and subcritical — based on the peak temperature each band reached during the weld thermal cycle.

Why does preheating help prevent hydrogen cracking?

Preheating raises the base metal temperature before welding, which does two things: it slows the cooling rate of the weld and HAZ, giving diffusible hydrogen more time to escape from the metal before it can build up at trapped sites and initiate cracking, and it reduces the tendency to form large amounts of brittle, crack-susceptible martensite in the HAZ by slowing the cooling rate through the critical transformation range. Both effects reduce the risk of hydrogen-induced cold cracking, especially in higher-carbon or thicker, more restrained steels.

What is the difference between hot cracking and cold cracking in welds?

Hot cracking (solidification cracking) happens while the weld metal is still solidifying, at temperatures near the melting point, when low-melting-point impurity films (sulfur, phosphorus) are pushed to grain boundaries and cannot resist the shrinkage strain of solidification — it typically runs along the centerline of the weld bead. Cold cracking (hydrogen-induced cracking) happens after the weld has fully cooled, often hours later, when diffusible hydrogen combines with a hard, brittle HAZ microstructure and residual tensile stress to initiate a delayed crack, usually in the HAZ rather than the weld metal itself.

Why might the HAZ be softer than the base metal in some steels?

In steels that were originally strengthened by cold work or by quenching and tempering, the portion of the HAZ that reaches an intermediate peak temperature — hot enough to over-temper or partially anneal the existing microstructure but not hot enough to re-austenitize and re-harden on cooling — can end up softer and weaker than both the base metal and the fusion zone. This local soft zone can become the location where a welded joint actually fails under load, even though the weld metal itself may be perfectly sound.

What is post-weld heat treatment for?

Post-weld heat treatment (PWHT), typically a stress-relief anneal below the transformation temperature, serves several purposes at once: it relieves residual stress built up from the weld thermal cycle, which reduces the risk of stress-corrosion cracking and brittle fracture in service; it tempers hard, brittle constituents like untempered martensite in the HAZ, restoring toughness; and for hydrogen-sensitive steels, holding at temperature accelerates hydrogen diffusion out of the joint, reducing the risk of delayed hydrogen cracking after the weld has cooled.

Engineering·12 min read·July 12, 2026

🛠️ Welding Metallurgy & Heat-Affected Zone Effects

A metallurgical guide to welding — what happens to the metal during welding, the heat-affected zone (HAZ) and its microstructural regions, effects on strength and toughness, common weld defects, and preheat and post-weld heat treatment rationale.

📖Materials Science & Metallurgy Handbook (Full Access)

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Welding Is a Metallurgical Process, Not Just a Joining Process

It is tempting to think of welding as simply melting two pieces of metal together. Metallurgically, it is far more consequential: welding subjects a narrow band of metal to one of the most extreme, rapid thermal cycles it will ever experience — heating from room temperature to well above its melting point in seconds, then cooling back down almost as fast — and that thermal cycle rewrites the local microstructure. The strength, hardness, toughness, and cracking resistance of a welded joint are set as much by what happens in the solid metal next to the weld as by the weld bead itself. Understanding that metallurgy is what separates a welding procedure that merely looks good from one that will actually perform in service.

Topics covered

welding metallurgyheat affected zoneHAZhydrogen crackinghot crackingpreheat weldingpost weld heat treatmentPWHTweld defectsweld microstructuregrain coarseningcarbon equivalentweld toughnesssolidification cracking