What is the heat-affected zone of a weld, and why do welded joints fail there?
The heat-affected zone of a weld is the narrow band of base metal alongside the weld deposit that was never molten, but was heated, cooled and metallurgically rewritten by an arc passing an inch away. When a welded joint fails, the failure frequently runs through the heat-affected zone rather than through the weld deposit. A welded joint is not one material but at least three, and practice usually specifies a filler metal at least as strong as the plate it joins, so the weld deposit is often the last part of the joint to give way.
What are the three metals in a welded joint?
A welded joint contains at least three metals: the fusion zone, the heat-affected zone and the unchanged base metal. The fusion zone solidified from a molten pool and has a cast structure, with grains that grew inward from the cold walls of the joint. The base metal beyond the joint is unchanged, still carrying whatever microstructure it was supplied with. Between the fusion zone and the base metal sits the heat-affected zone, which was never liquid but was driven through a rapid heating and cooling cycle that a metallurgist would recognize as an uncontrolled heat treatment.
The heat-affected zone of a weld is narrow — often a matter of millimeters — and it is not uniform across its width. Nearest the fusion line the metal reached temperatures high enough to coarsen the grain; further out it was merely tempered or partly transformed. Each sub-zone of the heat-affected zone behaves differently under load, and a crack tends to find the least favorable one.
Why is the weld metal often the strongest part of a welded joint?
The weld metal is often the strongest part of a welded joint because filler metals for structural and pressure work are commonly selected to overmatch the strength of the base metal, so that yielding occurs in the plate rather than in the joint. Pointing at the weld as the weak link is therefore often wrong. Where the weld deposit is sound and the filler metal correctly matched, the least favorable metal in the welded assembly is usually on either side of the weld deposit.
What does the welding arc do to base metal it never melts?
The welding arc’s thermal cycle can harden or soften base metal it never melts, depending on the steel. Rapid cooling in a hardenable steel can transform the coarse-grained region next to the fusion line into a hard, low-toughness microstructure. The same thermal cycle can do the opposite in a quenched-and-tempered steel, over-tempering a band of parent plate and leaving a softened zone that yields before anything around it.
Both of these heat-affected zone outcomes follow from heat input and cooling rate rather than from manual skill, which is why a weld dispute so often moves from the person holding the electrode to the welding procedure that told them how much heat to use.
What is a hardness traverse, and what does it show about a weld?
A hardness traverse is a line of indentations run across a welded joint — base metal, heat-affected zone, weld metal, and out the other side — and it is the most economical measurement in weld metallurgy. A hardness traverse converts an invisible thermal history into numbers, and the shape of the hardness profile is diagnostic. A sharp peak at the fusion boundary indicates fast cooling and a hardened heat-affected zone; a trough indicates local softening.
Hardness is also the practical screen for susceptibility to hydrogen-assisted cracking, which is why several service standards place hardness limits on welded joints — NACE MR0175/ISO 15156 for sour service being the most cited. A measured hardness outside the limit for the service is a finding in its own right, independent of whether anything in the welded joint has cracked.
What does a macroetch section of a weld reveal?
A macroetch section — made by cutting through a welded joint, polishing the face and etching it — reveals in one image what no external inspection can: the fusion boundary, the extent and shape of the heat-affected zone, the number and sequence of weld passes, the depth of penetration, and the position of any crack relative to all of them.
Macroetch examination is itself a code requirement for certain weld qualification tests, so the macroetch technique is familiar to fabricators and inspectors rather than devised for litigation.
Which weld cracks form outside the weld metal, where a visual inspection will not find them?
Hydrogen-assisted cold cracking, lamellar tearing and reheat cracking are cracks that commonly sit in the heat-affected zone or the parent plate rather than in the weld metal, and a visual inspection of the weld face will not find any of them. Hydrogen-assisted cold cracking commonly sits in the hardened heat-affected zone beneath the final bead rather than in the weld deposit, and it can appear hours or days after the arc was extinguished. Lamellar tearing sits further out still, in the parent plate, following bands of non-metallic inclusions where a restrained joint pulled on the plate through its thickness.
Reheat cracking, in susceptible low-alloy steels, appears in the coarse-grained heat-affected zone during post-weld heat treatment or high-temperature service. What hydrogen-assisted cold cracking, lamellar tearing and reheat cracking share is a location that a visual inspection of the weld face cannot reach.
How do heat input, preheat and interpass temperature affect the heat-affected zone?
Heat input, preheat and interpass temperature affect the heat-affected zone of a weld by controlling how fast it cools: cooling rate through the transformation range is governed by heat input, section thickness, and the temperature of the surrounding steel when the arc arrives. Preheat and interpass requirements exist to slow that cooling, keeping the heat-affected zone softer and giving diffusible hydrogen time to escape. AWS D1.1 sets preheat requirements by material group and thickness for structural steel, and the site records are among the first documents an examiner asks for.
What do welding codes require of the heat-affected zone?
Where the construction code calls for notch-toughness testing, procedure qualification under ASME Section IX takes specimens with the notch in the heat-affected zone as well as in the weld metal — an explicit acknowledgment that the heat-affected zone and the weld metal behave differently. None of ASME Section IX, AWS D1.1 or AWS D1.5 guarantees a particular microstructure in production; they establish that the welding procedure was capable of producing an acceptable one under test. Procedure qualification under ASME Section IX tests the joint as a whole, and AWS D1.1 for structural steel and AWS D1.5 for bridge work impose their own qualification and preheat regimes.
What has to be preserved so a failed weld can be examined?
An investigation of a failed weld needs the welded joint intact with a generous margin of parent plate on both sides, because the heat-affected zone of interest extends beyond the visible weld. Grinding, gouging out a suspect area, or repair welding removes precisely the heat-affected zone microstructure in question, and a repair pass imposes a fresh thermal cycle over the record of the original weld. Where a section must be cut for transport, the cut should be well away from the joint and add no heat to it.
This article on the weld heat-affected zone is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.