Welds crack because of some combination of stress, material vulnerability, and temperature. Sometimes the crack forms while the weld is still cooling. Other times it appears days, months, or even years later under service conditions. Understanding which type of cracking you’re dealing with tells you exactly what went wrong and how to prevent it next time.
Cracking During Solidification
The most common type of weld cracking happens while the molten metal is still cooling and solidifying. As the weld pool transitions from liquid to solid, it passes through a “mushy zone” where solid grains are surrounded by thin films of liquid. If the weld is being pulled apart by shrinkage stresses at this exact moment, those liquid films can’t hold, and a crack opens up between the grains.
The composition of the metal is the single biggest factor here. Sulfur and phosphorus are the classic troublemakers in steel. Even small amounts create low-melting-point films along grain boundaries that stay liquid longer than the surrounding metal, widening the vulnerable temperature window. For this reason, controlling sulfur content in both the base metal and filler wire is one of the most effective ways to prevent hot cracking.
Grain shape matters too. When the weld solidifies with long, columnar grains pointing inward from the edges, liquid metal has a harder time flowing between them to fill gaps as the metal shrinks. Equiaxed (roughly round) grains are more forgiving because liquid can move more freely through the spaces between them, essentially healing small cavities before they become cracks. Research has shown that semi-solid metal can tolerate surprisingly large deformation, around 14% strain, before damage begins, but only when enough liquid remains mobile to fill opening gaps.
Why Aluminum Is Especially Prone
Aluminum alloys, particularly the 6xxx series used widely in automotive, marine, and aerospace applications, are notoriously susceptible to hot cracking. The chemical makeup of these alloys is the core problem. Studies of cracked regions in 6061 aluminum welds have found concentrations of alloying elements far exceeding normal levels: silicon at 2.3 times, chromium at 8.1 times, and iron at 2.7 times what the alloy specification allows. These elements segregate to grain boundaries during solidification, creating weak spots.
The rapid heating and cooling rates typical of welding make this worse. High energy concentrated on a small area creates steep thermal gradients, which produce the residual stresses that pull the solidifying metal apart. This is why filler metal selection is critical with aluminum. Using a filler wire specifically designed to offset the base metal’s cracking tendency (such as a silicon-rich 4043 filler with 6061 base metal) changes the chemistry of the weld pool enough to keep it out of the crack-prone composition range.
Hydrogen-Induced Cold Cracking
Unlike hot cracking, cold cracking happens after the weld has fully solidified, sometimes hours or even days later. It’s driven by hydrogen that dissolves into the molten weld pool during welding and then gets trapped as the metal cools. Hydrogen atoms are tiny enough to migrate through the steel’s crystal structure, collecting at areas of high stress. When enough hydrogen accumulates, it creates internal pressure that can initiate a crack, especially in hard, brittle microstructures.
Three things must be present simultaneously: hydrogen in the weld, a susceptible (typically hardened) microstructure, and tensile stress. Remove any one of these and the crack won’t form. This is why preheating works so well. Warming the steel before welding slows the cooling rate, which produces a softer, more ductile microstructure and gives hydrogen more time to escape. For mild carbon steel, minimum preheat temperatures start around 50°C for thinner material, rising to 65°C for plates between 38 and 65 mm thick, and 110°C for anything over 65 mm. Moisture on the steel surface, damp electrodes, and humid air are the primary sources of hydrogen contamination.
Lamellar Tearing in Thick Plates
Lamellar tearing is a distinctive failure that occurs in rolled steel plate, never in forgings or castings. When steel is rolled into plate, non-metallic inclusions (mostly sulfide and oxide particles) get flattened into long, thin layers parallel to the plate surface. If a weld then pulls on the plate in the through-thickness direction, these inclusion layers act like perforated lines on a sheet of paper. The fracture surface has a characteristic “woody,” stepped appearance.
Three conditions must all be present: the weld must create shrinkage strain acting through the plate thickness, the weld’s fusion boundary must run roughly parallel to the inclusion layers, and the plate itself must have poor through-thickness ductility. T-joints and corner joints are the classic setups for this problem, particularly with full-penetration welds. Butt joints are largely immune because the welding stresses don’t act through the thickness.
The risk climbs with thicker, stronger steel. Plates over 25 mm thick with high-strength grades are most vulnerable, and fillet or T-butt joints with leg lengths over 20 mm are where tearing typically shows up. Testing the plate’s through-thickness ductility gives a reliable prediction: steel with a short transverse reduction in area above 20% is essentially resistant, while anything below 10 to 15% should only be used in lightly restrained joints. Steel suppliers can provide plate guaranteed to meet that 20% threshold, and specifying low-sulfur steel (below 0.005% sulfur, treated with aluminum) dramatically reduces the risk.
Reheat Cracking
Some welds survive the initial cooling just fine, then crack when exposed to elevated temperatures during post-weld heat treatment or high-temperature service. This is reheat cracking, and it primarily affects low-alloy steels containing chromium and molybdenum, or chromium, molybdenum, and vanadium. The dangerous temperature range is 350 to 550°C.
What happens is that during reheating, carbide particles precipitate inside the grains, strengthening them. But the grain boundaries don’t get the same strengthening, so when residual stresses try to relax, all the deformation concentrates at the boundaries, and they crack. Not all alloy steels are equally susceptible. Among creep-resistant grades, 0.5Cr-0.5Mo-0.25V steel is known to be highly prone to reheat cracking, while 2.25Cr-1Mo, which offers similar creep resistance, is significantly less susceptible. If your application requires stress relief heat treatment, choosing the right alloy is one of the most important decisions you can make.
Stress Corrosion Cracking in Service
Welds can also crack long after fabrication when exposed to certain environments under sustained tensile stress. Stress corrosion cracking requires three simultaneous ingredients: a susceptible material, a specific corrosive environment, and tensile stress. Welded joints are especially vulnerable because they almost always contain residual tensile stresses from the welding process itself.
In chloride-containing environments, tensile stress causes the protective oxide film at the metal surface to rupture, exposing fresh metal to corrosive attack. Small pits form first, then transition to cracks under the influence of stress. Crack propagation continues because tensile stress weakens the metallic bonds at the crack tip and increases the number of vacancies in the crystal structure, making it easier for corrosion to advance. This mechanism has been well documented in steel alloys used in nuclear and marine applications, where chloride solutions and elevated temperatures (around 180°C) accelerate the process.
Filler Metal Mismatch
Using a filler metal that’s significantly weaker than the base metal creates what engineers call an “undermatched” weld. This doesn’t automatically cause cracking, but it changes how the joint responds to stress in ways that can be dangerous. When a flaw exists in an undermatched weld, the softer weld metal deforms more than the surrounding base metal, concentrating strain at any existing defect. Research by the Naval Surface Warfare Center found that the fusion zone width compounds this effect: as the weld zone gets narrower, the apparent resistance to crack growth decreases.
Overmatched welds (where the filler is stronger than the base metal) are generally safer from a fracture perspective because they push deformation away from the weld and any defects it might contain. But overmatching isn’t always possible or desirable, particularly with high-strength steels where matching filler metals may have their own toughness limitations.
Preventing Cracks Before They Start
Most weld cracking comes down to a handful of controllable factors. Preheating slows cooling to prevent hard, crack-prone microstructures and gives hydrogen time to diffuse out. Using low-hydrogen welding processes or properly stored, dry consumables eliminates the main source of cold cracking. Controlling heat input prevents excessive residual stress and keeps thermal gradients manageable.
Joint design plays a role that’s easy to overlook. Highly restrained joints concentrate shrinkage stresses, so designs that allow some movement during cooling reduce cracking risk significantly. In plate structures, avoiding T-joints and corner joints that load the plate through its thickness prevents lamellar tearing. And selecting base metals and filler metals with compatible compositions, appropriate sulfur levels, and suitable strength keeps the metallurgy on your side.
Cracks are never acceptable in structural welds. The AWS D1.1 Structural Welding Code specifies zero tolerance for cracks in production welds, regardless of size or location. Any crack found during visual inspection, ultrasonic testing, or radiography is grounds for rejection and repair.