A cracked weld is one of the most frustrating things to deal with — especially when the repair looked fine at first. Weld cracks don't always show up immediately. Some appear hours after welding. Others develop over months of service. Here's what causes them and how good shops prevent them.
Cause one
Hot cracking — happens right after welding
Hot cracks form while the weld metal is still cooling from liquid to solid. They happen when the weld pool solidifies with too much tension — either because the joint was restrained, the filler metal composition was wrong, or the cooling was too fast. They're usually visible along the center of the weld bead.
Prevention
Choose the right filler metal for the base material. Control heat input and cooling rate. Reduce joint restraint where possible by allowing the assembly to move slightly as it cools.
Cause two
Cold cracking — appears hours or days later
Cold cracks (also called hydrogen-induced cracking) are sneaky. The weld looks perfect when it cools, then a crack appears 24–72 hours later. The culprit is hydrogen — absorbed into the weld metal from moisture in the electrode coating, the base metal surface, or the air. Hydrogen diffuses through the metal as it cools and causes cracking in the heat-affected zone.
Wild fact
Hydrogen atoms are so small they can literally move through solid steel. Even a tiny amount trapped in the wrong zone can cause a crack that opens up days after the welder has gone home. This is why low-hydrogen electrodes and dry storage of welding consumables matter so much.
Cause three
Underbead cracking — hidden under the surface
Underbead cracks form just below the weld in the heat-affected zone — the area of base metal that got hot but didn't melt. This zone becomes harder and more brittle than the surrounding metal, and if there's hydrogen present and residual stress, it cracks. This type is particularly dangerous because it's invisible without NDT testing.
Prevention
Preheat the base metal before welding to slow cooling and allow hydrogen to escape. Use low-hydrogen electrodes. Apply post-weld heat treatment to relieve residual stress and further drive out hydrogen.
Cause four
Fatigue cracking — develops over time in service
Fatigue cracks aren't caused by the welding process itself — they develop in service, usually at the toe of a weld where the weld meets the base metal. Every cycle of loading and unloading is a tiny stress event. Over thousands or millions of cycles, that stress concentrates at the weld toe and a crack initiates. Poor weld profile makes this worse.
Wild fact
A crack can grow completely invisibly for a long time — extending just a fraction of a millimetre per loading cycle. By the time it becomes visible, it may already be through a significant portion of the weld cross-section. Regular inspection on high-cycle parts is not optional.
Prevention Checklist
Slows cooling, reduces hydrogen cracking and heat-affected zone hardness.
Post-weld heat treatment drives out hydrogen and relieves residual stress.
Finds hidden cracks before the part goes back into service.
Matched to the base material — not just whatever's on the shelf.