Welds fail more often than the plates and pipes they join, and for understandable reasons: a weld is a small casting, made outdoors or in a fabrication shop, inside a heat-treated component, under production pressure. It concentrates geometry, residual stress, microstructural change and potential defects into a few millimetres — which is also why a weld failure, properly read, says so much about how it was made.
The crack's position is the first verdict
- In the weld metal: points to the consumable or the welding itself — solidification cracking, porosity, lack of fusion between passes.
- In the heat-affected zone (HAZ): points to the thermal cycle and the parent material's response — hardened microstructures, hydrogen cracking, liquation effects.
- At the weld toe: the classic fatigue address — the geometric notch where the weld meets the parent surface concentrates every load cycle. Fatigue signatures →
- In the parent metal nearby: may not be a "weld failure" at all — or may reflect residual-stress fields reaching beyond the joint.
The usual mechanisms
- Fabrication defects found late. Lack of fusion, lack of penetration, slag and porosity are built-in crack starters; service load or fatigue finds them, sometimes years later. The investigation asks: was it detectable, was it inspected, was it acceptable to the specification?
- Hydrogen (cold) cracking. Delayed cracking in hard HAZs from moisture-contaminated consumables or inadequate preheat — hours to days after welding. Hydrogen damage →
- Fatigue at toes and terminations. Driven by the joint's geometry and any undercut; the fix is usually detail improvement, not just rewelding.
- Corrosion selectivity. Weld metal and HAZ can corrode preferentially — sensitised stainless HAZs, compositional differences, galvanic effects along the fusion line.
- Repair welds. Repairs of repairs concentrate all the above; an unrecorded repair discovered metallographically is a common plot twist.
What the investigation examines
Macro sections across the joint (profile, passes, penetration, defects); hardness surveys across weld, HAZ and parent (was the procedure controlled?); microstructure against the expected thermal history; fractography of the crack itself; and the paperwork — procedure qualification, welder records, NDT reports — compared with what the metal actually shows. That comparison is often the finding: the weld tells you what was done, the records tell you what was claimed.
When to contact MTIS
When a weld cracks in service on pressure equipment or structure; when new fabrication fails inspection and the acceptability argument needs independent facts; or when recurring weld failures suggest a procedure or detail problem rather than bad luck. Preserve the joint uncut through the crack (preservation guide →) and request an investigation. Related: Manufacturing & Fabrication →