12 Weld Crack Types: Causes, Detection and Fixes
Weld cracks can form while metal solidifies, after it cools, during heat treatment, or in service. Names such as solidification crack describe how a crack forms; toe, root, and transverse crack describe its location or direction. Use both kinds of information to plan inspection and find the cause before choosing a repair.
12 weld crack types at a glance
These are useful shop-floor and engineering terms, not twelve mutually exclusive failure mechanisms or an exhaustive classification. One crack may fit several rows. The clues below help you decide what to investigate; they do not confirm a diagnosis.
Swipe sideways to read the full comparison.
| Crack type | Typical clue | Investigate next |
|---|---|---|
| 1. Solidification crack | Crack within weld metal, often near its centerline | Weld chemistry, solidification pattern, bead shape, and restraint |
| 2. Crater crack | Crack at a weld stop, sometimes star-shaped | End-crater shape and the weld termination sequence |
| 3. Liquation crack | Crack near the fusion boundary or in reheated weld metal | Alloy, thermal history, and evidence of local grain-boundary melting |
| 4. HAZ hydrogen crack | Steel cracks beside the weld after cooling; detection may be delayed | Hydrogen sources, heat-affected zone hardness, and stress |
| 5. Weld-metal hydrogen crack | Crack inside susceptible steel weld metal, sometimes transverse | Deposited metal, hydrogen control, cooling, and restraint |
| 6. Underbead crack | Crack beneath a weld bead, usually in the HAZ | Subsurface extent; hydrogen-assisted cracking is a leading concern in steels |
| 7. Toe crack | Crack where the weld face meets the base metal | Formation time, local profile, HAZ condition, and service loading |
| 8. Root crack | Crack at or extending from the weld root | Root geometry, fusion, restraint, and the actual cracking mechanism |
| 9. Transverse crack | Crack running across the weld direction | Material and timing; orientation alone does not explain the cause |
| 10. Lamellar tearing | Stepped cracking in rolled plate beneath a restrained joint | Through-thickness strain, inclusions, and plate ductility |
| 11. Reheat crack | Crack associated with heat treatment or hot service in a susceptible alloy | Alloy condition, heat-treatment records, and grain-boundary damage |
| 12. Stress-corrosion crack | Cracking in a specific material/environment combination under tensile stress | Service chemistry, temperature, stress, and crack morphology |
Before grinding or welding over a suspected crack: identify and hold the affected part, preserve photographs and inspection evidence, and involve the responsible welding or inspection authority. A thin surface line does not reveal the crack's depth or remaining load capacity.
How should you describe a weld crack?
Write down what you can observe before assigning a cause. This gives an inspector useful information without turning an early guess into a repair decision.
- When was it first found?
- During welding, after cooling, after heat treatment, or after service? The discovery time is not always the formation time.
- Where is it?
- Inside the weld, at its toe or root, in the heat-affected zone (HAZ), or farther into the base metal? The HAZ is nearby metal changed by welding heat.
- Which direction does it run?
- Longitudinal means along the weld; transverse means across it. Record branching, length, and position relative to a clear reference point.
- What conditions surround it?
- Record the material grade, thickness, filler, joint, welding procedure, temperature history, and any service loads or chemicals.
An indication is a test response, not automatically a crack. Surface shape, scratches, or testing conditions can produce misleading indications. Once evaluated, the finding is assessed against the applicable acceptance criteria. Do not invent an allowable crack length from a photograph.
What causes hot cracking in welds?
Hot cracking occurs at high temperature when the metal cannot accommodate the strain imposed on it. Solidification cracks form in newly freezing weld metal; liquation cracks involve local melting in material beside a weld or reheated by another pass.
1. Solidification cracks
As a weld freezes, the last liquid between growing grains can be weaker than the surrounding solid. If contraction and restraint pull this region apart before it becomes strong enough, a crack forms. It often follows the weld centerline, but a centerline appearance alone is not proof.
What to check: base-metal and filler chemistry, contamination, weld shape, fit-up, travel speed, and restraint. The effective weld composition includes both melted base metal and filler. A wire change therefore needs a review of dilution—the proportion contributed by each—not just its label.
2. Crater cracks
A crater crack forms in the final pool at a weld stop. An underfilled or concave end crater can leave too little metal to accommodate shrinkage as it freezes.
What to check: the stop sequence, crater filling, and permitted run-off arrangements. Trial the termination on a representative sample. A smooth repair over the surface is not evidence that an existing crack has been removed.
3. Liquation cracks
Some constituents melt locally during heating, even when the surrounding material remains largely solid. Strain can separate these weakened boundaries near the fusion line or in weld metal reheated by a later pass.
What to check: the exact alloy and condition, filler interaction, thermal cycle, and a metallographic section when needed. A crack beside the weld in aluminum or a nickel alloy should not automatically receive a steel hydrogen-cracking remedy.
Published example: why the alloy and filler both matter
A TWI study used MIG welding on 80 mm 6082-T651 aluminum plate. Its butt welds made with 5356 filler contained HAZ liquation cracks; those made with 4043A did not. This is evidence for that study's material and procedure—not a universal instruction to replace 5356 in every aluminum joint.
The practical lesson is to record the full material–filler–procedure combination before choosing a fix. Read the original TWI study and test conditions.
Mechanism references: TWI on solidification cracking and AWS on weld-crack terminology and crater cracks.
What causes cold or delayed cracking in steel?
A major concern in ferritic steels is hydrogen-assisted cracking. Three conditions act together: diffusible hydrogen, a susceptible microstructure, and tensile stress. A weld can look sound at first and crack after cooling. This explanation should not be transferred unchanged to every metal or every service failure.
4. HAZ hydrogen cracks
These cracks occur in the heat-affected zone, often near the fusion boundary where welding has changed the steel's structure. A hard, susceptible region is vulnerable when hydrogen and tensile stress are also present.
What to check: material chemistry, thickness, cooling history, HAZ hardness where relevant, moisture or contamination, and compliance with the welding procedure.
5. Weld-metal hydrogen cracks
Hydrogen-assisted cracking can also occur inside the deposited steel weld metal. In higher-strength weldments, cracks may run across the weld rather than along it.
What to check: filler classification and storage, deposited-metal strength, hydrogen control, restraint, and thermal practice. A preheat plan designed only to protect the HAZ may not adequately protect the weld metal.
6. Underbead cracks
“Underbead” tells you that the crack is beneath a weld bead, usually in the HAZ. Hydrogen-assisted cracking is a common explanation in susceptible steels, but the name alone is not a complete diagnosis.
What to check: subsurface extent and the steel's hydrogen-cracking conditions. A clear top surface cannot rule out a buried crack.
Does preheating prevent every weld crack? No. Appropriate preheat can reduce hydrogen-cracking risk in steels by changing cooling and helping hydrogen escape. It does not automatically solve solidification cracking, lamellar tearing, or stress-corrosion cracking. Set temperatures and holding conditions through the approved welding procedure, not a universal internet value.
Technical basis: TWI's guides to identifying hydrogen cracks and prevention and temperature control.
What do toe, root, and transverse cracks tell you?
These terms describe position or direction. They help locate an examination, but they cannot tell you which welding parameter to change.
7. Toe cracks
The weld toe is the transition between the weld face and the base metal. A crack there may relate to the HAZ, local stress concentration, or service damage. Fatigue cracking from repeated loading is an important possibility, especially when the component has already been working.
Check the load history and crack origin as well as the weld profile. Repairing the weld without addressing a damaging load or detail can allow the failure to return.
8. Root cracks
The weld root can contain a crack formed during welding, after cooling, or in service. Root shape, local restraint, and incomplete fusion may influence the stress state, but lack of fusion and a crack are different findings.
Inspect the root condition and establish the mechanism before changing the gap or adding another pass. Our weld root gap guide explains the fit-up side of this investigation.
9. Transverse cracks
A transverse crack runs across the weld's direction. It may fit more than one mechanism, depending on the material, temperature history, and loading.
Record its orientation clearly. The inspection plan needs to address the expected crack plane; a setup optimized for longitudinal flaws may not provide equivalent coverage of transverse ones.
Terminology and location reference: AWS Inspection Trends: Weld Cracks.
What causes lamellar tearing, reheat cracks, and stress-corrosion cracks?
Some failures depend strongly on the parent material, later heat treatment, or service environment. Repeating the original weld with slightly different settings may not address them.
10. Lamellar tearing
Lamellar tearing can occur when weld shrinkage pulls through the thickness of rolled plate with limited ductility in that direction. It is associated with inclusions and may follow a stepped path beneath a highly restrained T-joint or corner joint.
What to change: investigate through-thickness material properties, joint arrangement, and shrinkage strain. The solution may require different plate quality or a revised joint—not simply a new operator. A stepped appearance is a clue that still needs confirmation.
11. Reheat cracks
Susceptible alloys can crack during post-weld heat treatment (PWHT) or high-temperature service. In some chromium–molybdenum steels, damage develops at grain boundaries as residual stress relaxes in coarse-grained regions.
What to check: the exact grade, weld and HAZ condition, heating record, and local stress concentrations. Do not assume that every crack found after PWHT was caused by PWHT: it could have existed earlier and escaped detection.
12. Stress-corrosion cracks
Stress-corrosion cracking (SCC) requires a susceptible material, a damaging environment, and tensile stress. The stress may come from loading or remain from welding and fabrication. Fine cracks can develop even when the surrounding surface shows little general corrosion.
What to check: actual chemicals, temperature, exposure history, material condition, and stress. A service crack is not automatically SCC; fatigue and other damage mechanisms must also be considered. A weld repair alone may fail again if the environment remains unsuitable.
AMPP: the material–environment–stress conditions behind SCC.
Which inspection method can detect a weld crack?
Select nondestructive testing (NDT) for the material, access, thickness, and suspected crack orientation. Surface tests and internal tests answer different questions. An NDT indication can help locate a flaw; it does not always establish how that flaw formed.
Swipe sideways to see each method's limitations.
| Method | Useful for | Important limitation |
|---|---|---|
| Visual testing (VT) | Visible cracks, crater shape, location, and surface condition | Cannot exclude fine or buried cracks; lighting and access matter. |
| Penetrant testing (PT) | Surface-breaking cracks in suitable nonporous materials | Does not reveal a closed subsurface crack or establish its depth. Surface preparation matters. |
| Magnetic particle testing (MT) | Surface and near-surface cracks in ferromagnetic materials | Not a general method for aluminum or austenitic stainless steel. Field direction affects sensitivity. |
| Ultrasonic testing (UT), including phased array (PAUT) | Internal planar flaws and, with a suitable technique, location and sizing | Coverage depends on material, geometry, access, calibration, and beam direction. No single setup detects every crack. |
| Radiographic testing (RT) | Internal examination where the technique and geometry are suitable | Tight planar cracks can be difficult to detect when unfavorably oriented to the radiation beam. |
| Metallography or fracture analysis | Crack path, local structure, and evidence supporting a mechanism | Often requires a removed sample. The sampling plan must preserve useful evidence and be authorized. |
When should the weld be inspected?
Use the timing required by the applicable specification and inspection plan. An immediate examination may not detect hydrogen cracks that develop later. There is no single waiting period that suits every steel, thickness, and fabrication code.
Document whether inspection happened before or after heat treatment, repair, and service. If a crack first appears in a later report, compare the earlier method and coverage before concluding it formed later.
Do not polish away the evidence. Grinding or wire brushing can smear metal over a fine crack and make surface testing less effective. Photograph the condition first, then use the preparation specified for the selected examination method.
Inspection references: TWI on penetrant and magnetic particle inspection, phased array and TOFD, and delayed-crack inspection considerations.
Can a cracked weld be repaired?
Some cracked welds can be repaired, but not by simply covering the crack with another bead. Repairability depends on the material, crack extent, remaining section, service duty, and governing requirements. The responsible authority must approve the disposition: repair, replacement, or a permitted engineering assessment.
- Hold the part and preserve the recordIdentify the joint and affected batch. Record photographs with scale, crack position, material identification, and the welding history before altering the surface.
- Determine the full extentChoose suitable examination methods to map the flaw. Do not equate its visible length with its complete dimensions or assume the other side is sound.
- Establish the likely cause and authorize the planReview the evidence with welding, inspection, and materials personnel as appropriate. Agree on acceptance criteria, removal method, inspection stages, and the repair welding procedure.
- Verify removal before reweldingUnder the approved repair plan, remove the affected material and examine the prepared area. Confirm that the remaining geometry and thickness are suitable. A visually clean excavation is not automatically sufficient.
- Correct the cause, then perform the approved repairAddress the demonstrated issue: material, filler, moisture, fit-up, temperature control, restraint, termination, or service conditions. A repair excavation may behave differently from the original joint.
- Inspect and release against the agreed criteriaComplete the required examinations and hold periods, document results, and obtain release. Repeated cracking calls for renewed investigation, not an endless series of identical repairs.
This is a repair-planning sequence, not a welding procedure. It does not set removal dimensions, preheat, filler, heat treatment, or acceptance limits. Those decisions must match the actual component and governing requirements.
How can you reduce cracking in laser welds?
Laser welding is still a fusion process. A narrow bead or small heat-affected zone does not make the joint immune to cracking. For structural steels, TWI identifies weld chemistry, speed, weld shape, preparation, fit-up, and restraint as relevant to solidification-cracking risk.
Match the base metal and filler to the joint
Verify the actual grade, coating, and surface condition. In welding without filler wire, the melted base materials determine weld chemistry. Where filler is used, assess its contribution and dilution rather than assuming any compatible-looking wire will prevent cracks. Use the filler-metal selection guide for a broader explanation.
Evaluate the whole parameter combination
Power, travel speed, focus, and beam movement interact. A change that reduces cracking may also change penetration, fusion, distortion, or surface shape. Compare representative samples and inspect the result instead of treating higher power or slower travel as a universal cure.
Validate starts, stops, and production variation
Include the actual joint gap, clamps, corners, tack welds, and termination sequence in procedure development. A straight coupon does not represent every production feature. The laser welding parameter guide explains which settings to record together.
Laser-specific reference: TWI on avoiding common defects in laser-welded structural steels. Findings for one alloy and process should not be generalized to all metals.
Need help reviewing a laser-welding application?
Send Oceanplayer Laser the material grade and thickness, joint drawing, filler details, process settings, and clear photos with a scale. Include when the crack was first found and any inspection report. This helps define the next trial or evidence needed; a remote photo review cannot certify a weld.
Technical references
These references explain the mechanisms and examination limits discussed above. Apply the current project specification and relevant construction, inspection, or repair requirements to the actual component.
- AWS: Weld Cracks — terminology, position, and crack characteristics.
- TWI: Solidification cracking — formation, chemistry, and weld-shape factors.
- TWI: Hydrogen-crack identification — susceptible steels, location, and timing.
- TWI: Hydrogen-crack prevention — process controls and inspection considerations.
- TWI: Liquation cracks in Al-Mg-Si plate — the attributed 6082-T651 study.
- TWI: Lamellar tearing — through-thickness strain and plate properties.
- TWI: Reheat cracking — susceptible steels and thermal history.
- AMPP: Stress-corrosion cracking — environmental and stress conditions.
- TWI: Surface examination methods — PT and MT capabilities and limits.
- TWI: Advanced ultrasonic examination — PAUT and TOFD principles.
- TWI: Laser welding of structural steels — crack-related process factors.
Oceanplayer Laser · Laser welding, cleaning, and marking application guides.