8 Types of Welding Cracks:Hot, Cold, Crater & More
The fastest safe diagnosis starts with three questions: where is the crack, when did it appear, and which metallurgical mechanism can act in that material? This guide maps eight common crack patterns to their likely causes, suitable inspection methods and prevention controls—without turning one shop-floor rule into a universal welding procedure.
When did it form?
During final solidification suggests a hot-cracking mechanism. After cooling or after a delay points toward hydrogen, residual stress, service or heat-treatment mechanisms.
Where is it located?
Centerline, crater, root, toe, HAZ, weld metal and parent plate each narrow the credible mechanisms and the best NDT orientation.
What changed?
Material heat, filler, fit-up, restraint, consumable storage, heat input, preheat, interpass, PWHT and stop-start technique are all evidence.
Do not weld over a crack
Stop, define the crack extent, obtain an engineering disposition, remove it completely and verify the excavation before an approved repair weld.
What are the main types of welding cracks?
Eight useful shop-floor patterns are longitudinal centerline hot cracks, transverse hot cracks, crater cracks, transverse cold cracks, root cracks, toe cracks, underbead or HAZ hydrogen cracks, and lamellar tearing. These labels are not mutually exclusive. “Hot” and “cold” describe when and how cracking forms; “longitudinal,” “transverse,” “root” and “toe” describe orientation or location.
A transverse crack, for example, may form hot in a restrained weld metal or cold through hydrogen-assisted cracking. Correct diagnosis therefore combines morphology with material, procedure history, temperature timeline and NDT evidence. Liquation cracking and reheat cracking are also important mechanisms; they are discussed separately because they do not fit neatly into a simple eight-pattern field list.
The 8 welding crack patterns at a glance.
Use this as a screening map, not a final disposition. The same visible line can have different causes, and internal cracking may have no visible surface clue.
| Crack pattern | Family | Typical location / orientation | When it forms | Leading mechanism | First verification |
|---|---|---|---|---|---|
| 1. Longitudinal centerline | Hot | Along weld centerline | Final solidification | Weak interdendritic film + contraction strain | VT/PT/MT if open; UT/RT if buried |
| 2. Transverse hot | Hot | Across weld metal | During solidification | Solidification pattern and high restraint | Metallography + suitable surface/internal NDT |
| 3. Crater | Hot | Run stop or arc termination | As terminal pool freezes | Unfilled concave crater under shrinkage | VT plus PT/MT where required |
| 4. Transverse cold | Cold | Across weld metal or HAZ | After cooling; may be delayed | Hydrogen + susceptible microstructure + tensile stress | Timing audit, MT/UT and hardness/procedure review |
| 5. Root | Often cold | At root notch or first pass | After root pass or cooling | Notch restraint, hydrogen or local lack of ductility | UT/RT chosen for geometry; macro if qualified |
| 6. Toe / HAZ | Often cold | Weld toe into HAZ | After cooling or in service | Hydrogen cracking or fatigue at stress concentration | MT/PT, timing and service-history review |
| 7. Underbead / HAZ | Cold | Below bead in hardened HAZ | After cooling; may be delayed | Hydrogen-assisted cracking | UT/MT as applicable; hardness and hydrogen controls |
| 8. Lamellar tear | Base metal | Stepped, parallel to plate surface | During weld contraction | Through-thickness strain + elongated inclusions | UT oriented to planar tear; sectioning if required |
They are sharp planar discontinuities with high stress concentration. Common fabrication standards treat cracking as rejectable, but acceptance, inspection timing and repair requirements must come from the current governing code, contract and engineering authority.
What each crack looks like, why it forms and what to check.
Look for a repeatable combination of position, orientation, timing and process history. Do not infer metallurgy from color alone.
Longitudinal centerline crack
Usually follows the last region to solidify along the weld center. It may be open or buried and often reflects an unfavorable solidification pattern, composition and contraction strain.
- Check
- Weld profile, travel speed, dilution, S/P and carbon, fit-up, contamination and restraint.
- Prevent
- Use qualified filler and parameters, control fit-up, create a feedable pool shape and reduce unnecessary restraint.
Transverse hot crack
Crosses the weld instead of following the centerline. It is less common and needs confirmation that it formed during solidification rather than later through hydrogen or service fatigue.
- Check
- Fracture morphology, weld metal chemistry, solidification direction, joint rigidity and thermal sequence.
- Prevent
- Qualify the weld metal and procedure for the actual alloy and restrained joint.
Crater crack
Appears at a weld stop, often as a star, radial line or short centerline crack. The terminal puddle is too small or concave to carry contraction strain while freezing.
- Check
- Arc termination, crater shape, downslope, filler addition and run-off practice.
- Prevent
- Use the qualified crater-fill function, back-step technique or run-off tab appropriate to the process.
Transverse cold crack
Runs across weld metal or the HAZ after cooling. In ferritic high-strength systems it can indicate hydrogen-assisted cracking, but fatigue and other mechanisms must be excluded.
- Check
- Delay between welding and detection, consumable hydrogen classification, preheat/interpass records, hardness and restraint.
- Prevent
- Use a code-qualified hydrogen-control and thermal procedure for the actual material and section.
Root crack
Starts at the root where notch severity, fit-up, restraint and first-pass conditions combine. It can be surface-connected from the back side or completely internal.
- Check
- Root gap, land, penetration, tack condition, heat input, electrode condition and root accessibility.
- Prevent
- Control joint preparation and first-pass technique under the approved WPS.
Toe / HAZ crack
Initiates where the weld cap meets the base metal. A post-fabrication toe crack may be hydrogen-related; a crack that grows in service may be fatigue driven by geometry, undercut or residual stress.
- Check
- Timing, toe profile, undercut, hardness, stress range, weld direction and prior NDT records.
- Prevent
- Address both metallurgical control and fatigue-detail quality; do not assume every toe crack has one cause.
Underbead / HAZ crack
Forms below the weld in a crack-susceptible HAZ and may not break the surface. The classic condition combines diffusible hydrogen, a hard microstructure and tensile stress.
- Check
- Material carbon-equivalent method required by the code, cooling rate, preheat, thickness, hydrogen source and HAZ hardness.
- Prevent
- Control hydrogen and cooling through a qualified welding procedure—not a universal preheat number.
Lamellar tear
Develops in rolled plate below highly restrained T, corner or cruciform joints. The stepped crack follows elongated inclusions and weak through-thickness ductility.
- Check
- Plate cleanliness, joint orientation, weld volume, through-thickness strain and short-transverse properties.
- Prevent
- Redesign the load path, reduce weld shrinkage, use buttering where qualified, or specify suitable Z-quality plate.
Bead shape records solidification behavior.
This image shows an arc-welded seam for visual context; it is not presented as a confirmed cracked weld.
Image: KOMATSU Ltd / Wikimedia Commons, CC BY-SA 2.1 JP.Solidification cracking is a feeding-and-strain failure.
Near the end of solidification, the remaining liquid may be confined to thin films between growing grains. If contraction strain opens a gap faster than liquid can feed it, a crack forms. Composition affects the freezing range and low-melting constituents; geometry and travel speed affect how the pool solidifies; restraint controls the applied strain.
Warning signs
- Longitudinal centerline or branching crater morphology.
- Occurrence in weld metal at or immediately after solidification.
- High travel speed, unfavorable bead profile, poor fit-up or high restraint.
- Crack-sensitive dilution, filler chemistry or contamination.
Controls that need procedure evidence
Clean the joint, control fit-up, use the approved filler, set a stable bead profile, avoid excessive travel speed and manage the sequence so shrinkage strain does not accumulate at the solidifying tail. A copied depth-to-width ratio is not a substitute for procedure qualification across carbon steel, stainless steel, nickel alloy and aluminum.
Three conditions must overlap.
Hydrogen cracking in ferritic steels is governed by the interaction of available diffusible hydrogen, a susceptible hard microstructure and tensile stress. The threshold changes with material, thickness, heat input, restraint and hydrogen level.
Diffusible hydrogen
Moisture, contaminated surfaces, fluxes, electrodes and process conditions can add hydrogen. ISO 3690 defines how diffusible hydrogen is measured in ferritic steel weld metal.
Susceptible microstructure
Rapid cooling and sufficient hardenability can create a brittle HAZ or high-strength weld metal. Composition and section are only part of the cooling-rate problem.
Tensile stress
Weld shrinkage, restraint, joint geometry and external loading provide the crack-driving stress. A notch at the root or toe can intensify it.
The governing method may consider composition, thickness, hydrogen scale, heat input and restraint. Use the current code or approved engineering method.
Hydrogen cracks can be delayed, but there is no universal “48-hour rule” for every material and joint. Follow the applicable fabrication standard.
Do not invent one rebake temperature or exposure time for every low-hydrogen electrode. Use its classification, packaging and manufacturer instructions.
The formula and limits must match the material and code. It cannot by itself predict a crack-free weld or prescribe a complete WPS.
Screen the most likely crack family.
This tool organizes evidence for a qualified inspector or welding engineer. It cannot identify fracture mechanism from appearance alone and must not be used to release or repair a code weld.
Describe the indication
Choose the closest observations. The screening result updates instantly.
Centerline solidification crack
The timing and centerline location are most consistent with a hot-cracking mechanism during final weld-metal solidification.
Liquation and reheat cracks deserve separate attention.
They may resemble one of the eight visible patterns, but their formation stage and material susceptibility are different enough to require metallurgical investigation.
Forms in the partially melted HAZ or fusion-boundary region when low-melting constituents liquate and strain opens the weakened boundary. Austenitic stainless and nickel alloys can be susceptible. Confirm with location, alloy history and metallography.
Can occur in susceptible alloy weld metal or HAZ during PWHT or later high-temperature exposure. It is linked to grain-boundary strain relaxation and alloy response—not simply to diffusible hydrogen after initial cooling.
Match NDT to crack orientation, depth and material.
ISO 17635:2025 states that NDT selection depends on quality requirements, material, weld thickness, process and testing extent. No single method reliably finds every crack orientation.
Best first step for open crater, centerline and toe indications, plus geometry and process clues. It cannot clear buried cracks.
Finds surface-breaking discontinuities in clean, nonporous materials, including nonferromagnetic alloys. It does not determine full depth.
Finds surface and near-surface indications in ferromagnetic materials. Magnetization direction must be suitable for crack orientation.
Useful for internal planar flaws and thick sections when the sound path and scan plan intersect the crack effectively.
Creates a projected image. Tight planar cracks can be difficult when their plane is poorly aligned to the beam; technique and geometry matter.
PT shows surface-connected indications.
Interpretation still depends on cleaning, dwell, developer, procedure and acceptance criteria.
Image: MARCELANOELIA251 / Wikimedia Commons, CC BY-SA 4.0.
Technique discipline is part of detection.
A qualified procedure controls surface preparation, application, timing, lighting, interpretation and documentation.
Image: Senior Airman Tiffany Trojca, USAF / Wikimedia Commons, U.S. public domain and CC BY 2.0.What to do when a weld crack is found.
Do not hide the symptom. A repair that leaves the crack tip, original mechanism or incompatible thermal cycle in place can fail again immediately.
Stop and preserve evidence
Identify the joint, heat, welder, WPS, time, temperature history and crack location. Prevent unapproved grinding or overwelding.
Map the full extent
Use the qualified NDT combination for surface and internal extent. Record orientation, depth, length and relation to weld/HAZ features.
Find the mechanism
Review material, filler, hydrogen controls, fit-up, heat input, restraint, PWHT and service history. Add hardness or metallography where needed.
Use an approved repair WPS
Excavate completely, verify removal, repair under authorized controls, then perform final NDT at the required time and acceptance level.
The excavation margin, end extension, repair procedure, heat treatment and final inspection are code- and component-specific. Pressure equipment, bridges, pipelines, aerospace parts and ordinary fabrication can have very different authorities and acceptance requirements.
Prevent the mechanism, not just the visible pattern.
A good checklist connects design, materials, consumables, procedure execution and inspection. It does not replace the WPS or applicable code.
Verify grade, thickness, heat treatment, coating, chemistry, hardenability and through-thickness requirements before welding.
Control weld-metal chemistry, strength, ductility, ferrite or crack-susceptibility requirements for the actual alloy system.
Use clean dry joints, correctly stored consumables, sound gas/flux systems and the qualified hydrogen class for susceptible steels.
Apply the approved preheat, heat input, interpass, cooling and any postheat/PWHT—with calibrated measurement and records.
Review fit-up, tack strategy, weld size, sequence, accessibility and fixture stiffness before adding unnecessary weld volume.
Use crater fill, run-on/run-off tabs or the approved technique; inspect starts, stops and repairs as high-risk locations.
For heavy T-joints, consider joint redesign, through-thickness ductility and Z-quality plate under the project specification.
Schedule inspection timing and scan direction for the credible mechanism; retain results for trend and root-cause analysis.
Validate the welding window before production.
Oceanplayer can review a laser-welding application using representative material, joint geometry, thickness, surface state and acceptance criteria. A useful trial records parameter stability and inspects the resulting weld—not only the top-bead appearance.
- Base metal grade, condition and certification
- Thickness, joint type, gap and restraint
- Filler and shielding-gas plan
- Required penetration and production speed
- Applicable welding and inspection standard
- Visual, dimensional and metallurgical acceptance
Welding crack FAQ
Concise answers for welders, inspectors, engineers and fabrication buyers.
What are the eight common types of welding cracks?
A practical eight-pattern list is longitudinal centerline hot cracks, transverse hot cracks, crater cracks, transverse cold cracks, root cracks, toe cracks, underbead or HAZ hydrogen cracks, and lamellar tearing. The categories can overlap because some describe mechanism while others describe location or orientation.
What is the difference between hot and cold welding cracks?
Hot cracks form while weld metal or a partially melted region is solidifying and has low ductility. Cold cracks form after cooling, often through the interaction of diffusible hydrogen, a susceptible hard microstructure and tensile stress in ferritic steels.
What causes a centerline crack in a weld?
Centerline solidification cracking occurs when the last liquid at the weld center cannot feed the separation created by contraction strain. Weld chemistry, pool shape, travel speed, fit-up, contamination and restraint can all contribute.
What causes crater cracks?
A crater crack forms when the terminal weld pool freezes with insufficient section or filler to carry contraction strain. Use the qualified crater-fill, downslope, back-step or run-off technique for the welding process.
How long after welding can hydrogen cracks appear?
Hydrogen-assisted cracking can be delayed until after the weld has cooled. The required inspection delay is material-, joint- and code-specific; there is no universal 48-hour rule for every weld.
Can you weld over a crack?
No. Overwelding does not prove that the crack tip or root cause has been removed. Obtain an approved disposition, map the crack, excavate it completely, verify removal and repair under an authorized WPS followed by required NDT.
Which NDT method is best for weld cracks?
No single method is best for every crack. VT, PT and MT address surface or near-surface indications; UT and RT address internal conditions with different sensitivity to orientation and geometry. Select the combination under the governing standard and qualified procedure.
Are all toe cracks caused by hydrogen?
No. A toe crack found after fabrication may be hydrogen related, but a crack that grows under service cycles may be fatigue driven by toe geometry, undercut, residual stress and load range. Timing and fracture evidence matter.
What causes lamellar tearing?
Lamellar tearing occurs in rolled plate when weld shrinkage loads weak through-thickness regions containing elongated inclusions. Heavy T, corner and cruciform joints are typical risks. Joint redesign and appropriate through-thickness plate quality can reduce susceptibility.
Does preheating prevent every welding crack?
No. Preheat can reduce hydrogen-cracking risk in susceptible ferritic steels by slowing cooling and aiding hydrogen diffusion, but it does not solve every solidification, liquation, lamellar, reheat or service-fatigue mechanism.
What hydrogen level is safe for welding steel?
There is no universal safe number independent of steel, strength, thickness, heat input and restraint. ISO 3690 defines measurement, while the approved welding method or code determines the hydrogen class and thermal controls for the joint.
Are cracks allowed under ISO 5817?
Cracks are sharp planar imperfections and are generally not permitted for common ISO 5817 quality levels. The exact contract, material scope, weld process and current standard revision still control acceptance and repair.
Move from diagnosis to a controlled laser-welding trial.
Use the next resource that matches your decision: machine selection, welding fundamentals or representative sample validation.
Standards and engineering references.
The page uses current standards pages and established welding-institute guidance. Confirm the governing code, edition and project-specific clauses before inspection or repair.
- ISO 5817:2023: quality levels for imperfections in fusion-welded joints in steel, nickel, titanium and their alloys.
- ISO 17635:2025: general rules for selecting and applying NDT to metallic welds.
- ISO 3690:2018: determination of diffusible hydrogen content in ferritic steel arc-weld metal.
- ISO 17641-1:2004: fundamentals and tests for hot cracking in arc-welded weldments.
- TWI — Defects: Solidification Cracking: identification, causes, prevention, detection and removal principles.
- TWI — Hydrogen Cracks in Steels: the hydrogen, microstructure and stress interaction plus prevention principles.
- TWI — Assessing Weldability: solidification, liquation, hydrogen, lamellar and reheat cracking mechanisms and relevant tests.
- TWI — Lamellar Tearing: through-thickness strain, inclusion morphology and design/material controls.
- AWS D14.8M:2009 / ISO/TR 17844: comparison of standardized methods for avoiding hydrogen-induced cold cracking.
- TWI — Common Laser-Welding Defects in Structural Steels: solidification cracking and process-shape considerations in laser welding.