12 Weld Crack Types Causes, Detection & Fixes
A weld crack is a fracture-type discontinuity, but its visible position is not its root cause. Classify the finding by formation mechanism, timing, location and orientation before choosing NDT or authorizing repair. Grinding and rewelding without diagnosis can reproduce the same crack—or erase the evidence needed to prevent it.
How did the crack form?
Solidification, liquation, hydrogen assistance, through-thickness strain, stress relaxation or environment-assisted cracking each needs a different corrective action.
When did it appear?
During solidification, near ambient after cooling, during PWHT, or after service exposure? “Delayed” describes timing; it is not automatically a separate crack mechanism.
Where does it start and run?
Centerline, crater, HAZ, underbead, toe, root or parent plate—and longitudinal, transverse, branching or stepped—narrow the candidate mechanisms.
Use the governing construction code
Cracks are normally unacceptable, but inspection method, hold time, evaluation and repair authority must come from the applicable code, contract and approved quality plan.
What are the main weld crack types?
The most useful engineering list contains solidification, crater, liquation, hydrogen-assisted HAZ, weld-metal hydrogen, underbead, toe, root, transverse, lamellar tearing, reheat and stress-corrosion cracks. These 12 names are not mutually exclusive. A transverse crack can be hydrogen-assisted; an underbead crack is commonly a HAZ hydrogen crack; a crater crack is usually a terminal form of solidification cracking; a toe crack describes initiation location rather than one unique metallurgy.
For a real investigation, begin with four questions: when was it found, where did it initiate, which direction does it run, and what material/thermal/environmental conditions were present? Then select an examination method that is sensitive to the expected position and orientation. Visual testing alone cannot clear a buried crack, while radiography may miss planar cracks unfavorably aligned to the radiation beam.
Do not treat ASME Section IX as a universal production-weld acceptance standard. It qualifies welding procedures and personnel. The component’s construction code, referencing standard, drawing and contract establish production examination and acceptance.
Quarantine the part, identify the applicable acceptance criteria, protect evidence, establish the full crack extent, review design loading and obtain authorized disposition before cutting, grinding, gouging or welding.
One crack can have four correct names.
Confusion arises because shop terminology mixes mechanism, location, direction and time. Record all four. “Delayed transverse weld-metal hydrogen crack” is far more actionable than “bad crack near the cap.”
Mechanism
Solidification, liquation, hydrogen-assisted, through-thickness ductility failure, stress relaxation, fatigue or environment-assisted cracking.
Answers: why did it form?Timing
During welding, while cooling, after reaching ambient, during PWHT, after repair, or during service.
Answers: when did it form?Location
Weld centerline, final crater, fusion boundary, coarse-grained HAZ, underbead, toe, root, weld metal or parent plate.
Answers: where did it start?Orientation
Longitudinal, transverse, radial, branching, stepped or parallel to the plate surface. Orientation affects NDT response.
Answers: how does it run?
Crack, indication, discontinuity or defect?
An indication is the response produced by an examination method. A discontinuity is an interruption in normal material structure. A crack is a fracture-type discontinuity. A defect is a discontinuity that fails the applicable acceptance requirements.
Inspectors should report the observed indication and characterization without inventing a metallurgical cause. Engineering disposition then applies the construction code and investigation evidence.
It classifies geometric imperfections in fusion welds. Metallurgical mechanisms require additional materials analysis; one classification document cannot replace a failure investigation.
Temperature helps, but mechanism decides the prevention route.
“Hot versus cold” is useful first sorting. It becomes misleading when it is treated as a complete diagnosis. Lamellar tearing, reheat cracking and service-assisted cracking need their own material, stress and environmental questions.
High-temperature fabrication cracking
Weak interdendritic or grain-boundary films cannot withstand strain during final solidification or local partial melting.
- Includes: solidification, crater and liquation cracks
- Look for: centerline, crater branches, fusion-boundary/PMZ cracking
- Control: chemistry, weld-pool shape, filler, heat input, speed and restraint
Hydrogen-assisted cold cracking
Diffusible hydrogen, a susceptible microstructure and tensile stress combine after cooling. Timing may be immediate or delayed.
- Includes: HAZ, underbead and weld-metal hydrogen cracks
- Look for: coarse-grained HAZ or transverse weld-metal cracks
- Control: hydrogen source, hardenability, cooling rate and restraint
Material, heat-treatment or service cracking
Through-thickness strain, stress relaxation during reheating, or an alloy-environment-stress combination creates cracking beyond normal solidification.
- Includes: lamellar tearing, reheat and SCC
- Look for: stepped parent-plate tears, coarse HAZ cracks, branched service cracks
- Control: material, joint design, PWHT and service environment
12 weld crack types and the first clue for each.
This matrix is a screening tool. The final classification may combine entries—for example, “transverse weld-metal hydrogen cracking” or “toe-initiated reheat cracking.”
| # / crack type | Typical position and timing | Leading mechanism or conditions | Useful examination | First corrective direction |
|---|---|---|---|---|
| 1. Solidification crack | Weld metal, often centerline; forms during final solidification | Low-melting interdendritic films plus contraction strain; chemistry, pool shape and restraint interact | VT/PT/MT if open; UT/RT depending orientation and geometry; macrosection | Review filler/base dilution, bead profile, speed, gap, restraint and termination |
| 2. Crater crack | Final depression at pass stop; radial, longitudinal or star-shaped | Terminal weld pool has insufficient volume/strength during solidification shrinkage | VT plus PT or MT where applicable | Use qualified crater-fill/ramp-down, run-off or termination sequence |
| 3. Liquation crack | Partially melted HAZ or reheated weld metal near fusion boundary | Low-melting grain-boundary constituents liquate and separate under strain | Metallography; PT/MT if surface-breaking; targeted UT for larger cracks | Review alloy susceptibility, filler, heat input, interpass, bead placement and restraint |
| 4. HAZ hydrogen crack | Coarse-grained HAZ, often after cooling; may be delayed | Diffusible hydrogen + susceptible hard microstructure + tensile stress | MT or PT for surface; UT for buried planar cracks; hardness and metallography | Recalculate preheat/heat input, reduce hydrogen, control consumables and restraint |
| 5. Weld-metal hydrogen crack | Weld deposit, often transverse in high-strength weld metal; after cooling | Hydrogen and a crack-sensitive weld-metal microstructure under residual stress | MT/PT when open; UT with suitable beam orientation; macro/metallography | Review deposited-metal strength/chemistry, hydrogen class, preheat/interpass and sequence |
| 6. Underbead crack | HAZ beneath a weld bead; commonly hidden | Usually a location form of hydrogen-assisted cracking in hardenable steel | UT or sectioning; surface methods only if it breaks through | Treat as an HACC investigation, not as a separate cosmetic flaw |
| 7. Toe crack | At the weld-to-base transition | Location can concentrate stress; possible HACC, reheat, fatigue or service mechanism | VT/MT/PT; UT for depth; profile and hardness review | Identify mechanism before blending; review toe geometry, HAZ hardness and service loading |
| 8. Root crack | Root or root HAZ, frequently inaccessible | Possible solidification, hydrogen, high restraint, sharp root geometry or service loading | UT/RT chosen for geometry; borescope/VT where accessible; macrosection | Review root opening/face, penetration, purge/backing, restraint and thermal cycle |
| 9. Transverse crack | Runs across the weld or HAZ | Orientation label; often hydrogen-assisted in high-strength weld metal, but other mechanisms exist | UT beam plan must intersect crack plane; MT/PT if surface; RT may be orientation-limited | Determine whether the crack is weld-metal HACC, reheat, fatigue or another mechanism |
| 10. Lamellar tearing | Parent plate beneath T/corner welds; stepped, broadly parallel to plate surface | Poor through-thickness ductility plus through-thickness shrinkage strain and inclusion bands | UT is normally favored; fracture/metallography and Z-direction property review | Redesign load path/joint, specify through-thickness quality, reduce restraint or use engineered buttering |
| 11. Reheat crack | Coarse-grained HAZ or weld metal during PWHT or high-temperature exposure | Stress relaxation at weakened grain boundaries in susceptible alloys/microstructures | UT/MT/PT as applicable; metallography; PWHT record review | Review material susceptibility, impurities, weld detail, bead refinement and heat-treatment cycle |
| 12. Stress-corrosion crack | Service-exposed region under tensile stress; often branched | Specific alloy + environment + tensile stress; weld residual stress may contribute | Surface and volumetric NDT selected for morphology; metallography/fractography and chemistry | Control environment/material/stress combination; do not repair before a corrosion investigation |
“Longitudinal” is an orientation, not a single formation mechanism. Solidification centerline cracks, HAZ hydrogen cracks and weld-metal cracks can all run parallel to the weld. Record orientation in addition to one of the mechanism/location classifications above.
Solidification, crater and liquation cracks.
These cracks form while the weld metal is freezing or while local grain-boundary constituents are partially molten. The cure is rarely “add more heat” or “slow down” in isolation.
1. Solidification cracking: the last liquid cannot carry the strain
During the final stages of solidification, alloying and impurity elements can segregate to interdendritic boundaries and depress the local freezing temperature. If a continuous weak liquid film remains where contraction strain concentrates, the boundary separates. The crack commonly follows the weld centerline but can be transverse when solidification structure and restraint favor that direction.
Risk is affected by base/filler chemistry and dilution, freezing range, weld-pool shape, bead size, speed, penetration profile, gap and external restraint. Fixed sulfur, phosphorus or depth-to-width limits copied from another alloy/process are not universal acceptance rules. Stainless steels, nickel alloys, aluminum alloys and high-speed beam welds each need alloy-specific filler and procedure guidance.
2. Crater cracking: a terminal solidification problem
Stopping the energy source can leave a concave final pool with too little metal to withstand three-dimensional shrinkage. Branching or star-shaped cracks may form inside the crater and extend into sound-looking weld metal. The qualified procedure may use programmed power/current ramp-down, crater fill, back-step, run-off tabs or a controlled termination position. A fixed manual travel distance is not a universal remedy.
3. Liquation cracking: partially melted boundaries beside the weld
Liquation cracks form in the partially melted zone of the HAZ or reheated weld metal when grain-boundary constituents melt at temperatures below the bulk solidus. Thermal contraction or restraint then opens those weakened boundaries. Heat-treatable aluminum alloys, precipitation-strengthened nickel alloys and some stainless/alloy systems can be susceptible.
Because liquation cracks are close to the fusion boundary, they can be misreported as lack of fusion, HAZ hydrogen cracks or solidification cracks. Metallography, knowledge of the alloy’s precipitation/segregation state, and the exact bead thermal history are often needed to separate them.
High travel speed and a deep, narrow keyhole profile can produce a simple centerline solidification boundary and high transverse strain. Beam shaping, oscillation, focus, filler, speed, power and joint fit-up must be qualified as one window.
Hydrogen-assisted cracks require three conditions at the same time.
In ferritic steels, hydrogen-assisted cold cracking occurs when diffusible hydrogen, a susceptible microstructure and sufficient tensile stress coincide. Remove or reduce any leg of this triangle and risk falls.
Diffusible hydrogen
Moisture in electrode coverings or fluxes, contaminated surfaces, wet gas systems, oils, paints, rust and some service histories can supply hydrogen.
- Control: approved low-hydrogen process and consumable classification
- Control: storage, reconditioning and exposure limits per manufacturer/WPS
- Control: dry, clean joint and verified gas delivery
Susceptible microstructure
Alloy hardenability, carbon equivalent, thickness, combined thickness, heat input and cooling conditions determine HAZ and weld-metal hardness.
- Control: material-specific preheat/interpass calculation
- Control: heat input inside the qualified window
- Control: correct filler strength and chemistry
Tensile stress
Weld shrinkage, joint restraint, fit-up force, tack sequence, external loading and stress concentration create the driving force.
- Control: joint detail and assembly sequence
- Control: balanced deposition and realistic fit-up
- Control: avoid loading before required inspection/hold
“Delayed” describes discovery time, not a second mechanism.
Hydrogen cracks may appear soon after welding or later as hydrogen redistributes. The required delay before final NDT is material-, thickness-, process-, code- and project-specific.
4. HAZ hydrogen-assisted cracking
Cracks often originate in the coarse-grained HAZ of carbon-manganese, high-strength and low-alloy steels. They can be longitudinal beside the weld, root-associated in fillet welds, or extend into the deposit. Review the actual heat chemistry, carbon-equivalent method required by the applicable standard, hardness, section thickness, heat input, preheat, interpass and hydrogen classification.
5. Weld-metal hydrogen cracking
High-strength weld deposits can be more crack-sensitive than the HAZ. Transverse cracks may cross the weld at an angle and remain buried. Filler overmatching, high deposit strength, diffusible hydrogen, thick restraint and insufficient thermal control can contribute. The answer is not automatically a weaker filler; design strength, toughness and procedure qualification still govern.
6. Underbead cracking
“Underbead” describes a crack in the HAZ beneath a bead. In hardenable ferritic steel it is commonly a manifestation of hydrogen-assisted cracking. It may not reach the surface, so a clean VT result does not clear the joint. Volumetric examination or destructive sectioning must be planned for the geometry and likely crack plane.
Preheat depends on composition, thickness, joint type, hydrogen potential, heat input, restraint, ambient conditions and applicable standard. Excessive heat can damage properties or create other problems. Use an approved WPS or engineering calculation.
Toe, root and transverse describe clues—not root causes.
Location-based names help inspectors communicate, but repair engineering must determine the mechanism. The same toe can host hydrogen, fatigue, reheat or stress-corrosion cracking.
7. Toe cracks
The weld toe is a geometric transition and often a local HAZ. A crack there may be fabrication hydrogen cracking, reheat cracking after PWHT, fatigue cracking from cyclic service, or SCC from a corrosive environment. Record whether the crack is in the weld metal, HAZ or parent metal and whether it follows the fusion boundary.
- Do not: blend the toe before establishing depth and mechanism.
- Check: profile, undercut, hardness, residual/applied stress and service cycles.
- Prevent: qualified profile, smooth transition, material/thermal control and fatigue detail design.
8. Root cracks
A root crack can arise from high restraint, sharp unfused geometry, inadequate root reinforcement, hydrogen-assisted HAZ cracking, solidification or service fatigue. Lack of penetration and lack of fusion are planar imperfections but are not automatically “cracks”; they can nevertheless create crack-like stress concentrations.
- Do not: assume increasing penetration solves every root crack.
- Check: root face/opening, alignment, backing/purge, access, heat input and sequence.
- Detect: use a beam/radiation plan suited to root orientation and geometry.
9. Transverse cracks
A transverse crack runs across the weld axis. In high-strength thick weld metal it may be hydrogen-assisted, but reheat, solidification, fatigue and service mechanisms can also produce transverse orientation. Characterize whether the crack stays in the deposit, crosses the HAZ, branches or follows prior-austenite grain boundaries.
- NDT issue: UT must direct sound approximately normal to the crack plane.
- Investigation: deposit chemistry/strength, hydrogen, PWHT and service loading.
- Correction: depends entirely on the confirmed mechanism.
Lamellar tearing, reheat cracking and SCC need a wider investigation.
These mechanisms cannot be solved by welder technique alone. Material quality, joint design, heat-treatment procedure, service environment and residual/applied stress become primary variables.
Lamellar tearing
Rolled plate with poor through-thickness ductility can tear beneath T, corner or cruciform joints when weld shrinkage strains act through the plate thickness. Elongated inclusion bands produce a characteristic stepped profile broadly parallel to the plate surface.
- Evidence
- Joint load path, plate Z-direction properties, UT and metallography.
- Prevention
- Redesign the joint, reduce through-thickness strain, specify through-thickness quality or engineer a buttering route.
Reheat / relaxation cracking
Susceptible low-alloy steels and some austenitic systems can crack in coarse-grained HAZ or weld metal while residual stresses relax during PWHT or high-temperature exposure. Grain interiors strengthen while boundaries accommodate strain.
- Evidence
- Alloy/impurity chemistry, coarse-grain location, PWHT charts and intergranular morphology.
- Prevention
- Material choice, refined HAZ, sound detail, controlled heat input and a qualified heat-treatment cycle.
Stress-corrosion cracking
SCC requires a susceptible material, a specific corrosive environment and tensile stress. Welding can contribute residual stress, sensitized/hardened microstructure, heat tint and local geometry, but the crack is an alloy-environment-stress problem.
- Evidence
- Service chemistry, deposits, temperature, stress, morphology, metallurgy and fracture surface.
- Prevention
- Change material, environment, stress or all three; use the governing corrosion standard and design basis.
Some materials and services use stress-relief treatments, but heat treatment can also alter microstructure, dimensions and corrosion behavior. NACE MR0175/ISO 15156 applies to material selection for H₂S-containing oil and gas production environments; it is not a universal instruction to stress-relieve every sour-service weld.
Choose NDT by crack position, orientation, material and acceptance rule.
No single method detects every weld crack. Examination must be performed by qualified personnel using an approved procedure, calibrated equipment and the acceptance criteria referenced by the construction code or specification.
| Method | Best use for crack detection | Important limitations | Planning question |
|---|---|---|---|
| VT — visual testing | Open surface cracks, craters, toe/root appearance where accessible, profile and repair documentation | Cannot clear buried cracks; lighting, access, cleanliness and magnification affect detection | Can the complete surface and root be viewed at the required stage? |
| PT — liquid penetrant | Fine surface-breaking cracks on nonporous ferrous and nonferrous materials | Only open-to-surface flaws; surface condition and cleaning are critical; does not give reliable depth | Is the material nonporous and is the crack open to the prepared surface? |
| MT — magnetic particle | Surface and near-surface cracks in ferromagnetic material, especially toe and HAZ indications | Not for austenitic stainless, aluminum or other nonferromagnetic materials; field direction affects sensitivity | Can two field directions cover the expected crack orientation? |
| UT / PAUT / TOFD | Buried planar flaws, underbead cracks, lamellar tears and crack sizing when geometry permits | Orientation, near-surface zone, attenuation, geometry and operator technique matter; procedure must be qualified | Will the sound beam intersect the expected crack plane and cover the volume? |
| RT — radiography | Volumetric overview and some crack orientations in suitable butt-weld geometry | Planar cracks may be missed when not aligned favorably to the beam; radiation controls and access required | Is RT sensitive to this crack plane, thickness and joint geometry? |
| Metallography / fractography | Mechanism confirmation: interdendritic, intergranular, transgranular, HAZ location and microstructure | Usually destructive or requires replication; small sample may not represent full extent | Which hypothesis must the microstructure or fracture surface confirm? |
MT makes leakage fields visible.
Useful only on ferromagnetic material; magnetization direction must intersect the crack effectively.
Photo: Wikimedia Commons, CC0.
UT needs a coverage and beam plan.
Technique, calibration, geometry, acoustic properties and crack orientation determine probability of detection.
Photo: Davidmack / Wikimedia Commons, public domain.Inspection timing must follow the applicable requirement
Hydrogen-assisted cracking can be delayed. Some construction codes and project specifications require a defined interval before final inspection for particular steels, thicknesses, heat treatments or repair conditions. There is no universal “always wait 48 hours” rule that safely covers every material and code.
Record the completion time, interpass/postheat history, cooling conditions and time of each NDT. If repair introduces another thermal cycle, the required hold and examination may restart.
Specifying “perform UT” or “perform RT” is incomplete without technique, extent, qualification and acceptance criteria. Method selection must consider expected discontinuity size, shape and orientation.
Weld crack candidate diagnoser
Select the closest observations. The output ranks a starting hypothesis and inspection direction; it is not a certified NDT interpretation or engineering disposition.
Solidification centerline cracking
A crack found hot along the weld centerline is consistent with final-solidification separation. Confirm interdendritic morphology and review chemistry, pool shape and restraint.
Six steps before a cracked weld returns to service.
The applicable code, owner, authorized inspector and engineering organization determine whether repair is permitted. The sequence below is a quality-planning framework, not a universal repair procedure.
Quarantine
Stop fabrication or service as required. Protect personnel, prevent loading and identify all potentially affected welds, heats and shifts.
Document
Photograph and map the indication. Record dimensions, orientation, discovery time, weld ID, WPS, welder/operator, consumables and thermal history.
Size & classify
Use qualified NDT to establish full length, depth, branching and additional buried indications. Form a mechanism hypothesis.
Authorize disposition
Apply the construction code and engineering assessment. Decide reject, repair, replace or further analysis through the approved authority.
Remove & verify
Use an approved removal method. Avoid creating another HAZ or contaminating the cavity. Confirm complete removal before rewelding.
Correct & re-examine
Change the causal variables, use the approved repair WPS, observe any hold time, repeat required NDT and update traceability.
Required excavation length and shape depend on verified crack extent, branching, material, thickness, access and repair procedure. Grind marks, arc gouging or thermal removal can hide, extend or create indications if uncontrolled.
Hydrogen cracking may require consumable, preheat and sequence changes; solidification cracking may require chemistry and pool-shape changes.
Repeated thermal cycles can alter HAZ toughness, hardness, sensitization, distortion and residual stress. Track every repair location.
Cracks can branch or remain buried. Use the qualified method to establish extent before and after removal.
When service failure or systemic cracking is possible, retain samples for metallography, fractography and chemistry before repair.
Prevent the mechanism instead of policing the symptom.
A strong crack-prevention plan combines design, material, consumables, procedure, execution and verification. Welder retraining alone cannot correct a susceptible alloy or a joint that imposes severe through-thickness strain.
| Control layer | Solidification / crater / liquation | Hydrogen-assisted cracking | Lamellar / reheat / service cracking |
|---|---|---|---|
| Design | Reduce harmful restraint and abrupt stop/start features; choose weld detail compatible with process | Limit restraint and stress concentration; plan balanced sequence and realistic fit-up | Redirect through-thickness strain; remove notch details; account for service stress and environment |
| Material | Confirm base/filler compatibility, impurity/segregation sensitivity and solidification range | Review hardenability, carbon equivalent, strength and thickness | Specify through-thickness plate quality, reheat-resistant chemistry or SCC-resistant alloy as needed |
| Consumables | Choose filler that produces acceptable solidification behavior and weld chemistry | Use specified hydrogen classification, storage and exposure control | Match strength, creep/corrosion requirements and impurity controls |
| Thermal procedure | Control pool shape, speed, heat input, interpass and crater termination | Use calculated/qualified preheat, heat input, interpass and postheat where applicable | Qualify PWHT ramp, soak and cooling; avoid harmful HAZ coarsening and stress concentration |
| Production control | Monitor fit-up, cleanliness, wire placement, arc/beam stability and termination | Verify temperature, consumable handling, joint dryness and sequence | Verify material traceability, plate orientation, heat-treatment chart and environment controls |
| Evidence | Macrosections, chemistry/dilution review and targeted surface/volumetric NDT | Timed NDT, hardness, hydrogen/process records and representative restraint tests when needed | UT/metallography, Z-direction data, PWHT records, corrosion/fracture investigation |
High speed changes the crack window, not the physics.
Laser welding reduces total heat input and distortion in many applications, yet deep penetration, rapid solidification and tight fit-up demands create their own crack controls.
Deep, narrow weld profiles can intensify centerline strain
In keyhole laser welding, opposed solidification fronts can meet at the centerline. A high depth-to-width ratio, local mid-depth bulging, chemistry segregation and high transverse restraint can make the last-solidifying boundary vulnerable. The correct response may involve speed, power, focus, beam oscillation, beam shape, joint fit-up, filler or a combination—not simply reducing speed.
Autogenous welding removes filler adjustment
Without wire, the fusion zone is defined by base-metal chemistry and dilution between the joined parts. That can be advantageous for cleanliness and speed but removes the ability to modify crack-sensitive composition or add bridge volume. Representative trials should include different material heats when chemistry variation matters.
Beam oscillation can help or hurt
Oscillation changes pool width, mixing, local energy distribution and solidification path. It may broaden the process window, but excessive dwell or an unsuitable pattern can increase heat input, underfill or instability. Qualify amplitude, frequency, shape, focus, speed and wire position together.
High-strength steel still needs hydrogen control
A laser source does not eliminate moisture, coatings, HAZ hardening or tensile stress. Joint preparation, shielding, material hardenability, heat input, preheat and delayed inspection remain relevant. Rapid cooling may increase HAZ hardness in susceptible steels.
Use cross-sections at parameter limits, starts/stops and gap extremes. Add surface/volumetric NDT, hardness, mechanical, leak, fatigue or corrosion testing according to the actual acceptance basis.
Validate the joint before production multiplies a crack mechanism.
Oceanplayer can review a laser welding application using representative materials, thickness, joint geometry, gap range, surface condition and acceptance criteria. A useful sample test challenges the intended production tolerance window—not only one perfectly prepared coupon.
- Base-metal and filler certificates
- Thickness range and joint drawing
- Production gap, mismatch and restraint
- WPS parameters or proposed laser window
- Observed crack location, timing and NDT report
- Mechanical, leak, fatigue or corrosion acceptance criteria
Weld crack types FAQ
Short answers for welders, inspectors, engineers and buyers. The applicable construction code, qualified procedure and authorized disposition always control the real joint.
What is the most dangerous weld defect?
Cracks are among the most serious weld imperfections because they are sharp planar discontinuities that concentrate stress and may propagate. Avoid claiming one crack size or type is always the most dangerous; criticality depends on material toughness, loading, orientation, environment, geometry and acceptance code.
What are the main weld crack types?
A practical list includes solidification, crater, liquation, HAZ hydrogen, weld-metal hydrogen, underbead, toe, root, transverse, lamellar tearing, reheat and stress-corrosion cracks. Some names describe mechanism, while others describe location or orientation.
What is the difference between hot and cold cracking?
Hot cracking occurs while weld metal is solidifying or local grain-boundary films are partially molten. Cold cracking usually refers to hydrogen-assisted cracking that develops after cooling in susceptible ferritic steel under tensile stress. Temperature boundaries are not universal fixed values.
Is delayed cracking different from hydrogen cracking?
Delayed cracking commonly describes hydrogen-assisted cracking discovered after a time interval. “Delayed” is a timing descriptor, not necessarily a separate metallurgical mechanism.
Why do crater cracks form?
The final weld pool can be concave and too small to withstand solidification shrinkage. A qualified crater-fill, ramp-down, run-off or termination technique leaves sufficient metal and controls the final solidification path.
What three conditions cause hydrogen-assisted cracking?
Diffusible hydrogen, a susceptible hard microstructure and sufficient tensile stress must coincide. Material chemistry, thickness, heat input, preheat, consumable hydrogen, surface condition, restraint and timing all affect risk.
Is an underbead crack a separate mechanism?
Underbead describes location in the HAZ beneath the weld. In hardenable ferritic steels it is commonly a form of hydrogen-assisted HAZ cracking, so the investigation should address hydrogen, hardenability, cooling rate and stress.
Can a weld toe crack be repaired by grinding?
Not until the mechanism and full depth are established and repair is authorized. A toe crack may be hydrogen, reheat, fatigue or stress-corrosion cracking. Grinding before examination can remove evidence or leave buried branches.
Which NDT method is best for weld cracks?
No single method is best for every crack. PT detects surface-breaking flaws on nonporous materials; MT applies to ferromagnetic materials; UT is powerful for buried planar cracks when the beam intersects the crack plane; RT can miss poorly oriented planar cracks. Use the applicable code and qualified procedure.
How long should I wait before inspecting for delayed cracks?
There is no universal interval. Required delay depends on material, thickness, strength, hydrogen potential, heat treatment, repair condition, construction code and project specification. Follow the approved inspection plan.
Can weld cracks be repaired?
Many can be repaired when the governing code permits it, but only after extent and cause are established. The authorized procedure must remove the full crack, correct the causal conditions, control the new thermal cycle and repeat required examination.
Why do laser welds crack along the centerline?
Rapid solidification, chemistry segregation, a deep narrow profile and transverse contraction can concentrate strain at the final-solidifying centerline. Speed, focus, beam shape, oscillation, filler, fit-up and restraint should be evaluated together.
Does preheat prevent every weld crack?
No. Preheat can reduce hydrogen-assisted cracking risk in susceptible steels by slowing cooling and aiding hydrogen diffusion. It does not automatically prevent solidification, liquation, lamellar, reheat, fatigue or stress-corrosion cracking and can be harmful if incorrectly applied.
Are all cracks automatically rejected?
Cracks are normally unacceptable under common weld quality and construction requirements, but the exact examination, evaluation and disposition must come from the governing code and contract. Do not use ASME Section IX alone as a universal production acceptance standard.
Continue from crack diagnosis to process control.
Use the next resource that matches the decision: welding process fundamentals, parameter development, heat-input comparison or representative testing.
Standards and engineering references.
This page prioritizes current standards organizations, recognized welding institutions and corrosion authorities. Confirm contractually applicable editions before fabrication, inspection or repair.
- ISO 6520-1:2007: classification and description of geometric imperfections in fusion welds.
- ISO 5817:2023: quality levels for imperfections in fusion-welded steel, nickel and titanium alloys; beam welding excluded.
- ISO 17635:2025: selection of NDT methods and evaluation framework for metallic welds.
- AWS D1.1/D1.1M:2025-AMD1: structural steel welding, qualification, inspection and acceptance framework.
- AWS B1.10M/B1.10:2016: guide to nondestructive examination method selection for welds.
- TWI — Solidification Cracking: identification, metallurgy, weld profile, restraint and prevention factors.
- TWI — Hydrogen Cracks: Identification: HAZ and weld-metal locations, morphology and three causal conditions.
- TWI — Hydrogen Cracks: Prevention: hydrogen control, preheat, interpass and postheat context.
- TWI — Lamellar Tearing: through-thickness strain, plate ductility, joint design, detection and mitigation.
- TWI — Reheat Cracking: susceptible alloys, coarse-grained regions, PWHT and stress-relaxation mechanism.
- TWI — Laser Welding Structural Steel Defects: centerline solidification cracking, high-speed profile and process controls.
- AMPP — Stress Corrosion Cracking: combined material, environment and tensile-stress mechanism.
- ISO 23277:2015: penetrant-testing acceptance levels for surface-breaking indications in metallic welds.