Guide
What causes a casting crack, and what should you do with the lot when one is found at inspection?
A casting crack is a linear, sharp-ended separation in the metal of a cast part, caused when stress in the casting exceeds the strength of the metal at that moment: while it is still solidifying (a hot tear), while it cools or is quenched (a cold or contraction crack), or later, when it is trimmed, ejected, straightened, machined or handled. When one turns up at inspection, the right response is the same every time: confirm it really is a crack, find out how many parts in the lot carry one, quarantine the lot, and decide the disposition in writing with the design authority before anyone grinds, welds or ships a part.
By Saola Manufacturing engineering team · Published · Updated
A crack is different from most casting defects because it is a stress raiser with a sharp tip. Porosity, a rough surface or a small cold lap may be acceptable under an agreed class; a crack in a loaded or pressure-retaining part almost never is, and it can grow in service. That makes the first hours after the find important. Grinding the indication away to “see how deep it goes,” welding it without approval, or sorting only the parts that look bad can each turn one rejected casting into a field problem. This guide is for the quality, design or sourcing engineer who has a cracked casting on the bench and a lot waiting for a decision. It walks through how to recognize the crack, which cause each crack type points to, how to confirm it and size the problem, how to choose between scrap, weld repair, rework and use as is, and what to write on the drawing, RFQ and nonconformance report so the next lot is handled by rule rather than by argument.
On this page
How do you recognize a casting crack and tell it from a cold shut, a seam or a scratch?
A crack is a narrow, sharp-walled line that usually wanders or branches and ends in a fine tip, and it holds penetrant or magnetic particles in a crisp line after the surface is cleaned. A cold shut or lap has rounded, unfused edges, and a scratch or tool mark is straight, shallow and follows the direction of handling or cutting.
The Investment Casting Institute defect atlas describes a casting crack as a jagged crack with an irregular path that forms at about 90° to the stress, typically where the geometry seriously restrains contraction or around a local volume of unfed metal, and usually revealed by visual or penetrant inspection (Investment Casting Institute, crack). Its entry for a hot tear adds that the fracture face is typically oxidized and that hot tears also form where thick and thin sections meet (Investment Casting Institute, hot tear). Three quick checks separate a real crack from look-alikes on the bench:
- Does it have depth? A crack bleeds penetrant back out after the excess is removed and the developer is applied; a scratch or a shallow lap shows a faint, wide or broken indication, or none.
- Does it follow the part or the process? Cracks follow stress: inside corners, junctions of thick and thin walls, gate and overflow stubs, ejector bosses and thin ribs. Tool marks follow the cutter path, and handling marks sit on exposed edges.
- Does it repeat? A crack at the same feature on part after part is a design or process cause. A single crack in a random place points to handling damage or a one-off event, but it still needs the lot to be checked before anyone concludes that.
If you are not sure, do not grind, file or blast the area to look closer. Grinding and wire brushing can smear metal over a crack opening and hide it from penetrant inspection, a risk TWI singles out for copper and aluminum alloys; it suggests acid etching to remove smeared metal before the penetrant is applied (TWI, penetrant and magnetic particle inspection). If someone has already ground the area, tell the inspector so the surface can be prepared before it is judged crack free. Photograph it, mark it, and tag the part.
What causes a casting crack, and which cause does each crack type point to?
Casting cracks commonly come from restrained contraction or uneven cooling at a junction or sharp inside corner; other causes are damage at trimming, ejection or straightening, and residual stress released by machining. Where the crack sits and what its fracture face looks like usually tell you which.
The usual causes, and where each one leaves its crack:
- Restrained contraction at a hot spot. A heavy section meets a thin one, or the casting shrinks against a core, die steel or stiff gating while the junction is still weak. Cracks that form while the metal is still weak at high temperature are hot tears; the atlas also lists gates and runners strong enough to block contraction, premature mold movement and an uneven cooling rate as causes.
- Sharp inside corners. The same atlas lists sharp internal angles as a cause and adequate fillet radii as the remedy, and die casting design guidance from NADCA says all sharp corners and edges should be filleted when the part is modeled (NADCA design, fillets).
- Uneven or rapid cooling. Water quenching and other rapid cooling set up high internal stress that can crack a casting in the solid state; the atlas lists both and recommends avoiding rapid cooling methods.
- Trimming, ejection and handling. In die casting, a part that sticks in the die, uneven ejection, or a trim press that tears a gate stub off can crack thin walls and bosses. These cracks sit at ejector pin bosses, gate and overflow locations and trimmed edges.
- Material removal. Machining can unbalance residual stress and open a crack that was closed or very fine on the raw casting; the atlas recommends a stress-relief cycle before machining where this is the cause.
Cold shuts and flash, which are often mistaken for cracks on a die casting, are covered in the die casting defects guide; the point here is to use the crack type to decide where to look and who has to act. The table maps what you see to the likely cause and owner.
| What you see | Likely crack type and cause | Who usually owns the fix |
|---|---|---|
| Jagged, branching crack at a junction or inside corner; oxidized face when broken open | Hot tear: restrained contraction during solidification | Design (fillets, section change) and casting method |
| Straighter crack of fairly even width, often through the whole section; face bright or only lightly tinted | Cold or contraction crack after solidification; quench or straightening | Casting partner: cooling, shakeout, heat treat practice |
| Crack at an ejector boss, gate stub or trimmed edge | Ejection, sticking or trim damage | Casting partner: die condition, draft, trim tooling |
| Crack visible only after a machined face or bore is cut | Existing crack exposed, or residual stress released by cutting | Casting partner first; machining sequence or stress relief second |
| Crack in or beside a previous weld repair | Repair weld cracking or heat-affected zone cracking | Whoever approved and performed the repair |
| Crack at random locations on a few parts, sharp-edged dents nearby | Handling or transit impact | Packaging and handling, at whichever stage the damage occurred |
How do you confirm a casting crack and find how many parts in the lot have one?
Confirm the indication with liquid penetrant on any alloy, or magnetic particle testing on ferromagnetic castings, then inspect the rest of the lot at the same feature with the same method. Use radiography only when the crack may be internal or the drawing already calls for it.
Liquid penetrant testing finds only discontinuities that break the surface; practice for general industry is set out in ASTM E165/E165M. Magnetic particle testing finds cracks at or near the surface but works only on ferromagnetic material, and ASTM E709 is the guide for it; TWI notes that magnetic particle sensitivity is greatest when the flaw cuts the field at right angles, so each area is magnetized twice, in directions about 90° apart. For aluminum and zinc die castings, which are not ferromagnetic, penetrant is the surface method that applies. To classify what you see, the reference photographs in ASTM E433 (penetrant) and ASTM E125 (magnetic particle on ferrous castings) help, but E433 says itself that the user must set the acceptance limits; the photographs do not.
Then size the problem before you decide anything:
- Map the crack. Photograph it with a scale, mark the feature on a copy of the drawing, and record the inspection method and who performed it.
- Inspect the feature on every part in the lot, not a sample, if the cause is not yet known. A crack from a design or die cause will repeat at the same place.
- Check upstream and downstream. Look at parts still at the supplier, in transit, in your stock and already assembled, and identify them by lot, cavity and date code where the casting carries one.
- Break one open if you can spare it. An oxidized face points to a hot tear; a bright or only lightly tinted face points to a crack that formed later, when the metal was cooler; beach marks point to fatigue growth in service or test. Do this before any rework, because a ground or welded part can no longer tell you the cause.
- Ask for the history. Request the casting partner’s process records for the lot: die or pattern change, heat treat and quench, any weld repair, and when trimming tools were last serviced.
Keep cracked parts physically separate and tagged. They are the evidence for the root cause, and once they are scrapped or reworked the argument about what happened gets much harder. The receiving inspection guide covers how to hold a lot while this is done.
Should a cracked casting lot be scrapped, weld repaired, reworked or used as is?
Scrap or return the cracked parts unless the drawing or a written concession allows something else; release the rest of the lot only after every part has been inspected at the cracked feature. Weld repair and use as is accept a part that does not meet the drawing, so only the design authority can approve them.
The terms matter, because each one moves responsibility. In the ISO 9000:2015 vocabulary that ISO 9000 defines, rework makes a part conform to the requirement, repair makes it acceptable for its intended use without making it conform, and a concession is permission to use or release a product that does not conform. Grinding out a shallow crack in excess stock that will be machined away anyway can be rework. Welding a crack, blending it below a dimension, or impregnating it is repair, and it needs a concession. Specifications for cast products treat repair as something to be approved: the abstract of ASTM B26/B26M for aluminum sand castings says castings should only be repaired using approved processes such as welding, impregnation, peening, blending and soldering.
Severity sets how far up the decision has to go. The US federal acquisition rules give a useful classification: a critical nonconformance is likely to cause hazardous or unsafe conditions, a major one is likely to cause failure or materially reduce usability, and a minor one is not likely to reduce usability (FAR 46.101). For government contracts the same rules say nonconforming supplies should normally be rejected, and that accepting critical or major nonconformances needs technical concurrence and a price reduction or other consideration (FAR 46.407). Most private buyers can borrow that logic: a crack in a pressure boundary, a load path or a safety part is critical or major by default.
| Disposition | When it can make sense | Who approves, and what must follow |
|---|---|---|
| Scrap or return for replacement | Any crack in a pressure boundary, load path, sealing face or safety part; any crack whose cause is unknown | Buyer quality; supplier root cause and replacement parts inspected at the same feature |
| Rework (crack removed within stock to be machined off) | Shallow surface crack in machining stock, confirmed gone by re-inspection after removal | Buyer quality with the method agreed; penetrant or magnetic particle re-inspection after machining |
| Weld repair | Larger or costly castings in weldable alloys where the drawing or a concession allows it | Design authority; qualified procedure, crack fully excavated and confirmed, post-weld treatment, re-inspection and repair records |
| Impregnation or blending | Rarely for cracks; impregnation targets porosity, not structural discontinuities | Design authority only; treat as repair under a concession |
| Use as is | Crack in a non-functional area of a lightly loaded part, with engineering analysis | Design authority (and end customer if the contract requires); concession number, quantity and lot recorded |
| Sort and release uncracked parts | Cause understood, crack confined to identified parts | Buyer quality; record of the full-lot inspection and the method used |
Two cautions apply to every crack disposition. First, a weld repair is a new process on the part: it brings its own heat-affected zone, possible distortion and possible new cracking, so the repaired area has to be inspected again and, for heat-treated alloys, the heat treatment question has to be answered before approval. Conventional high-pressure die castings are usually poor candidates, because gas trapped in their pores expands when heated; vacuum or low-porosity die castings can be different, so ask the casting partner before assuming either way. Second, sealing is not a crack fix. The vacuum impregnation guide explains what impregnation is meant for, and a crack is a structural discontinuity, not leak-path porosity.
What should the drawing, RFQ and nonconformance report say about casting cracks?
The drawing should say that cracks are not permitted, name the inspection method, standard and zones, and state whether weld repair is allowed and under whose approval. The nonconformance report should record the crack location, method, count in the lot, suspected cause and the disposition you require.
On the drawing or the casting specification:
- A clear acceptance rule. For example, “No cracks, hot tears or cold shuts permitted. Linear indications are rejectable.” Do not rely on a reference photograph set alone; ASTM E433 states that users must specify the limits.
- Method, standard and zone. “Liquid penetrant inspect zone A per ASTM E165/E165M” or “magnetic particle inspect per ASTM E709,” with the zones marked, and whether the inspection happens before or after machining. The porosity after machining guide shows why machined surfaces can reveal what the raw casting hid.
- Repair rule. “Weld repair, impregnation, blending or peening of castings is not permitted without written approval of [design authority],” or, where you allow it, the procedure requirements, re-inspection and repair map.
- Design notes that remove the cause. Fillet radii at junctions and ejector bosses, a note on section transitions, and draft on deep features, agreed with the casting partner before tooling.
In the nonconformance report for a cracked lot, include the part and revision, lot and date code, the quantity received, inspected and found cracked, a photo with scale, the feature on a marked drawing, the inspection method and standard, whether the crack was found on the raw or machined surface, the suspected cause from the table above, containment status at every location, and the disposition you require, with any concession reference. The NCR guide covers the general fields and when to escalate to a material review board or an 8D. Ask the supplier for root cause, the corrective action on the die, pattern, method or drawing, and the evidence that replacement parts were inspected at the cracked feature.
Who does what on a Saola order when a cracked casting is found?
Die castings and sand castings on a Saola order are made by partner plants we manage, and machining is done at our lead CNC plant, T.K Viet Nam Precision Company Limited in Vietnam. Each quote names the production site and the country of origin for every step.
The casting partner owns casting-side causes and fixes: die and pattern condition, gating, cooling, trim tooling and heat treatment. The design-side fixes, such as fillets and section changes, stay with you, and we bring the casting partner’s comments back as a manufacturability note. When castings reach the lead CNC plant for CNC milling or CNC turning and a crack is found or opened by a cut, the part is set aside, photographed with its location and reported to you, and the disposition is yours to decide in writing; we do not treat welding, blending or sealing a crack as a routine fix. See die casting and sand casting for the partner-plant programs. If your drawing calls for penetrant, magnetic particle or radiographic inspection, name the standard on the RFQ and we confirm before quoting whether and where it can be arranged. ISO 9001 certification is in progress, not held. Send the drawing with your crack and repair notes through the quote form.
Frequently asked
Can one cracked casting mean the whole lot is bad?
It can. If the crack sits at a junction, corner or ejector boss, the same cause acts on every part from that die or pattern, so inspect that feature on the whole lot before releasing any of it. A random crack beside impact damage is more likely handling, but confirm it.
Is it acceptable to grind out a crack and keep the part?
Only if the ground area stays within stock that will be machined off or the drawing allows blending, and the area passes re-inspection afterward. Grinding below a dimension is repair, not rework, and needs the design authority’s concession.
Can aluminum die castings be weld repaired?
Conventional high-pressure die castings usually cannot be, because gas trapped in their pores expands when heated and heat can distort thin walls; vacuum or low-porosity die castings may allow limited welding. Sand and permanent mold castings in weldable alloys are more often repaired, and only by a process the purchaser has approved.
Will a leak test catch a cracked casting?
A through crack in a pressure wall will usually leak, but a crack that does not go through will pass a leak test and can still grow in service. Use penetrant or magnetic particle inspection for surface cracks, and do not use a passed leak test as proof of no cracks.
Who pays for sorting a cracked lot?
That depends on your purchase order and quality agreement. Write the sorting, replacement and inspection cost responsibility into them before the first order, so the question is settled before a crack is found rather than after.
Sources
- Casting defect atlas: crack, Investment Casting Institute, accessed October 2026
- Casting defect atlas: hot tear, Investment Casting Institute, accessed October 2026
- Fillets, NADCA Design, North American Die Casting Association, accessed October 2026
- Non-destructive examination, part 1: liquid penetrant and magnetic particle inspection, TWI, accessed October 2026
- ASTM E165/E165M-23, Standard Practice for Liquid Penetrant Testing for General Industry (scope), ASTM International, 2023
- ASTM E709-21, Standard Guide for Magnetic Particle Testing (scope), ASTM International, 2021
- ASTM E433-71(2023), Standard Reference Photographs for Liquid Penetrant Inspection (scope), ASTM International, 2023
- ASTM E125-63(2023), Standard Reference Photographs for Magnetic Particle Indications on Ferrous Castings (scope), ASTM International, 2023
- ASTM E446-20, Standard Reference Radiographs for Steel Castings Up to 2 in. in Thickness (scope), ASTM International, 2020
- ASTM B26/B26M-25, Standard Specification for Aluminum-Alloy Sand Castings (abstract), ASTM International, 2025
- ISO 9000:2015, Quality management systems: fundamentals and vocabulary, ISO, 2015
- FAR 46.101, Definitions (critical, major and minor nonconformance), Federal Acquisition Regulation, accessed October 2026
- FAR 46.407, Nonconforming supplies or services, Federal Acquisition Regulation, accessed October 2026