Guide
What causes hot tearing in castings, and which design changes and inspections prevent it?
Hot tearing is cracking that happens while a casting is still solidifying: the last metal to freeze is a weak, partly liquid network of crystals, and if the shrinking casting is held back by the mold, a core or a stiffer neighboring section, that network is pulled apart before it can be fed with liquid metal. The result is a jagged, branching crack, usually at a junction, an inside corner or a heavy section, often with a darkened, oxidized fracture face.
By Saola Manufacturing engineering team · Published · Updated
Hot tears are also called hot cracks or hot shortness cracks, and they turn up in both ferrous and nonferrous castings, in sand molds, investment shells and steel dies. They are expensive because they are often found late: after shot blast, at penetrant inspection, or when a machined face or a pressure test opens a crack that was invisible on the raw casting. This guide is for the design, quality or sourcing engineer who has a cracked casting on the bench. It covers how to recognize a hot tear and tell it from other cracks, which causes to check first, how to find every tear in a lot, which design and process fixes match which cause, and what to write on the drawing and RFQ so the next lot does not repeat it.
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What does a hot tear look like, and how is it different from a cold crack or a cold shut?
A hot tear is a ragged, branching crack with an irregular path that follows grain boundaries, usually at a junction or inside corner, and its fracture face is often oxidized or discolored because it opened while the metal was still hot. A cold crack is straighter and cleaner, and a cold shut is a lap with rounded edges where two metal streams failed to fuse.
The Investment Casting Institute defect atlas gives the appearance as a “jagged crack with irregular path, typically with an oxidized fracture face” and places it in the slowest-freezing areas where contraction is held back, often where a thick section meets a thin one (Investment Casting Institute, hot tear). University of Iowa researchers working with the Steel Founders’ Society of America note that hot tears can be on the surface or internal, and can be large enough to see or small enough to need magnetic particle inspection (Monroe and Beckermann, 2005).
Three clues separate a hot tear from the defects it is most often confused with:
- Path. A hot tear wanders and branches between grains. A cold crack, which forms after the casting is fully solid (during cooling, shakeout, quench or straightening), usually runs straighter and often cuts across grains rather than around them.
- Fracture face. Break a sample open. A hot tear face is dendritic and usually darkened by oxidation; a cold crack face is cleaner and brighter. A cold shut shows smooth, rounded surfaces that never bonded.
- Location and repeatability. Hot tears sit where a hot spot meets restraint, so they repeat at the same junction or corner on part after part. A one-off crack in a random place points to handling or a cold crack.
Shrinkage porosity sits in the same hot spots and the two often appear together, but shrinkage is a cavity, not a crack. If you need to separate them on film, the steel casting reference radiographs give hot tears their own category, as the table shows.
| Discontinuity | What you see | When it forms | ASTM E446 category (steel) |
|---|---|---|---|
| Hot tear | Jagged, branching crack at a junction or inside corner; oxidized, dendritic face | End of solidification, under restrained contraction | Category E, hot tear |
| Cold crack | Straighter crack, clean or bright face | After solidification: cooling, shakeout, quench, straightening | Category D, crack |
| Shrinkage | Irregular cavity or sponge in a heavy section | During solidification, when feeding stops | Category C, shrinkage |
| Cold shut | Lap or seam with smooth, rounded edges, often far from the gate | During filling, when two streams meet too cold | Not a separate category |
What causes hot tearing at junctions, ranked by how often it explains the crack?
A hot tear needs two things at the same place and time: a region that is still partly liquid and has been cut off from feed metal, and tensile strain because the casting’s contraction is restrained. Geometry creates most of both; alloy choice and process conditions decide how much margin there is.
Monroe and Beckermann put the physics simply: a tear starts where the partly solid metal can no longer be fed with liquid and is being stretched at the same moment, so anything that cuts the strain or keeps the section fed longer lowers the risk (Materials Science and Engineering A, 2005). In practice the causes rank like this:
- A hot spot at a restrained junction. Where a thick leg, boss or flange meets a thinner wall, the junction stays liquid longest while the thin wall around it has already frozen and is contracting. The strain concentrates in the weakest, hottest spot. In the Steel Founders’ Society of America T-casting trials analyzed by the Iowa group, the two ends of the arm were fixed in the mold so the leg made an unfed hot spot at the junction. With the 36 in arm, widening the leg from 1 in to 2 in turned an untorn casting into a heavily torn one. In the simulation, putting a riser on that leg cut the average tear indicator from 1.53% to 0.78%, though the authors show the effect shrinks under a stricter assumption about when feeding stops (Monroe and Beckermann, 2005).
- Sharp inside corners. A sharp corner both concentrates strain and creates a local hot spot. Total Materia’s review for aluminum alloys locates hot tears at inside corners and fillets, where the mold cavity stops the casting from shrinking, and near re-entrant corners (Total Materia, 2016).
- Rigid restraint from the mold, die or cores. A casting that shrinks onto a core, or between two features locked in a steel die, cannot contract freely. The Iowa paper names cores in cylindrical castings as a typical restraint in steel. Total Materia notes that a die casting die is a comparatively rigid steel structure, so tearing is a bigger concern in die casting than in sand casting, where the mold gives way more. The Investment Casting Institute adds gates and runners strong enough to stop the casting contracting, and a shell or mold that is simply too strong.
- Alloy with a long freezing range. The longer an alloy spends partly solid, the longer the weak network is exposed. In a study of aluminum-silicon alloys, the eutectic A413, with essentially no freezing range, never hot tore whatever the melt quality, while A380.1, with the widest freezing range of the alloys tested, showed the highest tendency (Uludağ et al., Metallurgical and Materials Transactions A, 2018). In constrained-rod permanent mold tests at the Advanced Casting Research Center, A356 did not crack while the aluminum-copper alloy 206 did (ACRC, hot tearing in aluminum cast alloys).
- Process conditions. Prevention centers on a fine grain structure and casting conditions that keep temperature and stress gradients small (Total Materia, casting defects: hot tearing). In the ACRC tests on alloy 206, a hotter mold (slower cooling) cut tearing significantly, pouring temperature between 700 and 750 °C made little difference, and only a fine, globular grain structure prevented tears (ACRC). The Investment Casting Institute also lists moving the mold before the casting has solidified, and uneven cooling. In steel, composition matters too, which is why the Iowa paper lists it alongside hot spot size and restraint.
How do you confirm a hot tear and find every one in a lot?
Map where the cracks are, open one up to read the fracture face, then use a surface method (liquid penetrant on any metal, magnetic particle on ferromagnetic steel) to sort the lot, and radiography or sectioning when tears may be internal.
- Map the location. Mark each crack on a drawing or 3D view across several castings. Repeats at the same junction or corner point to geometry and restraint; scattered cracks point to handling, heat treatment or cold cracking.
- Read the fracture face. Break or section one cracked sample. An oxidized, dendritic face confirms the crack opened during solidification.
- Sort the surface. Liquid penetrant testing finds discontinuities open to the surface on nonporous metals per ASTM E165/E165M. ASTM E1417/E1417M, which replaced MIL-STD-6866, sets minimum requirements and is written to be called on a drawing or contract. On steel and iron castings, magnetic particle testing per ASTM E709 finds cracks at or near the surface of ferromagnetic material. Both methods need a clean surface free of scale, sand and coating, so agree with the supplier how the castings are cleaned before inspection.
- Look inside when needed. Hot tears can be internal, so radiograph or section the risk zone on sample castings. For steel castings, ASTM E446-20 shows hot tears as Category E with a single illustration, not graded severity levels like porosity or shrinkage, and the standard requires a prior purchaser-supplier agreement on the acceptance criteria, so write the rule yourself (many buyers simply allow none in stressed zones).
- Check the process record. Ask the foundry for the pour temperature, mold or die temperature, cooling and shakeout or ejection timing, ingot source and melt chemistry, and any changes in cores, gating or mold strength for the cracked lot. A change that lines up with the first cracks is the fastest route to the cause.
- Test a fix on a small run before the full lot. Change one thing at a time (fillet, core, gate, mold or die temperature), pour or shoot a short trial and inspect the same zone by the same method, so you know which change worked. If the supplier runs solidification simulation, ask for the predicted hot spot at the cracked junction before and after the change.
For a machined casting, also check whether the crack was opened or extended by machining or by a press fit. A crack that runs into a machined face from an as-cast junction started in the casting; a crack only at a press-fit bore may be a cold crack from assembly stress.
Which design and process changes prevent hot tearing?
Fix the geometry first: generous fillets, gradual section transitions, cored-out bosses and a layout that lets the casting shrink freely. Then tune the alloy and the process, since a design that sits on the edge will tear again whenever conditions drift.
From the SFSA trials, the fixes that worked were rounded corners, less abrupt changes in section and a casting and gating layout free to shrink, all of which help feeding and lower the tensile strain (Monroe and Beckermann, 2005). NADCA’s design guidance for die castings says all sharp corners and edges should be filleted (NADCA design, fillets). The Investment Casting Institute’s corrections add webs to strengthen the weak area, weaker molds or shells, slower or more even cooling, and leaving the mold undisturbed until the casting has solidified. Use the table to match the fix to what the diagnosis found.
| Cause found | Design fix (drawing owner) | Process fix (foundry or die caster) |
|---|---|---|
| Thick leg or boss on a thin wall | Core out the boss, taper the transition, keep walls closer to uniform | Feed or chill the hot spot so it freezes with its neighbors |
| Sharp inside corner at a junction | Add a fillet; where you can, replace X and multi-wall junctions with staggered T or L joints | Chill the junction; add a web across the weak area |
| Shrinkage onto a core or between locked features | Add draft, move or split features so the part can contract; allow a slot or curved spoke | Weaker, more collapsible molds, shells and cores; review die core temperature and ejection timing |
| Gating ties the casting down | Allow the foundry to choose gate locations | Re-gate so runners do not restrain contraction |
| Long-freezing-range alloy | Allow a more castable alloy where strength permits | Grain refinement, tighter chemistry and melt control, clean virgin ingot |
| Uneven cooling or early handling | None, if geometry is sound | Hotter mold or die, slower or insulated cooling; do not move the mold until solid; check shakeout or ejection timing |
Two cautions. First, a feeder only helps while it keeps the hot spot fed; it does nothing for restraint, and a riser or runner that ties two ends of the casting together can add restraint of its own. Second, changing the alloy is a drawing change with knock-on effects on strength, finish and machining, so weigh it after geometry, not before. If the part is going from a sand casting to a die casting, or the reverse, recheck every junction, because a steel die restrains far more than sand. The wall thickness guide and the draft angle guide cover the two die casting rules that most often interact with tearing.
What should the drawing and RFQ say so hot tears do not recur?
Put the fillet radii, the alloy, the crack inspection method and the acceptance rule on the drawing, and mark the junctions where cracks are not acceptable. A drawing that only says “no cracks” leaves the method, the extent and the sampling to someone else.
- Fillets. Dimension the fillet at every structural junction, or use a general note such as “Unless otherwise specified, inside corners R [value] min.” Do not leave corners sharp to save a CAD step.
- Alloy and temper. Name the alloy by its standard designation, and say whether an equivalent alloy is acceptable if it is more castable.
- Critical zones. Mark the junctions that carry load or pressure as Zone A, with no linear indications allowed, and allow the supplier more freedom elsewhere.
- Inspection method and extent. “Liquid penetrant inspect Zone A per ASTM E165/E165M, 100% of first article, then [sampling plan].” Use magnetic particle per ASTM E709 for steel, and add radiography only where internal tears are a real risk.
- Repair rule. State whether weld repair of cracks is allowed, with or without your written approval, and whether repaired castings must be re-inspected.
- Process freedom. “Gating, feeding and chill locations at supplier discretion; changes after first article approval require notification.”
On the RFQ, attach crack photos and the location map from the old lot if this is a resourcing or a corrective action, and ask the supplier for a manufacturability note on junctions before tooling is cut. A tear that is designed out on the model costs far less than one sorted out of every lot. The first article guide covers how to lock the approved process once castings pass.
Who does what on a Saola order when castings hot tear?
Die castings and sand castings are made by partner plants we manage, and any 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 the process fixes: gating, feeding, chills, die temperature and ejection timing. The design fixes stay with you, and we bring the casting partner’s junction and fillet comments back as a manufacturability note before tooling. When castings reach the lead CNC plant for CNC milling or CNC turning, a crack opened by a cut is recorded with photos and its location and reported back, so you and the casting partner can decide whether the fix belongs on the drawing or in the method. 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, the crack photos and the location map through the quote form.
Frequently asked
Is hot tearing the same as hot cracking?
In foundry use, yes: both names describe cracks that form late in solidification under restrained contraction. In welding, “hot cracking” describes the same mechanism in a weld bead, so check which process a source is talking about.
Can a hot-torn casting be weld repaired?
Steel castings with hot tears are commonly repaired by welding or scrapped, as the Iowa paper notes. Whether that is acceptable is your decision, so the drawing should say if weld repair is allowed, who approves it, and how the repair is re-inspected. For pressure-containing or safety parts, settle this before the first lot rather than after a crack is found.
Will impregnation seal a hot tear?
No. Vacuum impregnation seals interconnected porosity that forms a leak path; it is not intended to seal structural defects such as cracks (Foundry Management & Technology). A hot tear is a structural discontinuity, and sealing it would not restore strength. Fix the cause and reject or repair torn castings under an agreed rule.
Can casting simulation predict hot tears?
It can point to likely initiation sites. The Iowa hot tear indicator is described by its authors as a locator for where tears start, not a full prediction of the crack, so use simulation to change the design before tooling and inspection to confirm the castings.
Why did cracks appear only after we changed suppliers or processes?
Restraint, cooling and alloy chemistry all change with the mold, die, gating and melt practice. A junction that sat close to the limit can tear under a new process even though the drawing is unchanged, which is a sign the junction needs a design fix.
Sources
- Development of a hot tear indicator for steel castings, Materials Science and Engineering A 413-414, C. Monroe and C. Beckermann, University of Iowa, 2005
- Freezing Range, Melt Quality, and Hot Tearing in Al-Si Alloys, Metallurgical and Materials Transactions A 49A, M. Uludağ, R. Cetin and D. Dispinar, 2018
- Casting defect atlas: hot tear, Investment Casting Institute, accessed October 2026
- Hot tearing in aluminum cast alloys: measures and effect of process variables, Advanced Casting Research Center (S. Li, K. Sadayappan, D. Apelian), accessed October 2026
- Hot tearing susceptibility in aluminum alloys, Total Materia, accessed October 2026
- Casting defects: hot tearing, Total Materia, accessed October 2026
- Can vacuum impregnation seal this?, Foundry Management & Technology, accessed October 2026
- Fillets, North American Die Casting Association design resources, accessed October 2026
- ASTM E446-20, Standard Reference Radiographs for Steel Castings Up to 2 in. in Thickness (scope), ASTM International, 2020
- ASTM E165/E165M-23, Standard Practice for Liquid Penetrant Testing for General Industry (scope), ASTM International, 2023
- ASTM E1417/E1417M-21e1, Standard Practice for Liquid Penetrant Testing (scope), ASTM International, 2021
- ASTM E709-21, Standard Guide for Magnetic Particle Testing (scope), ASTM International, 2021