Hot shortness is when hot metal suddenly loses toughness and cracks during hot work—reheating, rolling, forging, straightening, welding, or casting. If you’ve heard aluminum shops say “hot short,” they usually mean hot cracking. Below we explain both in plain language and give practical steps for aluminum die-casting.
What is hot shortness
Hot shortness is a loss of ductility at high temperature caused by low-melting material at grain boundaries, so the metal can’t stretch under load.
In steel, the classic case is copper-rich liquid at the surface after oxidation (liquid-metal embrittlement) that leads to transverse cracks during hot working. In aluminum, the shop term “hot short” commonly refers to hot cracking during or right after solidification (or highly restrained welding), which is a different mechanism.
Why does hot shortness happen in steel
- During reheating and rolling, steel forms an oxide scale on the surface.
- If the steel contains copper, oxygen can push the copper out of solution and concentrate it near the steel/scale interface.
- That enriched layer can melt below the steel’s temperature and seep into grain boundaries.
- When the mill stretches or bends the steel, those boundaries separate, giving transverse surface cracks.
Other contributors (depending on grade and practice) include tin, antimony, arsenic, and issues with sulfur/phosphorus balance.
What temperatures cause hot shortness in steel
There is a risk window where the copper-rich film is liquid and the steel’s ductility is low.
In practice, many mills see the worst cracking just below peak rolling temperatures (often around 1100–1150 °C), with less risk when the slab is either cooler and fully solid or hotter and the Cu-rich liquid is tied up in the scale.
Exact thresholds depend on grade, copper level, furnace time, and strain rate.
What elements increase the risk in steel
- Copper above a modest level increases risk (commonly discussed around >0.2 % Cu depending on practice).
- Tin, antimony, arsenic aggravate by forming very low-melting films.
- Poor control of sulfur/phosphorus or manganese can reduce hot ductility in other ways and make cracks worse.
What elements reduce the risk in steel
- Nickel helps hold copper in solution and raises the melting point of the boundary liquid; many shops target a Ni:Cu ratio near 1:1 to 2:1 when copper is unavoidable.
- Silicon promotes fayalite in the scale, which can help trap copper away from the steel surface and reduce penetration.
- Clean steelmaking and desulfurization also support better hot ductility.
What is the difference between hot shortness and hot cracking
- Hot shortness (steel, classical): loss of ductility caused by liquid metal at grain boundaries (often copper-rich) during hot working after oxidation.
- Hot cracking / hot tearing (steel or aluminum): shrinkage and feeding problems near the solidus during solidification or during early solid-state, often driven by restraint, temperature gradients, and insufficient feeding—not necessarily copper related.
In aluminum fabrication, many technicians say “hot short” when they observe hot cracking. It’s okay to use the shop term, but separating the mechanisms helps you fix the real cause.
How is hot shortness tested and detected
- Hot tensile / hot bend after a controlled oxidation step to replicate mill scale.
- Zero-ductility temperature (ZDT) or ductility trough testing to find unsafe straightening windows.
- Process monitoring: slab reheating time/temperature, descaling quality, straightening strain, rolling schedule.
- Visual and NDT on edges and surfaces for transverse cracks after hot work.
How do you prevent hot shortness in steel production
- Control chemistry: limit Cu, avoid Sn/Sb/As, target Ni:Cu ≈ 1:1–2:1 when Cu is present, keep Si within spec to support protective scale, manage S/P/Mn.
- Manage oxidation: avoid long, aggressive furnace soaks that enrich Cu at the surface; descale effectively before heavy deformation.
- Pick safe temperature bands: schedule roughing/finishing and straightening outside the ductility trough.
- Control strain rate and reduction: do not impose large, sudden stretch while the boundary film is liquid.
- Quality gates: hot-ductility checks on heats with higher Cu; extra inspections when furnace cycles deviate.
Does aluminum have hot shortness
Aluminum doesn’t show the same copper-film mechanism as steel, so metallurgists usually talk about hot cracking instead. But in shops you will hear “hot short” for aluminum when parts crack while hot. The common cases are:
- Welding certain alloys (e.g., some 6xxx) without the right filler or with high restraint.
- Die casting thin-wall shapes that solidify unevenly or have poor feeding paths.
How do welders avoid it in aluminum
- Choose filler wisely:
- 4043 often reduces cracking in many 6xxx welds thanks to its silicon; color after anodize may differ.
- 5356 can give higher joint strength and better color match but may be less forgiving in some crack-sensitive joints.
- Reduce restraint and shrinkage stress: good fit-up, proper sequencing, adequate preheat when specified.
- Manage heat input and travel speed: keep a consistent puddle and avoid excessive dilution with a crack-sensitive base.
- Use procedures qualified for the alloy: follow WPS/PQR guidance rather than general advice.
How does this relate to aluminum die casting
In die casting the concern is mostly hot tearing/hot cracking during solidification—not copper hot shortness. To reduce risk:
- Select the right alloy (e.g., A380/ADC12/AlSi10Mg families have good castability).
- Balance die and metal temperatures so the part freezes directionally toward a feed path.
- Design for feeding and release: uniform walls, generous fillets, avoid sharp hot spots, provide overflows and vents.
- Tune fill profile: consistent, fast enough to avoid cold shuts but not so violent that it traps oxides.
- Post-cast machining: leave stock on critical datums so you can machine away small surface tears if they occur.
How can beginners tell if a crack is hot shortness or hot cracking
- Where it appears: steel hot shortness is commonly near surfaces after hot working; aluminum hot cracking often runs through hot spots or weld toes.
- When it appears: hot shortness shows during hot work; hot cracking forms while solidifying or shortly after.
- What fixes it: steel → chemistry/temperature/scale control; aluminum → filler/heat input/feeding control.
How should buyers specify and inspect to avoid surprises
- Material callouts: maximum Cu for steel, or Ni:Cu targets if Cu is present; full grade/temper for aluminum.
- Process notes: reheating/straightening windows for steel; die temperature and filling controls for die casting; qualified WPS for weldments.
- Inspection plan: hot-ductility tests on risk heats; surface NDT for transverse cracks in steel; casting X-ray/CT where appropriate; weld macro-etch for trial builds.
- Corrective path: if cracks appear, record temperature, time, strain, chemistry; run a short design of experiments to isolate the variable.
What are the key takeaways for non-experts
- In steel, “hot shortness” usually means a copper-related surface embrittlement during hot work.
- In aluminum, people often use “hot short” to describe hot cracking, which has different causes; fix it with filler choice, heat control, and sound feeding.
- Most problems are predictable and preventable with the right chemistry, temperature windows, and process checks.
Ready to de-risk hot cracking in your aluminum die-cast parts?
Send us your 3D model (STEP/Parasolid) and a short note on alloy and production target. We’ll review your design for hot-spot / hot-crack risk and reply with practical, die-casting-specific fixes:
- Alloy guidance: A380 / ADC12 / AlSi10Mg selection for crack resistance & flow.
- Thermal window: die and melt temperature bands to avoid hot-tear zones.
- Geometry tuning: uniform walls, fillets, local ribs, and radii to reduce restraint.
- Feeding strategy: runner/overflow/vent concepts to solidify toward a feed path.
- Process notes: fill profile, gate location, and post-cast machining allowances for clean datums.
Buttons you can use:
- Get DFM for Die Casting
- Upload CAD for Hot-Crack Check
- Ask an Engineer (Die Casting)
(If your current parts are sheet/CNC and cracking during welding, we can also suggest a cast-and-machine path to remove the hot-crack risk and lower cost at volume.)
