Hot Tears vs Cold Cracks in Stainless Steel Investment Casting: A Foundry Engineer's Guide to Identification, Causes & Prevention

Cracks on stainless steel Investment Castings fall into two fundamentally different families:hot tears and cold cracks. They look different, they happen at different times, they favor different steel grades, and — critically — they require different prevention strategies. Conflating the two is one of the most expensive mistakes a buyer or a junior foundry engineer can make.

I have been inspecting stainless steel investment castings at PINGHENG Machinery for thirty years. In that time I have dispositioned hundreds of cracked parts. This guide distills what I look for first, what causes each defect, and what to put on your purchase specification so the foundry cannot choose the wrong prevention path. If you remember nothing else, remember this: a black tortuous crack is a hot tear in an austenitic grade; a bright straight crack is a cold crack in a martensitic grade. The rest of this article explains why.

Hot Tears vs Cold Cracks: An 8-Dimension Side-by-Side Comparison

Before we dive into mechanisms, here is the comparison table I use internally when triaging a cracked casting. Eight dimensions, each one verifiable on the casting itself or in the metallographic lab. Use it as your first-pass checklist.

The table makes the operational point clearly: if you see a black tortuous crack on a 316 valve body, the issue is upstream design or pouring; if you see a bright straight crack on a 17-4PH pump housing, the issue is post-cast stress relief. Two different root causes, two different fixes. Confusing them wastes a quarter of your engineering budget.

What Are Hot Tears in Stainless Steel Investment Casting?

A hot tear is a crack that forms when the thin liquid film remaining between grains during the final stage of solidification is torn apart by tensile stress. Because the crack opens while the metal is still above approximately 1000 °C, oxygen from the air rapidly oxidizes the freshly exposed surface, giving the crack its characteristic black or dark reddish-brown color.

Austenitic stainless steel (304, 316, CF8, CF8M) is uniquely vulnerable to hot tearing for three metallurgical reasons. First, austenitic steels have a wide solidification temperature range, which means the vulnerable solid-plus-liquid coexistence stage lasts longer than in carbon steel. Second, the linear shrinkage rate of austenitic steel is higher than that of ferritic or martensitic steel, so the contraction strain at the end of solidification is larger. Third, the ceramic shell of the investment mold can clamp down on the contracting casting and add a reverse tensile stress on top of the solidification shrinkage stress. The combination is what makes austenitic stainless steel the canonical hot-tear victim.

Main characteristics of a hot tear

• Shape: tortuous, discontinuous, jagged — the crack follows grain boundaries rather than cutting through grains, because the fracture happens in the weak intergranular liquid film rather than in the solid metal.
• Color: black or dark reddish-brown inside the crack. This is a thick oxide layer formed by exposure of the freshly torn liquid film to air at high temperature. The color is your single fastest visual identifier.
• Location: almost always at the thickest central sections of a casting, at abrupt wall-thickness transitions, or at sharp internal corners. These are the locations that cool last, solidify last, and accumulate the largest tensile stress.

Causes of hot tears

The fundamental cause is excessive tensile stress during the final stage of solidification, but several contributing factors amplify the risk.

• Alloy composition. If sulfur exceeds about 0.015% by mass or phosphorus exceeds 0.020%, low-melting-point sulfide and phosphide eutectics form persistent liquid films that extend the vulnerable stage. We hold sulfur ≤ 0.015% and phosphorus ≤ 0.020% by mass on every heat we run.
• Pouring parameters. Pouring temperature that is too high promotes coarse grain growth, which reduces the casting's tensile strength at the end of solidification and makes the liquid film even more vulnerable to tearing.
• Shell collapsibility. If the ceramic shell uses too much refractory binder, it will not yield during cooling and will clamp down on the contracting casting. Combined with austenitic steel's high shrinkage, this produces exactly the reverse tensile stress that opens hot tears.
• Design geometry. Wall-thickness transitions sharper than 2:1 within 25 mm of length, or corners sharper than a 1×wall-thickness fillet radius, are the geometry triggers. The thin section solidifies and shrinks first; the contraction force is then transmitted through the still-liquid thick section, and the liquid film opens.

How to prevent hot tears

Hot-tear prevention operates on five levers, in roughly this order of leverage.

1. Specify a minimum fillet radius. The fillet radius at any thickness transition should be at least 0.5 to 1.0 times the adjacent wall thickness. Put this in writing on the drawing. Sharp 90-degree corners in austenitic stainless steel are an open invitation to hot tears.
2. Maintain uniform wall thickness. Where thickness transitions are unavoidable, taper them gradually. Avoid the classic 5 mm-to-25 mm step within 10 mm of length — this geometry will hot-tear in 304 or 316 nearly every time.
3. Cap sulfur and phosphorus. Sulfur ≤ 0.015% by mass and phosphorus ≤ 0.020% by mass. Verify with a spectrometer check on every heat.
4. Optimize shell collapsibility. Foundries should control refractory content in the ceramic shell so the shell yields slightly during cooling rather than gripping the casting.
5. Pour at the lowest temperature compatible with complete fill, and pour slowly. Lowering the pouring temperature reduces the time the liquid film persists and improves the casting's tensile strength at the end of solidification.
6. Position the gating system at the thickest section. This directs solidification from the edges toward the center, so the tensile stress generated by shrinkage concentrates in the riser instead of in functional geometry.

What Are Cold Cracks in Stainless Steel Investment Casting?

A cold crack is a crack that forms after the casting has fully cooled, driven by residual internal stress rather than by a torn liquid film. Because the crack opens at or near room temperature, the exposed surface shows no high-temperature oxidation — it has the silvery-white metallic luster of the parent metal. Cold cracks are most common in martensitic and precipitation-hardening stainless steels, particularly 410, 416, and 17-4PH.

The driving stress behind a cold crack has two components. First, thermal stress from uneven cooling — thin sections contract before thick sections, and the differential strain accumulates as a locked-in stress field. Second, and uniquely severe in martensitic grades, the austenite-to-martensite phase transformation that occurs as the casting cools through approximately 200 °C is accompanied by a roughly 4-5% volume expansion. When the thin and thick sections transform at slightly different times, the asynchronous expansion generates what metallurgists call microstructural stress. Add the two together and you have enough stored elastic energy to pop a crack open days or even weeks after the casting was poured.

Main characteristics of a cold crack

• Shape: straight, smooth, fine line. The crack cuts through grains (transgranular) rather than following grain boundaries, because it is a brittle fracture of fully solid metal rather than a tear in a liquid film.
• Color: bright, silvery-white metallic luster inside the crack. No oxidation, because the crack formed at or near room temperature.
• Location: stress concentrators — sharp internal corners with insufficient fillet, un-chamfered edges of drilled or machined holes, junctions between ribs and walls. In severe cases, the crack splits the entire casting.

Causes of cold cracks

• Uneven cooling. Thin sections lose heat and contract first, locking in tensile stress in the adjacent thick sections.
• Phase transformation in martensitic grades. Austenite-to-martensite transformation below 200 °C generates 4-5% volume expansion. If the thin and thick sections transform asynchronously, the differential expansion creates large microstructural stress.
• Missing or incomplete stress relief. If the casting skips the stress-relief anneal after cooling, residual thermal and microstructural stress remain in the part. The casting is then a ticking time bomb — it can crack during storage, during machining, or during low-temperature service.

How to prevent cold cracks

1. Add fillet radii at every internal corner. Cold cracks concentrate at stress risers, and a generous fillet radius (at least 0.5-1.0× wall thickness) redistributes stress across a curve instead of letting it spike at a point.
2. Engineer uniform cooling. Wrap thin sections of the ceramic shell with a layer of refractory insulation to slow their cooling rate. Position the sprue in the thin-walled area to raise its temperature. The goal is to make every section of the casting cross the martensite-finish temperature at roughly the same time.
3. Sand-bury hot shells immediately after pouring. For martensitic grades, place the freshly poured shell into a heat-retaining pit or bury it in high-temperature quartz sand or ash powder. The thermal mass of the sand keeps the entire casting at a uniform temperature as it cools, so the austenite-to-martensite transformation happens simultaneously throughout the part.
4. Run stress-relief annealing on every martensitic casting. Heat to 550-650 °C, hold for 2-4 hours, then cool slowly. This brings the residual stress to near zero and prevents delayed cracking during storage or service.

Which Prevention Strategy Should Your Buyer Specification Require?

The prevention strategy you specify must match the steel grade on the drawing. This is the single most common specification mistake I see: a buyer writes a generic "prevent all cracks" line into a purchase order and lets the foundry choose the lever. The foundry then optimizes for the wrong stress, and the cracks return.

For austenitic grades (304, 316, CF8, CF8M) — prevent hot tears

Your specification must include:

• Minimum fillet radius of 0.5-1.0× adjacent wall thickness at every transition. Verify the drawing before releasing it for quote.
• Wall-thickness transition ratio no greater than 2:1 within 25 mm of length.
• Sulfur ≤ 0.015% by mass, phosphorus ≤ 0.020% by mass, verified by spectrometer on every heat.
• Pouring temperature and shell-collapsibility controls documented by the foundry. Require the foundry to state both on the first-article inspection report.

For martensitic and PH grades (410, 416, 17-4PH) — prevent cold cracks

Your specification must include:

• Mandatory stress-relief annealing at 550-650 °C for 2-4 hours, with a furnace-chart record attached to each batch.
• Sand-burial or heat-retaining pit cooling immediately after pouring, before the casting drops below 200 °C.
• Fillet radii and uniform wall thickness rules identical to the austenitic set — they apply to cold-crack prevention as well, because stress concentrators are bad for both defect types.

For mixed-grade or unknown-grade situations

If you are not sure which grade you are receiving, default to the more conservative specification: fillet radii, sulfur/phosphorus caps, and stress-relief anneal. The cost of doing both is roughly 3-5% of part cost; the cost of a single scrapped batch is many multiples of that.

What Factory Data Tells Us About the Two Defects

Across 1,260 stainless steel investment casting batches we dispositioned between 2025 and H1 2026, 38 batches (3.0%) showed confirmed crack failures. The split between the two defect types is stark and aligns with the metallurgical expectation:

• Hot tears accounted for 27 of the 38 incidents (71%). Twenty-four of those 27 were in 304 or 316; the remaining three were in 316L with elevated sulfur. The most common root cause was inadequate fillet radius at a wall-thickness transition (16 incidents), followed by elevated sulfur or phosphorus (7 incidents), and shell-collapsibility issues (4 incidents).
• Cold cracks accounted for 11 of the 38 incidents (29%). Nine of those 11 were in 17-4PH and two were in 410. The most common root cause was missing or incomplete stress-relief annealing (8 incidents), followed by insufficient fillet radius at a stress concentrator (3 incidents).

The single highest-leverage change we made was requiring a fillet radius callout on every drawing released for quote. Hot-tear incidents on parts drawn after that rule took effect dropped by roughly 60% within six months. The second highest-leverage change was tightening the sulfur cap from 0.030% to 0.015% on all austenitic heats, which eliminated the "elevated sulfur" cluster entirely.

Need a Foundry That Solves Both Hot Tears and Cold Cracks at the Design Stage?

At PINGHENG Machinery we run spectrometer checks on every heat, fillet-radius verification on every drawing, and stress-relief annealing on every martensitic casting. We document every step of the prevention chain on the first-article inspection report, so you can audit the exact lever we used to keep your part crack-free.

If you are evaluating foundries for a stainless steel investment casting program, send us your drawing, your steel grade, and your service conditions. We will return a quote within 48 hours that includes the crack-prevention strategy in writing — not as a verbal side comment, but as a line item on the quote.

Email: info@thecastingfactory.com · Phone / WeChat: +86-15724260192 · Website: thecastingfactory.com

Frequently Asked Questions

1. What is the quickest way to distinguish a hot tear from a cold crack on a stainless steel casting?

Look at the color inside the crack. If the crack appears black or dark reddish-brown, it is a hot tear (oxidized at >1000 °C). If it shines with a bright, metallic, silvery-white luster, it is a cold crack (formed after the casting cooled). Shape is a secondary clue: hot tears are tortuous and jagged; cold cracks are straight and smooth.

2. Why do 304 and 316 stainless steel castings rarely experience cold cracking?

Because 304 and 316 are austenitic grades that remain fully austenitic at room temperature. They do not undergo the austenite-to-martensite phase transformation that drives the volume change responsible for cold cracking in martensitic grades such as 410 or 17-4PH. Austenitic steels also retain excellent ductility and toughness at sub-zero temperatures, which further suppresses cold-crack initiation.

3. What is the minimum fillet radius required to prevent hot tears in stainless steel castings?

As a design-for-manufacturing rule, the fillet radius (R) at any thickness transition should be at least 0.5 to 1.0 times the adjacent wall thickness. Sharp 90-degree corners must be avoided entirely. A transition that goes from a 5 mm wall to a 25 mm wall within 10 mm of length is virtually guaranteed to hot-tear in austenitic stainless steel.

4. Can cracks in stainless steel castings be repaired by welding?

It depends on the application. For pressure-retaining, safety-critical, or aerospace components, cracks are grounds for scrap — the crack almost certainly extends below the visible surface, and the heat of welding will re-introduce thermal stress that can re-crack the casting. For standard industrial components, a confirmed-through-grinding surface crack may be repair-welded only with mandatory preheat, post-weld heat treatment, and a second PT inspection.

5. Do you detect internal cracks in investment castings?

Yes. We use 100% liquid penetrant testing per ASTM E1417 for surface-breaking cracks, plus radiographic testing per ASTM E446 reference images on critical areas and on every first article of a new batch. For high-Precision Castings we also add a second PT after machining to catch sub-surface defects exposed by material removal.

6. How do you control sulfur and phosphorus to reduce hot-tear risk without harming corrosion resistance?

We run a spectrometer check on every incoming heat and every finished batch. Sulfur is held below 0.015% by mass and phosphorus below 0.020% by mass. These limits are low enough to suppress the low-melting-point sulfide and phosphide eutectics that prolong the liquid film and increase hot-tear risk, while still preserving the corrosion resistance required for ASTM A351 CF8/CF8M service.

7. What other defects besides hot tears and cold cracks occur in stainless steel investment casting?

The two most common additional defects are shrinkage porosity (interior cavities caused by inadequate feeding during solidification) and gas porosity (round interior voids from trapped gas). Both are typically detected by radiographic testing rather than PT. Inclusions, surface laps, and misruns are less frequent but still worth specifying on the inspection plan.