Porosity is the defect that hides. A casting can pass every dimension, look flawless on the outside, and be full of voids exactly where the wall carries the most stress. It gets discovered three ways — on a radiograph, on a machining table, or on a live line. The costs of those three discoveries are separated by orders of magnitude. This piece explains where porosity comes from, why the two main types behave differently, what it does in service, and how it gets prevented rather than detected. We deal with this daily at Rainbow Technocast, and prevention is a method question, not an inspection question.
Understanding Casting Porosity
Porosity is voids inside solidified metal. That is the whole definition. What varies is how the voids formed, what shape they take, and where they sit.
Shape matters more than most buyers expect. Rounded voids are relatively benign — metal flows around them and the stress concentration is mild. Jagged, branching voids act like cracks, concentrate stress at their tips, and link up under load. Same volume of missing metal, very different consequence.
Types of Porosity in Castings
Gas Porosity
Gas porosity forms when dissolved gas comes out of solution as the metal freezes. Molten steel holds far more hydrogen and nitrogen than solid steel does. As it solidifies, the excess has nowhere to go and forms bubbles.
Characteristics:
- Smooth, rounded, often spherical voids
- Frequently distributed through the section rather than concentrated
- Can appear as subsurface pinholes just below a machined skin
- Blowholes are the large version, often from mould gas rather than dissolved gas
Shrinkage Porosity
Steel shrinks as it goes from liquid to solid — several percent by volume. If the last region to freeze cannot draw liquid metal from a feeder, the volume it needs simply is not there, and voids form.
Characteristics:
- Jagged, branching, dendritic in appearance
- Concentrated in the thermal centre of heavy sections
- Predictably located — thick sections, junctions, bosses, the region behind a flange
- Far more damaging than gas porosity for the same volume
Shrinkage porosity is the one that causes most critical-service failures, because its shape acts like a crack and its location coincides with the highest stress.
How to Tell Them Apart
On a radiograph, gas porosity reads as discrete round spots. Shrinkage reads as diffuse, feathery, branching darkness. On a machined face, gas porosity shows as clean small holes; shrinkage shows as an irregular spongy patch.
The distinction is not academic. Gas porosity points to melting practice and mould gas. Shrinkage points to the feeding design. They need different corrective action, and a foundry that reports “porosity” without naming the type has not diagnosed anything.
Causes of Porosity in Metal Castings
Causes of Gas Porosity
- Wet or damp charge material, ladles or moulds — moisture dissociates in contact with molten steel and releases hydrogen
- Inadequate degassing or insufficient holding before pouring
- Excessive pour temperature, which increases gas solubility in the melt
- Low mould permeability, so mould gases cannot escape and get pushed into the metal
- Binder decomposition producing gas faster than the mould can vent it
- Turbulent pouring, which entrains air into the stream
Causes of Shrinkage Porosity
- Inadequate risering — the feeder is too small, badly placed, or freezes before the section it feeds
- Isolated hot spots — a heavy region with no feeding path, typically a boss or a junction
- No directional solidification — the casting should freeze progressively toward the feeders, not randomly
- Missing chills where a local heavy section needed accelerated cooling
- Pour temperature too low, freezing the feeding path prematurely
Design Choices That Cause Porosity
This is where buyers have more influence than they realise, and it is exercised on the drawing rather than in the foundry.
- Abrupt section changes. Thick meeting thin creates a hot spot at the junction. Taper the transition.
- Sharp internal corners. They concentrate heat during solidification and stress during service. Fillet them.
- Heavy isolated bosses. A thick pad in the middle of a thin wall will be porous unless it is deliberately fed or chilled.
- Uniformly heavy walls “for safety.” Extra thickness adds thermal mass without adding a feeding path, so it often lowers soundness rather than raising it.
Here is a pattern that surprises people: on pressure-containing parts, thickening a wall to be safe frequently makes it less safe. The metal you added freezes last, and the void ends up in the middle of the section you were trying to strengthen.
Effects of Porosity on Cast Components
Leak Paths
For any pressure-containing part, this is the headline. Individual voids are harmless until they link into a continuous path through the wall. Then the part leaks, and it usually starts leaking after some service — because cyclic pressure and thermal movement gradually connect what was disconnected.
That delay is why a hydrotest pass is not a soundness guarantee.
Mechanical Strength and Fatigue
Porosity reduces the load-bearing cross-section, which cuts static strength roughly in proportion to the area lost. Fatigue is worse, and it is not proportional. Fatigue cracks start at stress concentrations, and a jagged shrinkage void is an excellent stress concentration. A part with modest shrinkage porosity in a highly stressed zone can lose a large share of its fatigue life while showing near-normal tensile results.
Machining Surprises
Subsurface porosity is invisible until the cutter reaches it. Then a finished sealing face develops a spongy patch, and the part is scrap — after all the machining value has been added to it.
This is the most expensive routine outcome of porosity, because the cost lands after maximum investment.
Corrosion Initiation
Surface-breaking porosity traps moisture and process fluid. Trapped fluid concentrates, the local chemistry turns aggressive, and crevice corrosion starts. On stainless parts this is a common route to pitting in service that the material would otherwise resist comfortably.
How Porosity Is Detected
- Visual inspection — only surface-breaking porosity, and only what is exposed
- Radiography (RT) — the primary method for volumetric internal defects; shows type, size and distribution
- Ultrasonic testing (UT) — effective in heavy sections where RT loses sensitivity; good for planar defects and wall thickness
- Dye penetrant (DPI) — surface-breaking porosity on any material, including stainless
- Magnetic particle (MPI) — surface and near-surface on ferromagnetic materials only
- Pressure and leak testing — proves the pressure boundary today, but only detects porosity that has already formed a through path
No single method covers everything. RT plus a surface method on machined faces is the practical minimum for critical parts.
How to Prevent Porosity in Castings
Prevention happens before the pour. The list is not long, but every item is a discipline:
- Solidification modelling or documented method design — know where the last metal will freeze before you make the tooling
- Correct riser sizing and placement so feeders freeze after the sections they feed
- Chills at isolated heavy sections to force directional solidification
- Controlled pour temperature within a narrow window — hot enough to keep feeding paths open, not hot enough to dissolve extra gas
- Dry, clean charge material and ladles
- Degassing and controlled holding time
- Adequate mould venting and permeability
- Non-turbulent gating to avoid entraining air
- Hot isostatic pressing for the highest-integrity parts, closing internal voids under heat and pressure
Notice that eight of the nine happen before metal is in the mould. Inspection sorts good from bad. Method design decides how many of each you make.
Weld Repair Standards for Porosity
Weld repair of porosity is permitted under many standards, within defined limits, using a qualified procedure, with re-inspection afterwards and post-weld heat treatment where the material requires it.
It is not always acceptable, and the boundaries should be agreed before production:
- Some specifications prohibit repair in critical zones entirely
- Repeated repair in the same location usually indicates a method problem, not a bad pour
- An unqualified repair leaves a hard, brittle patch in the pressure boundary — worse than the defect it replaced
Write the repair policy into the purchase order. Zones where repair is prohibited, procedure qualification required, re-inspection scope, and who approves each repair.
Financial Impact of Late Defect Detection
Follow one defective casting through four possible discovery points:
- Caught in method design — cost is an engineering revision before tooling
- Caught at radiography — cost is one casting plus the pour
- Caught at machining — cost is the casting plus all machining value added, plus schedule disruption
- Caught in service — cost is the shutdown, the investigation, and a review of every part from that batch
The ratio between the first and last is enormous, and the deciding factor is almost always whether anyone did the feeding design properly at the start. That is why the cheapest quotation on a critical casting is often the most expensive part you will buy.
Frequently Asked Questions
Is all porosity a reason to reject a casting? No. Standards define acceptance levels because some porosity is tolerable depending on location, type and size. ASTM reference radiographs let you grade severity against defined levels. What matters is whether the porosity is in a critical zone, whether it is shrinkage or gas, and whether it exceeds the agreed level. Set that level in the purchase order.
Can porosity appear after the casting passed inspection? The voids do not form later — they were always there. What changes is that sub-critical porosity can link into a leak path under cyclic pressure and thermal loading, and machining can expose subsurface porosity that no surface method would have found. Both look like “new” defects and are not.
Does thicker wall thickness reduce porosity risk? Usually the opposite. Thicker sections carry more thermal mass, freeze later, and need more feeding. Adding thickness without adding a feeding path moves the void deeper into the section rather than removing it. Uniform sections with adequate feeding beat heavy sections almost every time.
What is HIP and when is it worth it? Hot isostatic pressing applies high temperature and high inert gas pressure simultaneously, collapsing internal voids. It closes internal porosity but does nothing for surface-breaking defects. It adds meaningful cost and lead time, so it is reserved for the highest-integrity applications — aerospace, some critical energy components — rather than routine oil and gas work.
How do I know whether a foundry actually controls porosity? Ask to see the method design for your part — riser placement, chill locations, the predicted last-to-freeze region. A foundry that controls porosity can show you where it expects the risk and what it did about it. A foundry that relies on inspection will talk about its NDT equipment instead.
Conclusion
Porosity is a method problem wearing an inspection problem’s clothing. Get the feeding design, the pour discipline and the section geometry right, and radiography becomes confirmation rather than a lottery.
Send us the drawing for a part that has given you porosity trouble. We will tell you where we expect the last metal to freeze, and what we would change to move the risk out of your critical zone.
About Rainbow Technocast
Rainbow Technocast manufactures precision investment castings for critical service in stainless steel, alloy steel and carbon steel. We design the feeding before we cut the tooling, so soundness is built in rather than sorted out. Our promise: castings that are sound where the stress is, with NDT reports that confirm it.
Send your drawing for a soundness review and quotation.