Finding casting defects through destructive and non-destructive testing
Every step in the process of metal casting, from pattern-making to heat treating, is done carefully to avoid problems with the soundness, surface finish, mechanical properties, and final dimensions of the finished casting. Yet even castings made with diligence should undergo inspection for quality control. Small problems can arise unexpectedly, and many castings have mechanical requirements that may be undermined by a hidden defect. Inspection allows foundry and clients to feel confident they have a quality casting.
Common casting defects and discontinuities
Surface defects are visible to an inspector. These include very rough or uneven surfaces; “veins” or “rat tails” caused by cracking of the mold at high temperatures; “elephant skin,” which is puckered due to quick cooling; burned sand; and stripping defects. Stripping defects are flakes of metal on the surface caused by gas being trapped in the mold.
Slag inclusion is a defect where non-metallic materials create pockets or ribbon-like entrainments in the casting. Slag is an important part of the melting process in the furnace, necessary for good quality castings, as chemical processes in silica and calcium-based slags change the viscosity of the metal. During pouring, however, the foundry worker must keep slag out of the mold itself. Just before pouring, slag is often skimmed, but not all the slag is be captured at this stage. The sprue or gates of the mold also need to be formed to capture it. Failures at any step in this process may allow slag to contaminate the metal. Further, oxidization of the metal can be a problem for metals that have stayed at high temperatures a long time; slag can form inside the mold. Oxides are not the only slag: carbides, sulfides, or nitrides can also be at fault.
Sand inclusion is very common. It is expected in small quantities at surface depths for all sand castings. However, too much sand inclusion can ruin a casting. In this defect, sand from the mold gets trapped in the metal.
If there is a lot of sand inclusion at the surface, it might indicate the need for a mold wash, or a different molding system. In foundry sand that has been baked for stability, perhaps the mold spent too long in the oven and became fragile. Perhaps a better choice would be no-bake molding or an investment casting. This defect may also suggest there was not enough binder, or not the right kind of binder, to create a strong shell. Another possibility is that the sand was insufficiently packed.
Molding and pouring defects
Underpour or misrun defects happen when not enough molten metal has been introduced to fill the mold. Sometimes, open risers are used to judge when a mold is full: if they are misplaced, the casting might be short poured. Sometimes, misruns are caused by early freezing during the pour. In misruns, some part of the casting is incomplete, usually with a rounded edge where the metal froze before it reached the mold’s wall.
A raised mold or floating core will cause the casting to have the wrong dimensions. Molten metal exerts pressure, and if the core is not stabilized or mold properly clamped down, they can lift, deforming the casting.
Flashing at joins is common in castings but must be removed. Flashing is a thin skin created when liquid metal seeps between closed joints on the mold. If there is instability in the mold, flashing can be bad enough to deform the casting.
Mold mismatch or mold shift can happen if the cope and drag, or top and bottom of the mold are misaligned when it is closed. The resulting casting is often comical, as the top and bottom parts of the mold are askew.
Sand instability, insufficient packing, or issues with the foundry sand mixture can cause a loss of detail on a casting. This issue can affect fine details, like letters or decorations, or can lead to wavering edges over the whole casting.
Cold lap or cold shuts happen when the temperature of the mold or of the molten material is too low. Rather than flowing quickly throughout the mold, part of the stream of metal slows and begins to harden. This freezing bulge becomes an obstacle that the rest of the metal eddies around: like a rock in the river, there is often a disturbance on the down-stream side of the obstacle. The visible round lip of the cold lap creates a permanent discontinuity in the surface of the casting. This defect is visible on the surface, and sometimes is small enough to be ground or filled, but a cold shut or lap can go deep enough to threaten the structural integrity of the casting.
Cold shots, similarly, are caused by the premature freezing of some of the metal. They look like little iron balls or teardrops that are held, suspended, in the material around them.
Pinholes and blowholes or gas porosity can be caused by gas in the mold pushing through the molten metal leaving voids or bubbles as it cools. Trapped gas can be caused by the conditions of the mold, if it’s not porous enough to allow gas escape. Rusty, hydrogen rich charges of scrap metal are more likely to cause pinhole defects as they bring more hydrogen with them.Gas porosity is different than shrinkage porosity, although both leave small holes in the casting and can be found in the same piece of metal. Gas porosity tends to leave regular, bubble shaped pinholes near the top of the mold.
Shrinkage porosity is caused by insufficient volume of metal in the mold. As the metal cools and shrinks, small holes are left behind throughout the casting. These holes are usually jagged, in comparison to the smooth floating pinholes caused by gas.
Shrinkage cavities are like shrinkage porosity in that there is not enough metal to fill the space as the casting cools and shrinks. However, this defect tends to be a more serious structural issue. Often, these shrinkage cavities will appear like large, ragged crevices in the center of a volume of metal. Castings cool from the outside in, and so the cavity can form in the last place to cool. That is not universally the case: shrinkage depressions visible on the outside of the casting can appear depending on the infrastructure of gates, runners, and risers supporting the casting. These depressions are not normally jagged or open, but rather places where the metal seems sunken compared to the intended shape. An object suffering from a lot of shrinkage might be fixed by changing the casting design, by tapering walls and smoothing corners, placing additional risers in the pattern, or by changing where the gates emerge.
Chilling defects may happen if the mold is too cool or the casting is removed too quickly from the sand. Rapid freezing can cause the surface of castings to become extremely brittle, as though they’d been air quenched. When the defect is mechanical but not structural, annealing the casting may be able to save it, by removing the brittleness through heat treating.
Hot tears or cracks can also arise in too rapid cooling. These are akin to shrinkage cavities but are more ribbonlike voids in the metal caused by rapid shrinkage. Castings with hot tears or cracks will underperform mechanically.
Cooling deformation is a common defect in very long, thin castings, but can happen in other shapes too. In this defect, the metal casting warps away from its intended shape during cooling, causing it to be outside of specified tolerances.
Dimensional tolerance or surface finish problems can often be seen with visual inspection and measurement. Tests for mechanical properties can also be done simply. Some of the other problems, like porosity and shrinkage cavities, are internal to the casting. Testing methods are available to catch internal problems so that a casting doesn’t unexpectedly fail under load. There are two main types of internal tests: destructive and non-destructive.
In every production run, the foundry will choose a few samples and submit them to destructive testing. The casting is cut, and the properties of the metal inspected closely. The tester will look for inclusions, porosity, and shrinkage. Although destructively testing one casting does not guarantee anything about the other castings in the run, it does give a sense of the overall quality of the process. Radiographic and ultrasonic technologies have decreased the importance of destructive testing, but it is still used to inspect the quality and make evaluations about a run.
Non-destructive testing (NDT)
Non-destructive testing is done by foundry workers, clients, and NDT technicians to verify the internal and external soundness of a casting without damaging the casting itself.
uses the human eye to identify surface defects, cracks, gas evolution, slag or sand inclusions, mis-runs, cold shuts, and molding flaws.
is undertaken to ensure a part meets dimensional requirements/tolerances. This can be done manually or with a coordinate measuring machine (CMM) that uses probes to get very precise measurements.
Liquid Dye Penetrant Inspection (LPI)
finds tiny cracks, pores or other surface imperfections in all types of metal castings which would be hard to see by looking. The tester first cleans the casting to remove any particles of grit or dust that may prevent the liquid dye from going into cracks in the metal.
Once clean and dry, the tester bathes the casting with a penetrant solution. Different types of LPI use different solutions, but generally it’s a brightly dyed oil with high capillary action and low viscosity, meaning it will run freely into cracks in the casting. This dye is left for a “dwell time” so that it can work its way into any invisible crevices.
After sufficient time has passed to allow the liquid to do its job, excess is removed from the surface. This is generally done by gently wiping with a damp cloth, making sure not to flood the casting which could remove the dye in the cracks.
The tester then applies a special developer and the casting defects become clearly visible.
Magnetic Particle Inspection (MPI)
is similar to LPI in that it is used to find small cracks and holes on the surface or shallow subsurface of a casting. However, this process can only be used in castings made of ferromagnetic metal that can create a magnetic field—metals like iron, cobalt, nickel, and some of their alloys. The casting is magnetized, usually with electromagnets, to start the test.
A magnetic field is stronger in metal than in air. Where there are discontinuities like cracks or holes in the surface or close subsurface of a casting, the magnetic field induced will be disrupted.
Very deep cracks will often not create enough of a magnetic distortion at the surface to be found in this way.
To find the disruptions, the casting is sprayed with a dust or liquid containing small particles of iron oxide or other substance that react in a magnetic field. This sprayed particulate will cluster near the edges of distortions, outlining places where the magnetic flux is low. Therefore, using magnets and magnetic powder can be used to show disruptions where there is more air than metal, in any cracks or above any holes. The MPI method is used to inspect castings and is also used in the field to test metal fatigue in already operational pipes and structures. It can detect stress cracking invisible to the naked eye.
Ultrasonic Testing (UT)
finds defects by using high frequency acoustic energy transmitted into a casting, in a technology similar to the ultrasounds used by medical technicians. Sound waves travel through a casting until they hit the opposite surface or an interface or defect. Any barrier reflects the sound waves, which bounce back and are recorded for an analyst to look at. The pattern of the energy deflection can indicate the location and size of an internal defect. This non-destructive test can also be used to examine wall thickness, and the nodule count of ductile iron. Extremely small flaws can be found with UT at very large depths, allowing for a great deal of accuracy and confidence. An experienced technician can even make estimates as to the nature of an alloy by looking at the acoustic signature of an unknown metal.
Ultrasonic testing requires knowledge and experience for an accurate interpretation of results. The part must be cleaned of loose scale and paint, and must not be too irregular, small, or thin. In most cases, a surface to be examine with ultrasound must be wet, and often water is used: if the surface will rust, then an anti-freeze solution with rust inhibitors can be used instead.
Radiographic Inspection (X-Ray)
creates images like those in a hospital that show broken bones. The ghostly images produced through casting X-Ray show dark spots where there are shrinkage cavities, the small breaks and crevices of heat cracking, or the pinhole dots of porosity. These images help an experienced metalworker decide if the casting’s mechanical properties are compromised by shrinkage, inclusions, or holes and whether they can be fixed before castings are shipped.
During the radiographic inspection process, a casting is exposed to radiation from an x-ray tube. The casting absorbs part of the radiation, and the remaining portion of the radiation exposes the radiographic film. Denser parts of the casting will withstand the radiation penetration, so the film is exposed to a lesser degree in those areas, giving the film a lighter appearance. Less dense parts of the casting allow more radiation penetration, resulting in greater film exposure. Every space in the casting therefore casts a “shadow” on the final x-ray, caused by radiation passing more easily through it, and the x-ray depicts any crack, void or inclusion as a dark area on the film.
After inspections are completed by the foundry, the inspected and accepted casting is sometimes used as-is. Common surface irregularities or discontinuities may not matter to the use of an otherwise sound product. Sometimes, finishing can address observed issues. The casting may go back to heat treatment, or on to further processing, which may include painting, rust preventive oils, other surface treatment like hot-dip galvanizing, and machining. Final preparations may also include electrodepositing plated metals or powder coating for cosmetic or operational requirements.