Introduction to NDT (Non-Destructive Testing) for Castings
When customers want assurance their castings are free from internal defects, they can request Non-Destructive Testing (NDT). NDT encompasses a range of techniques used for verifying the quality of metal castings.
Reasons For Inspecting Castings
Casting involves pouring molten metal into a cavity where it’s left to cool and solidify. As with any liquid, there’s always some variability in how the metal flows, which can give rise to defects. Careful design of patterns and management of the casting process minimizes their occurrence but can’t completely eliminate the risk.
Casting defects can be visible on the surface or contained within the body of the cast part. If located in a region subjected to high loads, they could lead to the part failing. Skilled patternmakers use their experience in metal solidification to ensure any defects that do arise are in permissible regions. However, in cases where this isn’t possible it’s prudent to carry out NDT.
In addition, when a casting will undergo extensive machining, (more common for sand castings than investment castings,) finding defects beforehand will save time and money.
Types of Defects Detected by NDT
Cooling leads to shrinkage that can cause cracking. Cracks may break the surface of the part or could be purely internal. Surface-breaking cracks are often extremely difficult to see with the unaided eye because they’re very thin and the textured surface of the casting, especially with sand castings, makes them virtually invisible.
Other defects that can cause a casting to fail are porosity and inclusions. Both may be visible but are often below the cast surface.
NDT Methods, Strengths and Limitations
There are many ways of detecting defects in castings. The most widely used methods, ranked approximately from easiest/least expensive to most difficult/expensive, are:
- Magnetic particle/dye penetrant
- Ultrasonic inspection
- Eddy current
- X-ray and CT scan
Here’s a closer look at each.
Magnetic particle/dye penetrant
These methods find surface-breaking cracks. The magnetic particle technique uses the concentration of magnetic flux along a crack to attract particles that make the crack more visible. It only works on ferrous materials.
Dye penetrant methods work on ferrous and nonferrous materials. They use a low viscosity liquid that gets drawn into cracks by capillary action. After being allowed to soak, the surface is wiped clean and the dye makes the cracks show up.
This technique involves sending very high frequency sound waves into the casting and listening for echoes or reflections. These are produced wherever the incident sound wave encounters a change in material density.
There are various ways of performing ultrasonic inspections: selection depends on the type of defects being sought and the speed or process rate needed. Ultrasound is generally very effective for finding internal cracks, porosity, and inclusions but the equipment can be expensive and needs a trained operator.
This finds defects on or just below the surface. It can also be used as an indication of material properties such as hardness. Eddy current works by creating a magnetic field that is disrupted by the presence of discontinuities.
Eddy current inspection is used mainly with ferrous metals but will also work with those that are nonferrous. Eddy current equipment is not expensive but does require a highly trained operator to get repeatable results.
X-ray and CT scan
For obtaining a view through a casting, x-ray is the “gold standard”. It reveals differences in density, like those caused by porosity. However, its ability to highlight cracks depends on how the crack is oriented with respect to the x-ray beam.
X-ray equipment is expensive and requires safety protocols and highly trained operators. In addition, seeing through a large, dense casting like one made from iron, needs high levels of power.
CT is a form of 3D x-ray. The part is rotated in the beam and the resulting images processed into a 3D picture. This can be rotated and viewed from any direction. It is an unparalleled method of inspecting castings but expensive and slow.
Agree Inspection Methods
Not all castings need NDT to verify the absence of defects; it depends on the application and degree of risk. Where a customer feels it is required, the specific details should form part of the purchase agreement. Contact us for more information about the options for NDT.