Nondestructive methods for testing casting quality. (Casting Facts).
Foundries must meet the ranges of quality deemed acceptable for each application by casting end users. Before a part can be sold or used the foundry must measure it on multiple criteria to ensure that its operational limits are not exceeded during normal use, or that detrimental defects are not present in the casting. Many different tests have been designed to examine mechanical, chemical, and aesthetic properties of castings.
Destructive vs. Nondestructive
Casting quality tests can be divided into two major categories: destructive and nondestructive. In destructive testing, the substrate or part being tested is in some way permanently altered or deformed. A destructive test can be performed only once, and if another measurement is needed, another part representative of the first must be tested and subsequently destroyed.
This destruction creates several problems. The first and most deterring is that a broken or damaged casting cannot be sold. Repeatability causes a problem because a suspected erroneous measurement cannot be taken again with the same casting. In any testing operation, multiple iterations of the same test will be performed and an average result is reported. This means that many castings or test substrates will be destroyed in an attempt to obtain a usable measure.
Nondestructive testing methods are used more widely for several reasons. The part can be tested without sacrificing its integrity. A nondestructive test can be performed over and over on the same part, or any number of parts, with no negative impact on the part itself.
The real value of these nondestructive methods has been realized in manufacturing environments where 100% inspection is a must. All parts can be tested prior to shipping. This gives the manufacturer and buyer trust that the parts they are utilizing are of the highest quality. This is particularly important in applications where a failure could cause a serious accident or loss of life Following is a look at some of the more common nondestructive testing methods used in the foundry industry today.
Many castings that are produced appear to be void-free with just a simple visual inspection. However upon subsequent machining operations, defects can be found just below the surface or deep within the casting. This presents the question: how many more defects have not been found? Engineers use a powerful x-ray unit to penetrate the casting and then develop film that will reveal these defects.
This is done in the same manner that doctors use to inspect human bones. A piece of film is placed under a casting, and x-rays are directed through a diaphragm into the casting, and eventually through the casting to the film, Any discontinuity through the casting cross-section is revealed on the film by either lighter or darker regions. If a hole or void exists in a part of the casting, more x-rays will be allowed to pass through, subsequently overexposing the film. If a very hard, dense region is evident in the casting, this will appear as an underexposed area.
More modern methods of x-ray inspection include placing castings on a belt or manipulator and moving them under an x-ray source, much like at an airport. A monitor displays images to the operator, allowing a thorough inspection. This provides fast analysis, which is convenient in 100% inspection applications.
Similar to x-ray inspection, ultrasound is used to check for voids or discontinuity within a casting (Fig. 1). Instead of directing x-rays through the casting, a sound wave is "bounced" through the part. An oscilloscope device is used to read the sound wave.
If any cracks, shrink voids, holes or other defects are present, they will appear on the screen as echoes of different frequency and amplitude. Typically a front echo and back echo will be seen as the sound wave bounces off the front and back surface of the casting. Any reflections in-between these echoes indicates discontinuity, and thus a defect.
Magnetic Particle Inspection
A failure often will occur because of a stress concentration created by a small crack in the surface of a casting. A small crack will propagate as load cycles continue to fatigue the part and eventually become large enough that it causes failure. This becomes a problem when dealing with applications where casting integrity is crucial to safety, such as aerospace and automotive structural applications.
Magnetic particle inspection is one method implemented to find cracks at or near the surface of a casting (Fig. 2). An operator uses an electromagnet to magnetize the casting. A magnetic powder is sprinkled onto the casting surface. Cracks or discontinuities in the surface of the casting will cause a magnetic field 'leak." The powder will form a pattern at the leak, making visual detection very easy.
One problem with this method is that only the cracks that lie in a direction transverse to the lines of magnetic flux will be detected. An easy way around this is to turn the part 900 and retest the part. Once the magnetic field is released, the particles can be brushed away.
Dye Penetrant Testing
Using a penetrating dye is another method of finding cracks on the surface of castings (Fig. 3). A part is first cleaned so that the surface is free of oils and other contaminates. It then is dipped or sprayed with a visible or ultraviolet dye. The dye will find and fill the cracks on the surface. The excess dye is removed with a washing step that does not wash the dye out of the crack. Often a developer is applied to help draw the dye to the surface.
The dyed cracks then are viewed under a bright light (or black light if an ultraviolet dye was used). After testing, the castings can be washed and sold, or retested if necessary.
Many of the castings manufactured today are designed for use in hydraulic, pneumatic and other "sealed" applications. These types of castings are required to maintain pressure or a vacuum and contain fluid. For these castings, a leak or pressure test will be performed, generally using two predominate pressure testing methods.
The "bubble" test is the easiest and least expensive to perform. The casting is pressurized and then submerged in a tank of water. The operator will watch for any air bubbles escaping from the casting. If air is leaking through the casting wall, the part will either have to be scrapped or repaired.
Another method uses a slightly more complex setup (Fig. 4). One side of a casting will be pressurized with a tracer gas (typically helium). The other side will be connected to a vacuum pump and a detector, which recognizes the tracer gas. If any of the tracer gas is allowed to pass through the casting wall, a leak exists.
These are only a few of the many nondestructive tests being utilized today. All tests from a visual inspection to the most expensive and complex testing apparatus will help foundry personnel produce higher quality metal castings. These tests assure both the manufacturer and end user that the castings are capable of performing as intended.
For More Information
Nondestructive Testing Handbook, 1982, American Society for Nondestructive Testing, Inc.
Handbook of Metal Treatments and Testing, R. Ross, 1977, E. & F.N. Spon Ltd.
Mechanical Testing of Materials, A.J. Fenner, 1965. Philosophical Library, Inc.
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|Date:||Aug 1, 2002|
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