Many different methods have been developed for inspecting castings for any defects that might arise during the production process. These inspections can either be finished product inspections or in-process inspections.
In-process inspections are conducted before too many castings are produced to detect any flaws that might have developed during the process. That allows for corrective measures to be taken to remove the defect from the remaining units. With finished product inspections, the castings are inspected after all of them are finished to ensure that the product meets the customer’s requirements.
Defective castings can either be salvaged or totally rejected and re-melted for the materials contained in them, depending on the defect’s extent and nature. These methods can also be divided into non-destructive and destructive categories depending on the extent of the damage that is sustained by the casting during the inspection process. Usually, destructive methods are related to breaking or sawing off parts of the castings in places where it is suspected there are internal defects or voids. During strength tests, castings can also become damaged. The disadvantage to destructive tests is that saw cus might miss the flaw, or a sample might not represent the whole lot. Due to these reasons, non-destructive tests are relied on more often than destructive tests.
The following are some of the most prominent non-destructive methods that are used:
This involves using the naked eye or a microscope or magnifying glass to inspect the casting’s surface. This method can only detect surface defects like mismatches, external cracks, swells, fusion, and blowholes. Some form of visual inspection is done on almost all castings.
This type of inspection is conducted to ensure that the casings have all of the necessary overall dimensions along with machining allowances. It might be necessary to break part of the casting to measure the inner dimensions.
This is a type of rough test used to indicate a discontinuity or flaw in a casting. It involves suspending the casing from an appropriate type of support that is free of obstructions. Then various areas on the surface of the casting are tapped using a small hammer. If there is any change in tone, that is an indication that there is a flaw. This method cannot indicate the extent or precise location of the flaw.
A hammer of a certain weight strikes or falls on the casting on this test. A defective casing will fail due to the impact of the hammer’s blow. However, this is a very unreliable and crude inspection method.
This type of test is conducted on castings that need to be leak-proof. The castings openings are all closed, and a fluid or gas under pressure is then introduced into them. This pressure will cause castings with porosity. To detect leakage, a casting can be submerged in a water tank, or a soap film can be used to apply the pressure if compressed air is used. Visual inspection can detect leakage if a liquid is used to apply pressure.
Radiography uses gamma rays or X-rays that penetrate through the casting to provide a shadow image on the photographic film placed in the back of the casting. The rays have very short wavelengths of 0.001 to 40 Angstrom units, to around 3 Angstrom units on gamma-rays in comparison to around 5500 Angstrom units n the middle of the visible spectrum.
These waves being able to penetrate through metal will also depend on how dense the metal is. They are to penetrate more easily in areas with less meal than those where there is more meal, leading to a shadow picture forming on the film. These pictures make it easy to identify any defects the casting might have. Gamma rays have shorter wavelengths which means they can penetrate through metal better and are used more frequently as a result.
Magnetic Particle Testing
This type of test is used to detect cracks in metals such as steel and cast iron which may be magnetized. The casting is magnetized to conduct the test. The fine particles of steel or iron are spread on the surface. If there is a void or crack in the casting, it will result in the magnetic field being interrupted, and magnetic flux will leak where the crack is.
The particles of steel or iron that are spread onto the surface of the casting are held by the leaking flux and provide a visual indication of the extent and nature of the crack. Voids or very small cracks close to the surface might not be detected by radioactive, but this method can easily reveal them.
This type of test is used to detect very small surface cracks. Compared to the magnetic particle test, it has the advantage of being able to use it with any material. Whatever parts are being tests are either covered with or dipped into a penetrant testing looking with good penetrating and wetting abilities. Capillary action is used to draw the liquid into the voids or cracks.
Once the penetrant is applied to the surface being tested, the excess penetrant gets wiped off of the surface and then dried. Next, a developer is applied. The developer helps to draw the penetrant out so that it will be visible on the surface. These penetrant liquids frequently contain materials that fluoresce under an ultraviolet die or light to indicate they are present.
This type of testing is used for detecting defects such as porosity, voids, or cracks within the casting’s interior. Transmission and reflection of high-frequency sound waves are used b this method. It produces ultrasonic sound waves that are much higher compared to the audible range and are passed through the casting.
A cathode-ray oscilloscope records the time interval between the reflected ray and the transmitted ray. Any void or crack in the casting will rest in some sound or reflection from the crack and appear as a pip in between the two pips that represent that casting’s thickness. It is easy to calculate the depth of the crack on the crack on the casting surface by the distance in between the pips.