Methods of Inspection for Finding Out Defects in Casting Process

In the casting process, the first few castings will be inspected dimensionally, and the pattern is qualified afterward; only a few random inspections will be done. Every casting must be inspected to determine the defects in the casting process.

Different methods of inspection for finding defects in the casting process are discussed below

1. Visual Inspection
2. Hydrostatic Pressure Test
3. Magnetic Particle Inspection
4. Radiographic Examination
5. Ultrasonic inspection
6. Dye Penetrant Inspection
7. Coin Testing

1. Visual Inspection

Common defects such as surface roughness, apparent shifts, the omission of cores and surface cracks can be detected by a visual inspection of the casting. Cracks may also be detected by hitting the casting with a mallet and listening to the quality of the tone produced.

Visual inspection with an articulating fiberscope.

2. Hydrostatic Pressure Test

• The Hydrostatic pressure test is conducted on a casting to be used as a pressure vessel.
• In this test, first, all the flanges and ports are blocked.
• Then, the casting is filled with water, oil or compressed air. After that, the casting is submerged in a soap solution, and any leak will be evident in the bubbles.

Hydrostatic pressure tester. Image source: Wikimedia.org(author:Z22)

3. Magnetic Particle Inspection

The Magnetic particle test is conducted to check for minimal voids and cracks at or just below the surface of a ferromagnetic material. The test involves inducing a magnetic field through the section inspection. This is done, the powdered ferromagnetic material is spread out onto the surface. The presence of voids or cracks in the section results in an abrupt change in the permeability of the surface; this, in turn, causes leakage in the magnetic field. The powdered particles accumulate on the disrupted magnetic field, outlining the boundary of a discontinuity.

The inspection specialist checks the integrity of the central thrust pin by performing a magnetic particle inspection. Image source: Flickr

4. Radiographic Examination

The radiographic method is expensive and is used only for sub-surface exploration. In this, both X-rays and γ-rays are used. With γ-rays, more than one film can be exposed simultaneously; however, x-ray pictures are more distinct. Various defects, like voids, nonmetallic inclusions, porosity, cracks, and tears, can be detected by this method. The defects are that the film is less dense and appears darker in contrast to the surroundings.

5. Ultrasonic Inspection

In the Ultrasonic method, an oscillator sends an ultrasonic signal through the casting. For example, a signal is readily transmitted through a homogeneous medium. However, on encountering a discontinuity, the signal is reflected back. This reflected signal is then detected by an ultrasonic detector. The time interval between sending the signal and receiving its reflection determines the location of the discontinuity. The method is unsuitable for a material with a high damping capacity (e.g., cast iron) because, in such a case, the signal gets considerably weakened over some distance.

Ultrasonic inspection

6. Dye Penetrant Inspection (DPI)

The dye penetrant inspection method detects invisible surface defects in a non-magnetic casting. The casting is brushed with, sprayed with, or dipped into a fluorescent dye. The surface to be inspected is wiped, dried and viewed in darkness. The discontinuity in the surface will then be readily discernible.

Liquid penetrant inspection. Image source: Flickr

7. Coin Testing

The defect can be estimated by hitting a coin onto the component and by hearing the sound coming from the casing.