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The performance of metal components can be notably affected by internal and surface defects that can not be detected through the regular course of visual inspection. Several nondestructive methods can be employed to inspect castings for these “invisible” flaws.

Visual Inspection

Visual inspection is based on the use of the human eye to identify surface defects, improper filling and molding errors. Casting defects that can be detected via visual inspection include sand holes, excessively rough surface, surface shrinkage, blowholes, misruns, cold shuts, and surface dross or slag.

Dimensional Inspection

To ensure a part meets dimensional requirements, such as tolerances, a metalcasting facility can check the dimensional accuracy of a part manually or with a coordinate measuring machine (CMM). Checking the dimensional accuracy of a part helps guarantee the customer won’t have to perform further costly machining on a part to meet the specified dimensions.

Dye Penetrant and Fluorescent Powder Testing

For tiny cracks, pores or other surface glitches that are hard to detect by the human eye, dye penetrant testing is used for both ferrous and nonferrous materials. In this method, a colored dye solution is applied to the surface of the casting. The dye, which is suspended in penetrating oil, will find its way into the surface defects. When a special developer is applied, the defects are clearly indicated.

A similar method involves fluorescent powder suspended in penetrating oil. Again, the solution penetrates the defects, so when the casting is dusted or sprayed drying powder, the solution is drawn from the defect and glows under an ultraviolet light where defects have occurred. Fluorescent powder testing only detects surface cracks and flaws but is more effective and economical than radiographic testing.

In general, dye-penetrant techniques identify defects on the surface of the casting and do not detect internal porosity or shrinkage that is not open to the surface. But it can detect rounded indications for porosity or gas on the casting surface.

Magnetic Particle Inspection

Magnetic particle inspection is quick, inexpensive and sensitive to defects, particularly shallow (0.003 in.) surface cracks and other lineal indications. It detects small cracks on or near the surface of ferrous alloys that can be magnetized (basically any ferrous alloy except austenitic material).

Ultrasonic Testing

Internal defects that are detected by radiography may also be detected by sound.In casting inspection, ultrasonic testing uses high frequency acoustic energy that is transmitted into a casting.

Radiographic Inspection

Another method used to detect internal defects is radiographic inspection. When done correctly, radiographic inspection is the best nondestructive method for detecting internal defects, such as shrinkage and inclusions.

Eddy Current Inspection

The eddy current inspection method is applied to the detection of cracks at or near the surface.

Pressure Leak Testing

When a casting is specified to be pressure tight or leak-proof, it is often tested by sealing openings in the casting and pressurizing it with air, inert gas or water.