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Differences in Ultrasonic Testing Methods for Castings and Forgings
Release Date:
2022/05/26
Due to the differing manufacturing processes for castings and forgings, the internal microstructures of these two types of workpieces vary significantly, resulting in substantial differences in their inspection methods.
Forgings are produced by subjecting metal to external pressure, causing plastic deformation to achieve the desired shape. This external pressure is typically applied using a hammer or a press. The forging process also refines the grain structure within the metal and enhances its mechanical properties. The primary advantages of forgings include their ability to elongate along the longitudinal axis while reducing in cross-sectional area.
Forgings are metal-shaped components produced by various casting methods, in which molten metal obtained through smelting is poured, die-cast, vacuum-injected, or otherwise introduced into a pre-prepared mold. After cooling, the workpieces undergo subsequent processing steps such as grinding and shot blasting to achieve the desired shape, dimensions, and mechanical properties.
Whether casting or forging, ensuring product quality requires nondestructive testing of the workpiece; however, due to the differences in their internal microstructures, the inspection methods and associated precautions also differ. The following discussion will highlight the distinctions between ultrasonic testing when applied to castings and to forgings.
I. Forging Inspection:
There are only two sources of defects in forgings: one is defects originating from the ingot, and the other is defects generated during the forging process and heat treatment.
Currently, the main defects in forgings include shrinkage cavities, looseness, slag inclusions, cracks, folds, and white spots. Due to the deformation induced during forging, these defects generally exhibit a certain degree of anisotropy. Therefore, to achieve optimal inspection results, the selection of the ultrasonic beam incidence surface and incidence direction must take into account both the forging-induced deformation and the flow-line orientation, and the ultrasonic beam direction should, whenever possible, be oriented perpendicular to the forging flow lines.
A major advantage of forgings is their fine-grained internal microstructure, which results in very low ultrasonic attenuation and scattering. Consequently, higher inspection frequencies can be used on forgings, while still meeting the requirements for high-resolution inspection and enabling the detection of small-sized defects.
II. Casting Inspection
The main types of casting defects include cavity-type defects (such as shrinkage cavities, looseness, porosity, and gas pores), cracks and cold shuts (including cold cracks, hot cracks, white spots, and heat-treatment cracks), and slag inclusion defects. Among these, cracks are the most hazardous, particularly in castings with rapid cooling rates, complex geometries, or significant variations in cross-sectional dimensions, as they often arise due to residual stresses.
Due to the non-uniform and porous microstructure, rough surface finish, and complex geometry of castings, ultrasonic penetration is poor and clutter interference is severe. However, defects that are generally unacceptable in castings tend to be relatively large in size and may occur in significant numbers.
To summarize, the main similarities and differences in the application of ultrasonic testing to castings and forgings are as follows:
Common points:
1. The instruments are identical, including the probe, connecting cable, and display.
2. The coupling agent is the same.
3. The detection theory is the same (physical theory).
4. The purpose is the same (both are to ensure the quality of the workpiece).
Differences:
1. Due to variations in the fineness of the internal grain structure, ultrasonic attenuation in the casting is severe, making high-frequency flaw detection impractical (as excessive clutter would obscure defect echoes and make it difficult to distinguish them from background noise).
2. The requirements differ: forgings are subject to stringent quality-grade specifications, whereas castings have much lower requirements; consequently, the evaluation criteria used during inspection also vary.
3. The causes and types of defects vary.
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