Common non-destructive testing methods for forgings 

Ultrasonic testing (main method for detecting internal defects in forgings) 

Principle: Ultrasound waves penetrate the base material of the forging. When they encounter internal defects, they will generate reflected echoes. The instrument can then determine the location, size and equivalent value of the defect. 

Applicable to: Various forged parts of carbon steel, alloy steel, and stainless steel, specifically for detecting internal defects - shrinkage cavities, porosity, internal cracks, inclusions, large slag inclusions, white spots, and forging delamination. 

Advantages: High penetration depth, capable of inspecting thick valve bodies and valve covers; capable of quantitative rating, for pressure class I forged parts, 100% UT is mandatory. 

Limitations: Minor defects near the surface are prone to go undetected; for coarse-grained forgings, the waveforms are more complex, increasing the difficulty of defect identification. 

2. Magnetic Particle Testing (Surface / Near-Surface of Ferromagnetic Forgings) 

Principle: The forging is magnetized. Surface defects will cause magnetic leakage. After spraying magnetic powder, the magnetic powder gathers to reveal the defect patterns. 

Applicable to: Forgings of ferromagnetic materials such as carbon steel and chromium-molybdenum steel, only inspect surface and shallow micro-cracks, folds, streaks, and grinding cracks. 

Valve scene: Surface screening of high-temperature and high-pressure carbon steel, chromium-molybdenum steel valve bodies, and flange forgings. 

Limitations: Austenitic stainless steel (304/316) is non-magnetic and cannot be tested using MT; it can only be used for shallow layer testing, and deep layer defects are not applicable. 

3. Penetrant Testing (PT) (Surface Opening Defects on Stainless Steel / Non-magnetic Components) 

Principle: After cleaning the workpiece, apply the penetrant liquid. The liquid penetrates the surface opening defects. Then, wash away the excess penetrant liquid. The developer absorbs the colored traces to reveal the cracks. 

Applicable to: Non-magnetic forged parts made of stainless steel, copper alloy, aluminum alloy, etc. Detect all surface defects: surface cracks, folds, pinholes, sand holes. 

For the stainless steel valve body forgings of ball valves, the surface flaw detection method is the preferred choice. 

Limitations: Can only detect defects from the opening to the surface, internal and buried defects cannot be detected; High requirements for the surface roughness of the workpiece. 

4. Radiographic testing RT (Auxiliary inspection for forgings, rarely used for full inspection) 

Principle: X/γ rays penetrate the workpiece, and the film captures the image. The defect location appears with a darker blackness. 

Applicable to: thin-walled forgings, complex flow channel areas, for detecting dense slag inclusions, pores, and large-scale porosity; for thick-walled pressure-bearing forgings, RT (due to insufficient penetration power) is generally not used. 

Comparison between UT and RT: UT is more suitable for thick and large components, while RT is mainly used for thin-walled small parts, weld seams. For forgings, ultrasonic testing is the main method, and radiographic testing is used as a supplement. 

Disadvantages: It has radiation, low detection efficiency, high cost, and large forgings are rarely used in large quantities. 

5. Eddy Current Testing (Screening for Thin and Small Components with High Precision) 

Principle: An alternating magnetic field induces eddy currents in the workpiece. Defects alter the eddy current signals, and the instrument detects the abnormalities. 

Application: Thin-walled small precision forgings, small shafts, for detecting extremely fine surface cracks and surface defects. 

Valve forgings are rarely used as the main inspection method; they are only employed for spot checks on small and precision valve stems. 

6. Visual inspection VT (the first inspection step at the base level) 

Principle: Visual inspection using a magnifying glass and a strong flashlight. 

Applicable to: The first non-destructive screening of all forgings, to detect obvious surface cracks, folds, impact pits, oxide scales, and dimensional defects. 

This is a basic non-destructive testing procedure, and it must be carried out for all forging inspections.