Forgings and castings are important blank parts for various mechanical equipment and boiler pressure vessels.

I. Ultrasonic testing of castings
Due to the coarse grain structure, poor sound transmission, and low signal-to-noise ratio of castings, ultrasonic testing is difficult. It uses a high-frequency sound beam that propagates through the casting and reflects when encountering internal surfaces or defects, thereby detecting flaws. The magnitude of the reflected sound energy is a function of the directivity and nature of the internal surface or defect, as well as the acoustic impedance of the reflector. Therefore, the reflected sound energy from various defects or internal surfaces can be used to detect the location of defects, wall thickness, or the depth of subsurface defects. As a widely used non-destructive testing method, ultrasonic testing has the following main advantages: high detection sensitivity, capable of detecting fine cracks; and large penetration capability, capable of inspecting thick-section castings. Its main limitations are: difficulty in interpreting reflected waveforms from discontinuities with complex contours and poor directivity; interference from undesirable internal structures such as grain size, microstructure, porosity, inclusion content, or fine dispersed precipitates, which also hinder waveform interpretation; and the need for reference standard blocks during testing.

II. Ultrasonic testing of forgings
(I) Forging processing and common defects
Forgings are produced by hot working steel ingots through forging deformation. The forging process includes heating, deformation, and cooling. Forging defects can be classified into casting defects, forging defects, and heat treatment defects. Casting defects mainly include: shrinkage cavity residue, porosity, inclusions, and cracks. Forging defects mainly include: folding, white spots, and cracks. Heat treatment defects are mainly cracks.

Shrinkage cavity residue is the remnant of shrinkage cavities in the ingot that remain due to insufficient cropping during forging, often found at the ends of forgings.

Porosity is the lack of density and cavities formed during the solidification shrinkage of the ingot, which are not fully welded due to insufficient forging ratio, mainly existing in the center and head of the ingot.

Inclusions include intrinsic inclusions, extrinsic non-metallic inclusions, and metallic inclusions. Intrinsic inclusions are mainly concentrated in the center and head of the ingot.

Cracks include casting cracks, forging cracks, and heat treatment cracks. Austenitic steel axial interdendritic cracks are cracks caused by casting. Improper forging and heat treatment can form cracks on the surface or in the center of forgings.

White spots are cracks caused by high hydrogen content in the forging, combined with excessively rapid cooling after forging, where dissolved hydrogen in the steel cannot escape in time, resulting in excessive stress. White spots are mainly concentrated in the center of large-section forgings. White spots always appear in groups in steel.

(II) Overview of testing methods
Classified by testing time, forging inspection can be divided into raw material inspection, in-process inspection during manufacturing, product inspection, and in-service inspection.

The purpose of raw material inspection and in-process inspection is to detect defects early so that timely measures can be taken to prevent defect propagation and avoid scrap. The purpose of product inspection is to ensure product quality. The purpose of in-service inspection is to monitor possible new defects or the growth of existing defects after operation, mainly fatigue cracks.

1. Inspection of shaft-type forgings
The forging process for shaft-type forgings mainly involves drawing out, so most defects are oriented parallel to the axis. For detecting such defects, longitudinal wave straight beam probes applied from the radial direction give the best results. Considering that defects may have other distributions and orientations, inspection of shaft-type forgings should also be supplemented with axial straight beam probing, as well as circumferential and axial angle beam probing.

2. Inspection of disc-type and bowl-type forgings
The forging process for disc-type and bowl-type forgings mainly involves upsetting, and defects are mainly distributed parallel to the end faces. Therefore, using a straight beam probe on the end face is the best method for detecting defects.

3. Inspection of cylindrical forgings
The forging process for cylindrical forgings involves first upsetting, then piercing, and then rolling. Therefore, the orientation of defects is more complex than in shaft-type and disc-type forgings. However, since the poorest quality central part of the ingot is removed during piercing, the quality of cylindrical forgings is generally better. The main orientation of defects is still parallel to the outer cylindrical surface. Therefore, inspection of cylindrical forgings mainly uses straight beam probes on the outer cylindrical surface. However, for cylindrical forgings with thicker walls, angle beam probes should also be used.

(III) Selection of testing conditions
1. Probe selection
For ultrasonic testing of forgings, longitudinal wave straight beam probes are mainly used, with element sizes of Φ14–Φ28mm, commonly Φ20mm. For smaller forgings, smaller element probes are generally used due to considerations of near-field zone and coupling loss. Sometimes, to detect defects at an angle to the inspection surface, angle beam probes with a certain K value can be used. For near-surface defects, due to the blind zone and near-field effect of straight beam probes, dual-element straight beam probes are often used.

The grain size of forgings is generally fine, so higher testing frequencies can be used, commonly 2.5–5.0MHz. For a few forgings with coarse grain structure and severe attenuation, in order to avoid "forest echo" and improve the signal-to-noise ratio, lower frequencies, generally 1.0–2.5MHz, should be used.