Large castings and forgings play an important role in fields such as machine tool manufacturing, automotive manufacturing, shipbuilding, power plants, weapons industry, and steel manufacturing. As very important components, they have a large volume and weight, and their processes and processing are relatively complex. The commonly used process is to melt and then cast ingots, which are forged or remelted and poured into shape. The required shape, size, and technical requirements are obtained through high-frequency heating machines to meet their service conditions. Due to its processing characteristics, there are also certain application skills for ultrasonic testing of castings and forgings.
1、 Ultrasonic testing of castings
Due to the coarse grain size, poor sound transmission, and low signal-to-noise ratio of castings, it is difficult to detect defects. It uses a high-frequency sound beam to propagate inside the casting, causing reflection when encountering internal surfaces or defects. The magnitude of reflected sound energy is a function of the directionality and properties of the inner surface or defect, as well as the acoustic impedance of this reflector. Therefore, various defects or the reflected sound energy of the inner surface can be applied to detect the location, wall thickness, or depth of subsurface defects. Ultrasonic testing, as a widely used non-destructive testing method, has the main advantages of high detection sensitivity and the ability to detect small cracks; It has great penetration ability and can detect thick section castings. Its main limitations lie in the difficulty in interpreting the reflection waveform of disconnected defects with complex contour dimensions and poor directionality; Unsatisfactory internal structures, such as grain size, organizational structure, porosity, inclusion content, or small dispersed precipitates, also hinder waveform interpretation; In addition, reference to standard test blocks is required for testing.
2、 Ultrasonic testing of forgings
（1） Forging processing and common defects
Forgings are formed by forging and deformation of hot steel ingots. The forging process includes heating, deformation, and cooling. Forging defects can be divided into casting defects, forging defects, and heat treatment defects. The main casting defects include residual shrinkage, porosity, inclusions, cracks, etc. The main forging defects include folding, white spots, cracks, etc. The main defects in heat treatment are cracks.
Shrinkage residue refers to the residual shrinkage in the ingot caused by insufficient cutting during forging, commonly found at the end of the forging.
Looseness is the non density and porosity formed during solidification shrinkage of steel ingots. During forging, due to insufficient forging ratio, incomplete fusion occurs, mainly in the center and head of the ingot, where there are internal inclusions, foreign non-metallic inclusions, and metal inclusions. The internal inclusions are mainly concentrated in the center and head of the steel ingot.
Cracks include casting cracks, forging cracks, and heat treatment cracks. Axial intergranular cracks in austenitic steel are cracks caused by casting. Improper forging and heat treatment can cause cracks to form on the surface or center of the forging.
White spots are caused by high hydrogen content in forgings, which cools down too quickly after forging, causing hydrogen dissolved in the steel to escape in time, resulting in cracking caused by excessive stress. The white spots are mainly concentrated in the center of the large cross-section of the forging. White spots always appear in groups in steel.
（2） Overview of Flaw Detection Methods
According to the classification of inspection time, forging inspection can be divided into raw material inspection and manufacturing process inspection, product inspection, and in-service inspection.
The purpose of raw material inspection and manufacturing process inspection is to detect defects as soon as possible, in order to take timely measures to avoid the development and expansion of defects, resulting in scrapping. The purpose of product inspection is to ensure product quality. The purpose of in-service inspection is to supervise defects that may occur or develop after operation, mainly fatigue cracks+ 1. Flaw detection of shaft forgings
The forging process of shaft forgings is mainly based on elongation, so the orientation of most defects is parallel to the axis. The detection of such defects using a longitudinal wave straight probe from the radial direction has the best effect. Considering that defects may have other distributions and orientations, the inspection of shaft forgings should also be supplemented by straight probe axial detection, oblique probe circumferential detection, and axial detection.
2. Flaw detection of cake and bowl forgings
The forging process of cake and bowl forgings mainly relies on upsetting, and the distribution of defects is mainly parallel to the end face. Therefore, using a straight probe to detect defects on the end face is the best method.
3. Flaw detection of cylindrical forgings
The forging process for cylindrical forgings is first upsetting, then punching, and then rolling. Therefore, the orientation of defects is more complex than that of defects in shaft forgings and cake forgings. However, due to the fact that the center part with the worst quality in the ingot has been removed during punching, the quality of cylindrical forgings is generally better. The main orientation of its defects is still parallel to the outer circular surface of the cylinder, so the detection of cylindrical forgings is mainly based on the outer circular surface of the straight probe. However, for cylindrical forgings with thicker walls, an angle probe must be added for detection.
（3） Selection of detection conditions
Selection of probes
When ultrasonic testing forgings, longitudinal wave straight probes are mainly used, and the chip size is Φ 14~ Φ 28mm, commonly used Φ 20mm. For smaller forgings, considering the near-field region and coupling losses, small chip probes are generally used. Sometimes, in order to detect defects with a certain inclination angle to the detection surface, an angle probe with a certain K value can also be used for detection. For close range defects, due to the influence of the blind zone and near-field zone of the straight probe, double crystal straight probes are often used for detection.
The grain size of forgings is generally relatively small, so a higher inspection frequency can be chosen, commonly ranging from 2.5 to 5.0 MHz. For forgings with severe attenuation of coarse grain size in a few materials, in order to avoid the occurrence of "forest shaped echoes" and improve signal-to-noise ratio, a lower frequency, usually 1.0-2.5MHz, should be selected.