Large castings and forgings play an important role in machine tool manufacturing, automobile manufacturing, shipbuilding, power station, weapon industry, steel manufacturing and other fields. As a very important part, it has a large volume and weight, and its process and processing are relatively complex. Usually, the ingot is cast after melting, forged or remelted, and the required shape, size and technical requirements are obtained by high-frequency heating machine to meet the requirements of its service conditions. Due to the characteristics of its processing technology, the ultrasonic flaw detection of castings and forgings also has certain application skills.
1、 Ultrasonic inspection of castings
It is difficult to detect the inner defects of the casting due to the high-frequency transmission of sound beam or the high-frequency reflection of the casting. The reflected sound energy is a function of the directivity and properties of the inner surface or defect and the acoustic impedance of the reflector. Therefore, the acoustic energy reflected by various defects or inner surfaces can be used to detect the defect location, wall thickness or depth of defects under the surface. As a widely used non-destructive testing method, ultrasonic testing has the following advantages: high detection sensitivity, it can detect small cracks; it has large penetrability, and can detect thick section castings. Its main limitations are: it is difficult to interpret the reflection waveform of the fracture defect with complex contour size and poor directivity; for the undesirable internal structure, such as grain size, microstructure, porosity, inclusion content or fine dispersed precipitates, the waveform interpretation is also hindered; in addition, reference should be made to the standard test block when testing.
2、 Ultrasonic inspection of forgings
（1） Forging processing and common defects
The forging is made of hot ingot by forging. The forging process includes heating, deformation and cooling. Forging defects can be divided into casting defects, forging defects and heat treatment defects. Casting defects mainly include shrinkage cavity residual, porosity, inclusion, crack and so on. Forging defects mainly include folding, white spots, cracks, etc. The main defect of heat treatment is crack.
Shrinkage cavity residual is the shrinkage cavity in the ingot during forging, which is often found in the end of forging.
Porosity is the non compact and cavity formed during solidification shrinkage of ingot. It is not fully dissolved due to insufficient forging ratio during forging, which mainly exists in the center and head of ingot.
The inclusions include internal inclusion, external non-metallic inclusion and metal inclusion. The inclusions are mainly concentrated in the center and head of ingot.
The cracks include casting cracks, forging cracks and heat treatment cracks. The axial intergranular crack of austenitic steel is caused by casting. Improper forging and heat treatment will form cracks on the surface or center of forgings.
The white spot is caused by the high hydrogen content of forgings, the rapid cooling after forging, and the hydrogen dissolved in the steel can not escape in time, resulting in the cracking caused by excessive stress. The white spots are mainly concentrated in the center of large section of forgings. White spots always appear in groups in steel.
（2） Overview of flaw detection methods
According to the classification of inspection time, the flaw detection of forgings can be divided into the flaw detection of raw materials and manufacturing process, product inspection and in-service inspection.
The purpose of flaw detection of raw materials and manufacturing process is to detect defects as soon as possible, so as to take measures in time to prevent defects from expanding and causing scrapping. The purpose of product inspection is to ensure product quality. The purpose of in-service inspection is to supervise the 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 drawing, so the orientation of most defects is parallel to the axis. The detection effect of longitudinal wave straight probe from radial direction is the best. Considering that the defects will have other distribution and orientation, the detection of shaft forgings should be supplemented by axial detection with straight probe, circumferential detection with oblique probe and axial detection with oblique probe.
2. Flaw detection of cake and bowl forgings
The forging process of cake and bowl forgings is mainly upsetting, and the distribution of defects is mainly parallel to the end face. Therefore, it is the best method to detect the defects with a straight probe on the end face.
3. Flaw detection of cylinder forgings
The forging process of cylinder forgings is upsetting first, then punching and then rolling. Therefore, the orientation of defects is more complex than that of shaft forgings and cake forgings. However, the quality of cylinder forgings is generally better because the center part of ingot with the worst quality has been removed during punching. The main orientation of the defects is still parallel to the cylindrical surface of the cylinder, so the detection of Cylindrical Forgings is still based on the cylindrical surface of the straight probe, but for the cylinder forgings with thick wall, the angle probe must be used.
（3） Selection of detection conditions
The longitudinal wave straight probe is mainly used in ultrasonic flaw detection of forgings. The chip size is Φ 14 ~ Φ 28mm, and commonly used is Φ 20 mm. For small forgings, considering the near-field region and coupling loss, the small wafer probe is generally used. Sometimes in order to detect the defects with a certain inclination angle with the detection surface, the angle probe with a certain K value can also be used for detection. Due to the influence of the blind area and near-field area of the straight probe, the double crystal straight probe is often used to detect the short-range defects.
The grains of forgings are generally fine, so a higher frequency of flaw detection can be selected, usually 2.5 ~ 5.0mhz. In order to avoid "forest echo" and improve signal-to-noise ratio (SNR), a lower frequency (1.0-2.5mhz) should be selected for a few forgings with coarse grains and serious attenuation.