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Application skills of ultrasonic testing for forgings and castings

Jul 27, 2018

Large castings and forgings play an important role in the fields of machine tool manufacturing, automobile manufacturing, shipbuilding, power station, weapon industry, steel manufacturing, etc. As a very important part, it has a large volume and weight, and its process and processing are more complicated. The commonly used process is followed by smelting the ingot, forging or remelting and casting, and obtaining the required shape size and technical requirements by the high frequency heater to meet the requirements of its service conditions. Due to the characteristics of its processing technology, it also has certain application skills for ultrasonic flaw detection of castings and forgings.

First, the ultrasonic testing of castings

Due to the coarse grain size, poor sound permeability and low signal-to-noise ratio, it is difficult to detect. It uses the sound beam with high-frequency sound energy to propagate inside the casting. When it encounters the internal surface or defects, it will reflect and find defects. The magnitude of the reflected acoustic energy is a function of the directivity and properties of the inner surface or defect and the acoustic impedance of such a reflector, so that various defects or acoustic energy reflected from the inner surface can be applied to detect the presence of the defect, wall thickness or surface. Deep ultrasonic testing of lower defects is a widely used non-destructive testing method, and its main advantages are: High detection sensitivity, can detect fine cracks; has large penetration ability, can detect thick-section castings. The main limitation is that it is difficult to explain the reflection waveform of the broken defects with complex contour size and poor directivity; Desirable internal structures, such as grain size, microstructure, porosity, inclusion content or finely dispersed precipitates, also impede waveform interpretation; in addition, reference to standard test blocks is required for testing.

Second, forging ultrasonic testing

(1) Forging processing and common defects

Forgings are forged by hot forged 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 casting defects are mainly: shrinkage hole residual, loose, inclusion, crack and so on. Forging defects mainly include: folding, white spots, cracks and other heat treatment defects are mainly cracks.

The residual of the shrinkage cavity is that the shrinkage hole in the ingot is insufficient to be cut when forging, and is often found at the end of the forging.

Looseness is the intimacy and void formed by the ingot during solidification and shrinkage. It is not fully fused due to insufficient forging ratio during forging, mainly in the center and head of the ingot.

Inclusions are inherently mixed, foreign non-metallic inclusions and metal inclusions. The inner 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 intergranular cracks are cracks caused by casting. Improper forging and heat treatment can form cracks on the surface or core of the forging.

The white point is that the forging has a high hydrogen content, and the cooling after forging is too fast, and the dissolved hydrogen in the steel cannot escape, causing cracking caused by excessive stress. The white point is mainly concentrated on the center of the large section of the forging. White spots always appear in groups in steel. * x-H9 [:

(2) Overview of flaw detection methods

According to the time of flaw detection, forging flaw detection can be divided into raw material flaw detection and flaw detection in the manufacturing process, product inspection and in-service inspection.

The purpose of flaw detection in raw material flaw detection and manufacturing is to detect defects early so that measures can be taken in time to avoid the development of defects and the retirement. 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.

(3) Selection of detection conditions

Probe selection

For ultrasonic flaw detection of forgings, the longitudinal wave straight probe is mainly used. The wafer size is Φ14~Φ28mm, and Φ20mm is commonly used. For smaller forgings, small wafer probes are typically used for near field and coupling loss reasons. Sometimes, in order to detect a defect with a certain inclination angle to the detection surface, it is also possible to detect with a declining probe with a certain value. For close-range defects, due to the influence of the blind zone and the near-field of the straight probe, the double-crystal straight probe is often used for detection.

The grain of the forging is generally small, so a higher detection frequency can be used, usually 2.5~5.0MHz. For forgings with a large amount of grain coarse and large attenuation, in order to avoid "forest-like echo" and improve the signal-to-noise ratio, a lower frequency should be selected, generally 1.0~2.5MHz.