Large castings and forgings play an important role in machine tool manufacturing, automobile manufacturing, shipbuilding, power station, weapon industry, iron and steel manufacturing and other fields. As very important parts, they have large volume and weight, and their technology and processing are complicated. The process usually used after smelting ingot, forging or re-melting casting, through the high-frequency heating machine to obtain the required shape size and technical requirements, to meet the needs of its service conditions. Because of its processing technology characteristics, there are certain application skills for ultrasonic flaw detection of casting and forging parts.
I. Ultrasonic inspection of casting
Due to the coarse grain size, poor sound permeability and low signal-to-noise ratio of the casting, it is difficult to detect defects by using the sound beam with high frequency sound energy in the propagation of the casting, when it encounters the internal surface or defect, the defect is found. The amount of reflected sound energy is a function of the directivity and properties of the inner surface or defect as well as the acoustic impedance of such a reflective body. Therefore, the reflected sound energy of various defects or inner surfaces can be used to detect the location of defects, wall thickness or depth of defects under the surface. Ultrasonic testing as a widely used nondestructive testing means, its main advantages are: high detection sensitivity, can detect fine cracks; Has a large penetration capacity, can detect thick section castings. Its main limitations are as follows: it is difficult to interpret the reflected waveform of disconnection defect with complex contour size and poor directivity; Undesired internal structures, such as grain size, microstructure, porosity, inclusion content or fine dispersed precipitates, also hinder waveform interpretation. In addition, reference to standard test blocks is required.

2.forging ultrasonic inspection
(1) Forging processing and common defects
Forgings are made of hot steel ingot deformed by forging. The forging process includes heating, deformation and cooling. Forgings defects can be divided into casting defects, forging defects and heat treatment defects. Casting defects mainly include shrinkage residual, loose, inclusion, crack and so on. Forging defects mainly include folding, white spot, crack and so on. The main defect of heat treatment is crack.
Shrinkage cavity residual is the shrinkage cavity in the ingot in forging when the head is not enough to remain, more common at the end of the forgings.
Loose is the ingot solidification shrinkage formed in the ingot is not dense and holes, forging due to the lack of forging ratio and not fully dissolved, mainly in the ingot center and head. e
Inclusion has internal inclusion, external non-metallic inclusion and metal inclusion. The inner inclusions are mainly concentrated in the center and head of the ingot.
The cracks include casting cracks, forging cracks and heat treatment cracks. Intergranular cracks in austenitic steel are caused by casting. Improper forging and heat treatment will form cracks on the surface or core of the forging.
The white point is the high hydrogen content of the forgings, cooling too fast after forging, the dissolved hydrogen in the steel too late to escape, resulting in cracking caused by excessive stress. White spots are mainly concentrated in the center of the large section of the forging. White spots always appear in clusters in steel. * x- H9 [:
(2) Overview of flaw detection methods
According to the classification of flaw detection time, forging flaw detection can be divided into raw material flaw detection and manufacturing process, product inspection and in-service inspection.
The purpose of defect detection in raw materials and manufacturing process is to find defects early so that measures can be taken in time 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 the defects that may occur or develop after operation, mainly fatigue cracks. + 1. Inspection of shaft forgings
The forging process of shaft forgings is mainly based on drawing, so the orientation of most defects is parallel to the axis. The detection effect of such defects is best by longitudinal wave straight probe from radial direction. Considering that the defects will have other distribution and orientation, so the shaft forging flaw detection, should also be supplemented by straight probe axial detection and oblique probe circumferential detection and axial detection.
2. Inspection of cake and bowl forgings
The forging process of cake and bowl forgings is mainly upset, and the distribution of defects is parallel to the end face, so it is the best method to detect defects by straight probe on the end face.
3. Inspection of cylinder forgings
The forging process of cylinder forgings is upsetting, punching and rolling. Therefore, the orientation of defects is more complex than that of shaft and cake forgings. But because the center part of the worst quality ingot has been removed when punching, the quality of cylinder forgings is generally better. The main orientation of the defects is still parallel to the cylindrical surface outside the cylinder, so the cylindrical forgings are still detected mainly by straight probe, but for the cylindrical forgings with thick walls, oblique probe should be added.
(3) Selection of detection conditions
Probe selection
Forgings ultrasonic inspection, the main use of longitudinal wave direct probe, wafer size of φ 14 ~ φ 28mm, commonly used φ 20mm. For small forgings, the chip probe is generally used considering the near field and coupling loss. Sometimes in order to detect the defects with a certain Angle of the detection surface, also can use a certain K value of the inclined probe for detection. Due to the influence of the blind area and near field area of the direct probe, the double crystal direct probe is often used to detect the near distance defects.
The grains of forgings are generally small, so higher flaw detection frequency can be selected, usually 2.5 ~ 5.0mhz. For a few forgings with coarse grain size and serious attenuation, in order to avoid "forest echo" and improve signal-to-noise ratio, a lower frequency, generally 1.0 ~ 2.5mhz, should be selected.