Forging processing and forging inspection methods

Forgings are made by deforming steel ingots or forging blanks into part blanks of a certain shape and size under the pressure of a forging hammer or die. The forging process includes heating, deformation and cooling. Forging methods are broadly classified into forging, drawing and rolling. Forging is when forging force is applied to both ends of the blank, causing the cross section to deform. Deep drawing refers to applying forging force to the outer circle of the blank to deform the blank. Portrait. Rolling involves first reaming and punching the blank, and then inserting a mandrel to apply forging pressure to the outer circle. There are two types of rolling: longitudinal deformation and transverse deformation. Among them, rough drawing is mainly used for cake forgings, and long drawing is mainly used for shaft forgings. In order to improve the structural properties of forgings, heat treatments such as normalizing, annealing or quenching and tempering must be carried out after forging. Therefore, there are two main sources of defects in forging grain forgings: one is caused by defects in the ingot; the other is caused by defects in the ingot itself. The other is the forging process and shrinkage cavities, which are defects formed in the head of the casting when the cracks cool and shrink. The amount of cut head is insufficient and remains. It is more common in the heads of shaft forgings. It has a larger volume, is located in the center of the section, and has a larger axial extension. Shrinkage cavities are pores and cavities formed when the ingot solidifies and shrinks. Not eliminated due to insufficient deformation during the forging process. According to their origin or nature, inclusions can be divided into intrinsic non-metallic inclusions, extrinsic non-metallic inclusions and metallic inclusions. Intrinsic non-metallic inclusions are the reaction products of deoxidizers, alloying elements, etc. contained in steel ingots and gases. Small in size, they often float on the melt and finally gather in the center and head of the ingot.

Foreign non-metallic inclusions are refractory materials or impurities mixed in during the smelting and pouring processes. They are larger and therefore often found mixed in the lower part of the ingot. Accidentally dropped non-metallic inclusions have no definite location. There are many reasons for cracks in forgings, such as adding more alloys and larger sizes during smelting, small particles splashed during pouring, or dissimilar metals falling into metal inclusions. According to the cause of formation, the types of cracks can be roughly divided into the following categories:

Cracks formed due to the expansion of metallurgical defects (such as shrinkage cavity residues) during the forging process. Cracks formed during heat treatment due to improper forging process (such as too high heating temperature, too fast heating speed, uneven deformation, excessive deformation, cooling speed). If the heating temperature is high during quenching, the structure of the forging will be coarse and cracks will easily occur during quenching. Cracks appear; cracks caused by improper cooling, untimely or improper tempering, and cracks caused by residual stress inside the forging. The defect caused by the protruding part of the molten iron being bent and embedded into the surface of the forging is called folding, which mostly occurs in forgings. Internal rounded and sharp corners. The oxide layer on the folded surface can prevent metal connections in that part. Small cracks in forged steel caused by the presence of hydrogen are called white spots. White spots have a great influence on the mechanical properties of steel. When the white spot plane is stressed in the vertical direction, it will cause the steel part to suddenly break. Therefore, white spots are not allowed in steel. White spots mostly appear in high carbon steel, martensitic steel and bainitic steel. Aus white spots generally do not appear in sternitic steels and low carbon ferritic steels. The characteristics of forging defects are related to their formation process. The casting defect structure is elongated along the metal extension direction during the forging process, and the resulting fibrous structure is usually called metal streamlines. The direction of metal streamlines generally represents the main direction of metal extension during the forging process. In addition to cracks, most defects in forgings, especially those caused by casting defects, tend to follow the direction of metal flow lines. Directional distribution, which is one of the important characteristics of forging defects.

Forgings can be inspected by contact method or water immersion method. With the development of computer technology and people's deepening understanding of the characteristics of the water immersion method, such as easy automatic detection, less human factors, and high detection reliability, the water immersion method is used for important forgings that require high resolution, high sensitivity, and high reliability detection. Increasingly used for testing. The structure of the forging is very fine, and the resulting sound wave attenuation and scattering effects are also relatively small. Therefore, sometimes higher detection frequencies (such as above 10MHz) can be applied to forgings to meet high-resolution detection requirements and achieve smaller size detection. The purpose of defect detection.
Due to forging deformation, defects in forgings generally have a certain directionality. Usually the distribution and direction of metallurgical defects are related to the direction of the forging flow lines. Therefore, in order to obtain the best detection effect, the selection of the incident surface and direction of the sound beam during forging inspection needs to consider the deformation process and streamline direction of the forging, and the direction of the ultrasonic sound beam should be perpendicular to the forging. The direction of forging flow lines should be as consistent as possible. Take die forgings as an example. The deformation streamlines of the die forging are parallel to the outer surface. Therefore, during inspection, it is generally required that the direction of the ultrasonic sound beam should be perpendicular to the outer surface. The scan needs to follow the shape of the outer surface. Water immersion is usually required. Forgings with method or water jacket probes are often used in critical components with higher safety requirements. Therefore, their surfaces and shapes usually need to be processed to ensure that the forgings have a smooth sound incident surface to meet the needs of high-sensitivity detection, while making their appearance as close as possible to the timing. Ultrasonic over-forging detection. In principle, it should be carried out after heat treatment and before finishing processes such as punching and grooving. Because complex

shapes such as holes, grooves, steps, etc. will form areas that the ultrasonic sound beam cannot reach, increasing detection blind areas, and at the same time, defect-free interference waves caused by the shape may affect the detection and identification of defects. Post-heat treatment inspection can help detect defects generated during the heat treatment process, such as heat treatment cracks.

Commonly used technologies for ultrasonic testing of forgings include: longitudinal wave direct incidence detection, longitudinal wave oblique incidence detection, transverse wave detection, etc. Since the shape of the forging may be complex, sometimes in order to find defects in different directions, it is necessary to conduct longitudinal and transverse wave inspections on the same forging at the same time. Among them, longitudinal wave direct incidence detection is the most basic detection method.

The standard flaw detection range of the BSN900 series ultrasonic flaw detector is 2.5-10000mm (longitudinal wave for steel). If there are special needs, the maximum flaw detection range that can be provided is 2.5-280000mm.