X-ray Flaw Testing for Metal Castings

In the process of penetrating the casting, X-ray or γ-ray interacts with the material, and its intensity is attenuated by absorption and scattering. Images with varying degrees of blackness corresponding to the internal structure and defects of the material can be obtained on photographic film. The nature of the defect is judged from factors such as the shape, quantity, size, orientation, distribution and blackness of the image, and then the defect is classified and rated according to the nature, size and quantity of the defect. In this way, we can know the type and severity of the internal defects of the casting.

Radiographic sensitivity refers to the ability to find the smallest defects in a casting. Due to the influence of the nature, location, orientation, quantity, size and other factors of the defects in the casting, the flaw detection sensitivity during the photographic process is determined by the quality of the photographic image. An image quality meter (otherwise known as a penetrometer) is an indicator. It is made of the same material with the same attenuation coefficient as the casting. Common image quality meters are wire-type image quality meters, hole-type image quality meters and slot-type image quality meters. The line (hole, groove) diameter of the image quality meter is represented by the image quality index. The larger the index value, the worse the image quality. In this way, the radiographic flaw detection sensitivity can be indirectly expressed by the image quality index. An image quality meter is a tool to measure the quality of a photograph, it does not represent the true size of defects within a casting that can be detected. The ASTM reference radiographic film is the current internationally recognized standard.

Features of Radiographic Inspection:

1. The biggest advantage of radiographic inspection is that it is intuitive and easy to judge the nature of defects. Internal defects in castings can be seen on film as long as they are within the photographic sensitivity range.

2. Radiographic inspection has a high detection sensitivity for volumetric defects (such as pores, shrinkage cavities, shrinkage porosity, sand inclusion, and slag inclusion); it has a certain sensitivity for planar defects (such as cracks, lack of fusion, etc.). However, when the thickness of the casting is greater than 40mm, it is difficult to find large-area shrinkage-like defects in radiographic inspection, and the detection sensitivity of micro-cracks is also low.

3. Images can be archived and stored for a long time for later reference and rechecking.

4. Radiographic inspection requires special equipment and sites, the cost is high, and the inspection cycle is long, which is not suitable for rapid and batch inspection of castings.

Descriptions of Castings Defect Classification and Grade:

The macroscopic internal defects of castings found by radiographic inspection can be divided into five categories: porosity, sand inclusion and slag inclusion, shrinkage cavity and shrinkage porosity, unfused inner iron and unfused core, hot crack and cold crack.

1. Stomatal defects. Stomata appear as round or oval dark spots, sometimes with tails, distributed in groups or singly. When distributed in groups, the images tend to overlap and have irregular shapes. Blowholes often appear in the final solidification of the casting, where the gas collects and cannot escape. The formation of needle-shaped pores belongs to the reactive intrusion type, and the surface layer of the casting is arranged in rows and dispersedly distributed perpendicular to the surface.

2. Defects of sand inclusion and slag inclusion. Sand and slag inclusions are distributed in irregular dot or line shape. When they are in line shape, they have a certain width and can be randomly distributed inside the casting. Slag inclusion often occurs around the bottom of the shrinkage cavity, and sand inclusion is sometimes distributed in the surface of the casting.

3. Shrinkage cavities and shrinkage porosity defects. According to the three-dimensional shape, the shrinkage cavity defects of castings can be divided into tubular, dendritic and large-area shrinkage cavities. Such defects are generally distributed at the bottom of the riser and at the final solidified hot node. Shrinkage cavities usually occur at the same time as pores, slag inclusions and shrinkage porosity.

4. Unfused defects. The images of unfused defects are similar to cracks, and they are all dark lines, but one side of the lines is a straight line segment, which only occurs at the location where the inner chiller or core support is arranged.

5. Crack-like defects. In radiographic inspection, crack-like defects show dark lines on the image with irregular shapes, some are straight, some are basically straight, but their ends are sharp and the head is not round. Crack-like defects generally appear at the hot node of the casting or the junction of abrupt changes in the section.

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