The generation and appearance pattern of white spots in stainless steel parts: Although ultrasonic flaw detection is not intuitive enough, it provides the three-dimensional distribution of defects in the workpiece and can offer an equivalent concept of defect size. Therefore, the distribution pattern of white spots in steel and the occurrence pattern of white spots in different steel grades are of great significance for the determination of white spots. Thus, a brief introduction is provided first.

　　First of all, different steel grades have different sensitivities to white spots. Carbon content0.30%All the above-mentioned carbon steels, alloy structural steels and alloy tool steels may develop white spots. Practical experience has shown that alloy steel is more sensitive to white spots than carbon steel. Among them, alloy structural steel and alloy tool steel containing elements such as chromium, nickel and manganese have the greatest sensitivity to white spots, while austenitic steel, ferritic steel and ledestenitic steel do not have white spots.

　　There are many hypotheses about the formation mechanism of white spots, but several more convincing viewpoints all hold that it is related to the presence of a relatively high amount of hydrogen in steel. Practice has proved that the hydrogen content in steel is lower than2.0One milliliter means one milliliter.100When iron is removed, no white spots will be produced. The generally accepted view is that hydrogen in the atomic state of steel makes it brittle, and the stresses during the cooling of steel after rolling, such as thermal stress, organizational stress, deformation stress, and the pressure generated by the transformation of atomic oxygen into molecular hydrogen, are the driving forces that cause steel to break. These stresses may act together or separately. Under the action of argon and fracture force, a group of internal cracks - white spots - were formed in the steel.

　　The main measures to prevent white spots are to reduce the hydrogen content in steel, among which allowing oxygen to escape from the steel is a very important link.

　　Given that the formation of white spots is related to the escape of oxygen and the material, the distribution of white spots in steel has the following characteristics.

　　The appearance of white spots often has batch characteristics, that is, in the same smelting furnace, the same treatment process, and the same post-cooling process of forgers, white spots often appear in batches.

　　White spots tend to occur in large cross-sections. Due to the diffusion and escape of hydrogen, no white spots will appear near the end or surface of the forging billet (except in cases where white spots are exposed to the surface after machining). In shaft forgings, white spots are mostly distributed on the shaft center or the cross-section of the shaft1/3-1/2Within the annular range of the radius. For stepped shafts, white spots mostly occur at the parts with larger diameters. The larger the diameter of the shaft, the more white spots there are. When the diameter of the axis decreases, the distribution range and number of white spots also become relatively smaller. The distribution range of white spots on the same shaft forging varies with the diameter of the shaft. When there are few white spots, they usually only occur in areas with larger diameters. When white spots are severe, their distribution range expands from large diameters to small diameters. The more severe the white spots are, the larger the expanded range will be. However, on different diameters, the depth of the white dots from the surface of the axis changes almost little.

In large forgings, white spots mostly occur in the segregation zone of the forgings.

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