① Shrinkage: During the condensation process of molten steel after pouring, a hole is formed in the center of the steel ingot due to volume shrinkage, which is called shrinkage. In order to reduce the harm of shrinkage steel, reasonable processes should be adopted during the pouring and crystallization process of steel liquid, so that the holes formed by volume shrinkage move towards the head of the steel ingot. After the steel ingot is opened, the shrinkage part is cut off. However, due to improper pouring and cooling processes, such as improper positioning, insufficient insulation of the steel ingot head, and insufficient cutting of the ingot head after opening, shrinkage holes remain in the steel. During low magnification inspection, It will be displayed.

② White spots: Short, discontinuous hairline like open seams in the center or nearby area of the transverse section of the steel sample after acid washing, generally distributed in a radiating state, or silver white spots with smooth surfaces and shapes similar to circular or elliptical on the longitudinal fracture surface of the steel, are called white spots. The reasons for the formation of white spots are twofold: firstly, the presence of hydrogen gas in the steel; secondly, the steel is not slowly cooled at 600-300 ℃ after forging, and hydrogen gas is not fully diffused, resulting in structural stress and cracking. Steel or parts with white spots have significantly decreased longitudinal and transverse mechanical properties, so steel or parts with white spots have no practical value.

③ Overburning: When steel ingots or billets are heated during forging, the temperature is too high, and the surface layer is invaded by oxygen along the grain boundaries, producing oxides. Some low melting point compounds at grain boundaries and between dendrite axes melt, resulting in the formation of cracks or pores after condensation. This phenomenon is called overburn. After the steel is overheated, it will cause cracking during forging. Even if it does not crack, the strength and impact toughness under the condition of repairing the weather will be greatly reduced, so it cannot be used.

④ Bubbles: Steel has a greater ability to dissolve gases in liquid state than in solid state. During the condensation process of steel, gases escape from the steel, and if there is no time to discharge them, pores are formed. In addition, poor baking of the steel ingot mold can result in the presence of moisture or gas on the surface of the steel ingot mold, as well as poor coating on the inner surface of the steel ingot mold, forming a large amount of gas. If these moisture or gas cannot be discharged from the steel liquid in time, subcutaneous bubbles will form. The presence of bubbles greatly reduces the strength of steel.

⑤ Segregation: The phenomenon of uneven chemical composition formed during the condensation process of steel due to the different crystallization and diffusion rates of various chemical components such as carbon, chromium, tungsten, phosphorus, etc. in the steel is called segregation. The presence of segregation can cause difficulties in subsequent deformation and processing, with sulfur segregation prone to thermal embrittlement and phosphorus segregation prone to cold embrittlement. The presence of segregation can easily cause metal fatigue fracture.

⑥ Loose: The small pores caused by volume shrinkage during the condensation process of molten steel are called loose. The dispersed distribution of small pores is called general porosity. The small pores distributed in the center of the steel are called central porosity. Looseness reduces the density of steel, significantly reduces mechanical properties, and reduces the service life of bearings.