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Bearing steel quality requirements and shortcomings (2)

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2. Metallurgical defects of bearing steel (1), appearance defects of bearing steel 1. Crack: subcutaneous air bubbles of steel ingot; severe non-metallic inclusions and steel during forging and rolling; heating temperature is too high; forging and rolling After the cooling is fast, the final rolling and final forging temperature are too low. The cause of the crack is likely to occur. 2. Folding: the flash, burrs, wrinkles and sharp edges of the steel during the forging and rolling process; Pressed into the interior of the metal during rolling; it constitutes a fold. 3, crusting: because of the slag inclusions and pits on the outer surface of the ingot; it forms a thin, flat layer during the forging and rolling; it is called crusting. Scratch: due to the metal particles on the guide plate of the rolling mill; the guide plate device is not properly ordered; the outer surface of the steel is scribed as a groove; it is called scratch or scratch. 5. Slag inclusion: slag and various refractory materials; During the pouring process, it does not float on the head of the steel ingot; it accumulates on the outer surface of the steel ingot; when the steel ingot is trimmed; it is not trimmed; thus; the slag inclusion is formed on the outer surface of the steel. 6. Decarburization: the steel is heated during the process; Oxygen in the furnace gas and carbon on the outer surface of the steel The composition of the gas; the carbon content of the steel surface is lower than the regular value called decarburization. Decarburization is a serious disadvantage for high carbon bearing steel; often constitutes the decarburization of the bearing parts; the hardness after quenching does not meet the skill requirements .
(2) Low-fold defects of bearing steel 1. Shrinkage hole: The condensation process of molten steel after pouring; forming a hole in the middle part of the ingot due to the shortened volume; it is called shrinkage hole. In order to reduce the damage of shrinkage steel; Therefore, in the pouring of molten steel; a reasonable process should be selected in the crystallization process; the hole formed by shortening the volume is moved to the head of the steel ingot; after the ingot is opened; some of the shrinkage holes are cut off; however; because the casting and cooling processes are not If the Dingqi is unreasonable; the steel ingot head is lack of heat preservation; after the blanking, the ingot head is removed less; the shrinkage hole remains in the steel; when viewed at low magnification; it will be displayed.
2. White point: the middle of the transverse section of the steel sample after pickling is short and non-continuous; the hairline-like opening seam usually spread in the radiation state; or the surface lubrication on the longitudinal fracture of the steel; the shape is similar Circular or elliptical silver-white mottle; known as white point. The reason for the formation of white spots; one is the presence of hydrogen in steel; the second is that there is no slow cooling at 600~300 °C after casting; the hydrogen is not scattered; Stress and cracking. Steel or parts with white spots; their longitudinal and transverse mechanical properties are significantly reduced; therefore, steel or parts with white spots have no application value.
3. Over-burning: When the steel ingot or billet is heated by casting; the temperature is too high; the outer layer is invaded by oxygen at the grain boundary to attack the oxide. Some low melting point compounds between the grain boundary and the dendrite axis melt and melt; causing condensation After the formation of cracks or holes. This appearance is called over-burning. After the steel is over-burned, it will cause cracking when re-forging; even if it does not crack; the strength and impact toughness under the Tiantian bath day are greatly reduced;
4. Bubble: The ability of steel to dissolve gas in liquid state is larger than that in solid state; molten steel is in the process of condensation; gas escapes from molten steel; if it is too late to discharge; it constitutes pores. In addition; poor ingot baking; There is moisture or gas on the surface of the steel mold; and the outer and outer coatings of the ingot mold are poor; many gases are formed; these moisture or gas can't discharge the molten steel; it constitutes subcutaneous bubbles. The presence of bubbles greatly reduces the strength of the steel.
5, segregation: in the process of steel liquid condensation; because the chemical composition of various chemical components in the carbon, chromium, tungsten, phosphorus and other elements of the crystallization and dispersion speed is not the same as the chemical composition of the phenomenon is called segregation. The existence of segregation will give The deformation processing in the future is difficult; the precipitation of sulfur is prone to hot brittleness; the segregation of phosphorus is prone to cold brittleness. The presence of segregation can easily lead to fatigue cracking of metals.

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