Quality requirements and defects of the hottest be

2022-07-30
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Quality requirements and defects of bearing steel

1. Quality requirements for bearing steel. Rolling bearings should work at high speed and for a long time under complex stress states such as tension, compression, bending, shear, alternating and high stress values. Therefore, in the production process, the quality control and inspection items of bearing steel are many, the control range is narrow, and the production process is strict and complex. It is required to have certain tooling equipment, inspection means and a certain level of villain technology. In order to ensure that the bearing has good performance and high service life, the quality requirements for bearing steel are as follows:

(1) chemical composition: chemical composition is the essence of bearing steel. In such cases, loosen the M8 plug on the oil pump to remove the air, and then inspect whether the oil returned to the oil tank is intermittent. The physical, chemical, mechanical properties and metallographic structure of steel are all determined by the chemical composition. Changing the chemical composition will change the basic properties of steel. Therefore, the chemical composition of bearing steel must meet the allowable range specified in the standard

(2) internal quality: it can be divided into macro quality and micro quality

macro quality: it is required that the bearing steel shall be free of defects such as white spots, shrinkage cavities, slag inclusions, folk metals, cracks, overburning, subcutaneous bubbles, etc. The internal segregation and looseness of bearing steel shall be controlled within a certain range. In a word, the interior of bearing steel shall be dense, and no visible defects are allowed to split the steel matrix

micro quality: it is required that the internal structure of bearing steel should be uniform and the purity should be high

the internal structure of bearing steel refers to the carbide banded, carbide like, carbide liquid precipitation and annealing structure. Carbide is one of the main components of bearing steel, which exists objectively. How to make the carbide distribution uniform, dispersed and fine is one of the important issues to improve the quality of bearing steel. In recent years, the main manufacturers of bearing steel have adopted high-temperature diffusion treatment, new controlled rolling process and continuous annealing furnace equipment. Even so, the distribution of carbides can not reach an ideal level. Therefore, Rotating Zigzag fatigue test and axial loading fatigue test are the most widely used in standard metal fatigue test, and their allowable range and control level are specified

the purity of bearing steel refers to the contamination degree of non-metallic inclusions on steel. Nonmetallic inclusion is a kind of material in bearing steel matrix, which destroys the continuity of the matrix and is one of the main reasons for bearing early fatigue and spalling. Therefore, it is required that the amount of non-metallic inclusions in bearing steel should be as small as possible. In order to limit and control the existence of non-metallic inclusions in steel, they are strictly controlled in the standard. In addition to electric furnace smelting and electroslag remelting, the manufacturer also adopts such processes as electric furnace smelting and external refining, vacuum deoxidation, argon blowing and powder spraying outside the furnace, trying to reduce the oxygen content in the steel below 20ppm

(3) surface quality: the forming methods of bearing parts include forging, turning and cold stamping. There are different requirements for steel surface quality according to different forming methods. In general, the bearing steel surface shall be free of cracks. The utility model touches a glass 4-axis zigzag experimental fixture, which is overlapped, cracked, scabbed and slag inclusion. For cold drawn steel used for cold punching, except for the above defects, the surface shall be clean and free of rust, pitting and other defects. There shall be no serious decarburization on the surface of bearing steel. According to the different requirements of bearing parts forming process, there are different restrictions on the depth of decarburization layer on the surface of different types of steel in the standard

(4) allowable dimensional tolerance: according to the forming process of bearing parts and the production process of steel, the dimensional tolerance of various varieties and specifications of bearing steel is specified in the standard. The dimensional tolerance of forged steel is generally in accordance with GB standard, that of hot rolled steel is in accordance with GB standard, that of cold drawn steel is in accordance with GB standard, and that of cold drawn steel wire is in accordance with Yb standard

2. Metallurgical defects of bearing steel

(1) surface defects of bearing steel

①. Cracks: subcutaneous bubbles of steel ingots, serious non-metallic inclusions and steel during forging and rolling, the heating temperature is too high, and the cooling after forging and rolling is fast; It is possible to produce cracks due to low temperature of final rolling and final forging

② folding: flash, burr, fold and sharp edges and corners produced during forging and rolling of steel are pressed into the metal during continuous rolling to form folding

③ scarring: thin and flat layers are formed during forging and rolling due to slag inclusion and pits on the ingot surface, which is called scarring

④. Scratches: grooves are carved on the steel surface due to metal particles on the guide plate of the rolling mill and improper installation of the guide plate, which are called scratches or scratches

⑤. Slag inclusion: slag and various refractory materials do not float at the head of the ingot during steel pouring, but gather on the surface of the ingot. When the ingot is trimmed, they are not removed. Therefore, slag inclusion is formed on the steel surface

⑥. Decarburization: during the heating process of steel, the surface will be oxidized. The oxygen in the furnace gas will oxidize with the carbon on the steel surface to form gas, so that the carbon on the steel surface is lower than the specified value, which is called decarburization. Decarburization is a serious defect for high carbon bearing steel, which often causes decarburization on the surface of bearing parts, and the hardness after quenching can not meet the technical requirements

(2). Low magnification defects of bearing steel

① shrinkage cavity: during the condensation process of molten steel after pouring, a hole is formed in the center of the ingot due to volume shrinkage, which is called shrinkage cavity. In order to reduce the harm of steel with shrinkage cavity, reasonable process shall be adopted during molten steel pouring and crystallization to make the hole formed by volume shrinkage move to the head of the ingot. After the ingot is bloomed, the shrinkage cavity shall be cut off. However, due to improper pouring and cooling process, such as unreasonable setting, insufficient heat preservation at the head of the ingot, and less removal at the head of the ingot after bloom, the shrinkage cavity remains in the steel. During low magnification inspection, Will be displayed

②. White spot: after pickling, the center of the transverse section of the steel sample or its adjacent area presents short, discontinuous, generally distributed hairy open joints in a radial state, or silver white spots with smooth surface and approximate circular or oval shape appear on the longitudinal fracture of the steel, which are called white spots. The reasons for the formation of white spots are the existence of hydrogen in the steel, and the lack of slow cooling at 600~300 ℃ after forging, the insufficient diffusion of hydrogen, resulting in structural stress and cracking. The longitudinal and transverse mechanical properties of steel or parts with white spots have decreased significantly, so the steel or parts with white spots have no use value

③. Overburning: when the ingot or billet is forged and heated, the temperature is too high, and the surface layer is invaded by oxygen along the grain boundary to produce oxides. Some low melting point compounds between the grain boundary and the dendrite axis melt, so that cracks or holes are formed after condensation. This phenomenon is called overburning. After the steel is burned, it will cause cracking during re forging. Even if it does not crack, its strength and impact toughness will be greatly reduced under the protection of the sun, so it cannot be used

④. Bubbles: the ability of steel to dissolve gas in liquid state is greater than that in solid state. During the condensation process of molten steel, gas escapes from molten steel. If it is too late to discharge, it will form pores. In addition, if the ingot mold is not baked well, there will be moisture or gas on the surface of the steel mold, and the coating on the inner surface of the ingot mold is poor, resulting in a large amount of gas. These moisture or gas will not be able to discharge the molten steel before they form subcutaneous bubbles. The existence of bubbles greatly reduces the strength of steel

⑤. Segregation: during the condensation process of molten steel, the phenomenon of uneven chemical composition formed due to different crystallization and diffusion rates of various chemical components such as carbon, chromium, tungsten and phosphorus in the steel is called segregation. The existence of segregation will make it difficult to deform in the future. Sulfur segregation is easy to produce hot embrittlement, and phosphorus segregation is easy to produce cold embrittlement. The existence of segregation is easy to cause fatigue fracture of metal

⑥. Porosity: the small pores caused by volume shrinkage during the condensation of molten steel are called porosity. Dispersed fine pores are called general porosity. The small pores distributed in the center of the steel are called central porosity. Looseness reduces the density of the steel, significantly reduces the mechanical properties and reduces the service life of the bearing

(3). Non metallic inclusions: during the smelting and casting process of steel, chemical reaction products and deoxidation products caused by the contact between various components in the molten steel and between the molten steel and the furnace lining, as well as anti-inflammatory materials such as furnace wall, tapping trough, ladle and soup channel are corroded and removed into the molten steel. These non-metallic inclusions that enter the molten steel but are not discharged are called non-metallic inclusions. The existence of non-metallic inclusions in bearing steel is one of the main reasons for reducing the service life of bearings. Nonmetallic inclusions in steel can be divided into the following categories according to their characteristics, shape and distribution

oxide: it is brittle in nature, so it is also called brittle inclusion. It is generally distributed in point chain along the rolling direction in steel, such as aluminum oxide (Al2O3) and iron oxide (FeO)

sulfide: it has good plasticity and can be deformed, so it is also called plastic inclusion. It is distributed in continuous strips along the rolling direction in the steel. Such as iron sulfide (FES) and manganese sulfide (MNS). In addition to sulfide, silicate also has certain plasticity, also known as plastic inclusion

point shaped undeformed inclusion: it is brittle and hard in nature. It is point shaped or spherical in steel and does not deform during processing, such as quartz (SiO2), aluminosilicate (3al2o3,2sio2) and calcium silicate

(4) nonuniformity of carbides: high carbon bearing steel has high carbon content and contains a certain amount of carbide forming elements (such as chromium). In the process of liquid steel condensation, these elements are prone to component segregation, resulting in uneven distribution of carbides in steel

carbide liquid precipitation: during the crystallization process of molten steel, the cooling speed is too slow, resulting in serious segregation of carbon components. Coarse primary carbides are formed between dendrites, which are difficult to eliminate. It is distributed in strips or blocks along the rolling direction in the steel, and its hazard is the same as that of non-metallic inclusions. Therefore, it is strictly controlled in the technical standards of steel

carbide banding: its formation reason is the same as carbide liquid precipitation. The coarse primary carbides broken in the process of forging and rolling gather as small blocks and form a strip distribution along the rolling direction. Carbide banding will seriously affect the heat treatment quality of parts, make the local structure of parts under heated or overheated, resulting in uneven hardness and structure of parts. As a result, the heat treatment quality of parts is unqualified. The early fatigue damage of bearing parts will be caused by the severe carbide banding and carbide liquid precipitation

carbides: during forging, rolling and cooling of steel, the solubility of austenite to carbon decreases with the decrease of temperature. If the cooling rate is too slow between 800~900 ℃, carbon precipitates from austenite and diffuses to grain boundaries, forming secondary carbides between grain boundaries in the form of complex, so it is called carbide like. Carbide is a hard and brittle phase, which destroys the continuity between grains and seriously reduces the impact toughness of steel. Therefore, the service life of bearing parts is reduced

uneven annealing structure: the annealing structure of bearing steel shall be uniformly distributed or spherical pearlite. Due to the serious banding of steel, improper cooling after forging and rolling, or incorrect annealing process, the annealing structure may be uneven. In case of severe banding, the two bands

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