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Bearing steel quality requirements and shortcomings
Source: China Bearing Network Time: 2018-01-15
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(1) Chemical composition: Chemical composition is the most essential element of bearing steel. The physical, chemical, mechanical functions and metallographic arrangement of steel are determined by chemical composition; the chemical composition is changed; the fundamental properties of steel are changed. Therefore, the chemical composition of bearing steel is necessary to meet the scale of the normative rules.
(2) Internal quality: can be divided into micro-quality and micro-quality.
Micro-quality: the bearing steel inside does not agree to white spots, shrinkage holes, slag inclusions, civilian metals, cracks, over-burning, subcutaneous air bubbles and other shortcomings. The internal segregation and loose control of the bearing steel are required to be within the certain scale. The interior of the steel is fine; it does not allow the defects visible to the naked eye to split the base of the steel.
Micro-quality: the internal arrangement of the bearing steel should be uniform; the purity should be high.
The internal arrangement of bearing steel refers to carbide strip, carbide net, carbide liquid precipitation and annealing arrangement. Carbide is one of the primary components of bearing steel; it exists objectively; how to make carbide spread evenly and loosely In the past few years, the primary producers of bearing steels have chosen high-temperature dispersing treatment; new processes for handling rolling and successive annealing furnace equipment. Even so; the dispersion of carbides cannot be The degree of reaching the ambition. Thus; in the norm; rules their scale of consent and level of control.
The purity of the bearing steel refers to the degree of contamination of the steel by non-metallic inclusions. The non-metallic inclusions are the civilian substances of the bearing steel matrix; the connectivity of the matrix is ​​damaged; it is one of the primary causes of the early fatigue and shedding of the bearing. Therefore, the amount of non-metallic impurities in the bearing steel is as small as possible. In order to restrain and control the presence of non-metallic inclusions in the steel; in the specification; they are subjected to strict level control. In addition to the electric furnace exercise, the manufacturer Electroslag remelting; also used the electric furnace to enhance the external essence of the furnace, vacuum deoxidation, argon blowing treatment, powder spraying outside the furnace and other processes; strive to reduce the oxygen content of the steel to below 20ppm.
(3) Appearance quality: the forming method of bearing parts; currently there are casting, turning and cold punching. According to the different forming methods, there is a different degree of demand for the quality of steel appearance. In general; the bearing steel surface must not have cracks, Folding, cracking, crusting and slag inclusion. Cold drawn steel for cold punching; except for the above shortcomings; the appearance should be clean; there should be no defects such as rust and burrs. The bearing steel should not have a severe decarburization appearance. According to the different requirements of the forming process of bearing parts; in the specification; there are different constraints on the depth of decarburization of different types of steel.
(4) Scale to promise public service: according to the forming process of bearing parts and the production process of steel; in the specification; the rules for the various types of bearing steel and the standard scale of public service are carried out. The standard for casting steel is usually according to GB908-72. Specification; hot-rolled steel scale public service according to GB702-86; cold-drawn steel according to GB905-82 specification; cold drawn steel wire according to YB245-64 specification.
2. Metallurgical defects of steel for bearings
(1) Disadvantages of steel for bearing 1. Crack: subcutaneous air bubbles of steel ingot; severe non-metallic inclusions and steel in the process of forging and rolling; heating temperature is too high; cooling after forging and rolling is fast, finishing and final The forging temperature is too low and the cause of the crack is capable.
2, folding: the burrs, burrs, wrinkles and sharp edges of the steel in the forging and rolling process; pressed into the metal inside during continuous rolling;
3. Scarring: Because of the slag inclusions and pits on the outer surface of the steel ingot; it forms a thin, flat layer in the forging and rolling process; it is called crusting.
4, scratch: due to the metal plate on the guide plate of the rolling mill; the guide plate device is not appropriate; the steel surface is carved out of the groove; called scratch or scratch.
5, slag inclusion: slag and various refractory materials; in the process of steel pouring does not float on the head of the steel ingot; and gathered in the appearance of the steel ingot; steel ingot trimming; not trimmed; thus; in the steel surface constitutes slag.
6. Decarburization: The steel is in the process of heating; the surface should be oxidized; the oxygen in the furnace gas is oxidized with the carbon on the surface of the steel; the gas is formed; the carbon content of the steel surface is lower than the regular value called decarburization. High carbon bearing steel is a serious shortcoming; it 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: 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 mottled; 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 sufficiently scattered; Stress and cracking. Steel or parts with white spots; their longitudinal and transverse mechanical functions 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 in the Tiantian bath day are greatly reduced;
4. Bubble: Steel can dissolve gas in liquid state more than solid state; steel liquid 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 various chemical components of steel, such as carbon, chromium, tungsten, phosphorus and other elements of crystal, the speed of dispersal is not the same, the chemical composition of the uneven appearance 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 is prone to metal fatigue cracking.
6. Looseness: The molten steel is in the process of condensation; the fine pores caused by the shortening of the volume are called loose. The fine pores scattered and scattered are called loose. The fine pores scattered in the middle part of the steel are called intermediate loose. Loose and lowered steel. The fine density; the mechanical function is significantly reduced; the life of the bearing is reduced.
(3) Non-metallic inclusions: steel in the process of exercise and casting; chemical reaction products and deoxidation products caused by the touch between the components in the molten steel, between the molten steel and the lining; and the furnace wall and the tapping tank , steel ladle; soup and other anti-inflammatory materials are eroded and fall into the molten steel. These non-steel liquid materials that enter the molten steel and are not discharged are called non-metallic impurities. The presence of non-metallic inclusions in the bearing steel; One of the primary reasons for the use of bearing life.
Non-metallic inclusions in steel are classified into the following types according to their characteristics, shape and dispersion.
Oxide: It is more brittle; it is also called brittle noisy; it is usually distributed in a chain in the rolling direction; such as aluminum oxide (Al2O3) and iron oxide (FeO).
Sulfide: has good plasticity; can be deformed; it is also called plastic noisy. It is distributed in the steel in the rolling direction. For example, iron sulfide (FeS) and manganese sulfide (MnS) belong to this category. Outside the material; silicate also has a certain plasticity; also known as plastic inclusions.
Point-like non-deformed inclusions: brittle and hard; in the steel, point or spherical; not deformed during processing; such as quartz (SiO2), aluminosilicate (3Al2O3, 2SiO2) and calcium silicate .
(4), carbide non-uniformity: high carbon bearing steel carbon is high; and is enriched with a certain amount of carbide constituent elements (such as chromium). In the process of steel liquid condensation; these elements are prone to segregation of components; Involving the unevenness of carbide dispersion in steel.
Carbide liquid precipitation: molten steel in the crystallization process; because the cooling rate is too slow; constitutes a severe segregation of carbon components. It forms a coarse primary carbide between the dendrites; this primary carbide is difficult to eliminate. The direction of the strip is strip or block spread and its damage is the same as that of non-metallic inclusions; thus; there is severe control in the skill specification of steel.
Carbide ribbon: its constituents are the same as carbide liquid precipitation. The coarse primary carbides that are broken during the casting and rolling process are gathered in small pieces and form a strip-like dispersion along the rolling direction. Carbide ribbon Severe will affect the quality of the heat treatment of the parts; make the parts of the parts less hot or overheated; the hardness of the parts is not uniform; the arrangement is uneven. The quality of the heat treatment of the parts is unqualified. When the carbide strips and carbides are severely analyzed; Will cause the bearing parts to be damaged in the early stage.
Carbide mesh: steel in the process of casting and rolling cooling; the solubility of austenite to carbon decreases with temperature. If the cooling rate is too slow between 800 and 900 °C, carbon is separated from austenite. And the grain boundary is formed; the secondary carbide is formed between the grain boundaries; it exists in the form of a network; so it is called a carbide network. The carbide is a hard and brittle phase; the connectivity between the grains is damaged; The impact toughness of the steel is reduced. Thus; the life of the bearing parts is reduced.
Uneven annealing schedule: The annealing arrangement of bearing steel is required to spread even or spherical pearlite. Because the steel strip is severe; the cooling after casting and rolling is not proper; or the annealing process is not correct, the annealing arrangement is uneven. When the shape is severe; the carbon-poor zone between the two zones undergoes overheating of the coarse layered pearlite during the annealing process. The cooling after casting is too slow to not only attack the mesh; and the hand-punched layered pearlite is uniquely large; The carbide center is cut; then some of the annealing arrangements are saved to preserve the original layer and to spread the extremely coarse coarse grain carbide. If the original arrangement of the steel is normal; and the annealing temperature is too high, the coarse layered pearlite will occur. Annealing temperature When it is low, it will show fine-grained or fine-grained pearlite. The annealing arrangement is uneven; the cutting function of the steel is deteriorated; the parts after quenching are unevenly arranged; then the life of the bearing parts is decreased.
Quality inspection of bearing steels
1. Appearance of the appearance quality of steel: The quality of the steel surface is usually viewed with the naked eye; the annealed and unannealed heats steel; if necessary, it can be ground with a pneumatic or electric portable grinding wheel. The cold drawn annealing steel and steel pipe can be fine. The flat file is cut into three or four circumferential smooth surfaces on the whole data; it is visually inspected. The steel wire material is usually also viewed with the naked eye; in addition, it can also be viewed by pickling; that is, 250 mm long at each end of the disk; After pickling by low-pick pickling process, the appearance of the skin is checked by the naked eye.
2, steel scale view: annealed and non-annealed hot-rolled round steel; with a reading value of 0. 1mm vernier caliper or caliper for scale viewing. Cold drawn steel and steel wire with a scale value of 0. 01mm micrometer for viewing. The outer diameter scale and wall thickness difference of the hot-rolled and cold-drawn steel tubes are respectively read with a reading value of 0.1 mm. The vernier caliper and the graduation value are 0. 01 mm micrometer. The thickness of the cold-rolled steel sheet and the steel strip is viewed with a micrometer. The length of the steel. The width is viewed with a steel ruler. The bending of various steels (except the coil) can be viewed with a double ruler or feeler gauge.
3. Identify steel types. Viewing steel types is usually viewed with a portable spectroscope and spark discrimination. The portable spectroscope is a semi-quantitative spectrometer; it can semi-quantitatively view the primary alloying elements in the steel; Chromium, manganese, nickel, molybdenum, tungsten, vanadium, etc. Spectral view can identify the steel number of the steel to be inspected and the presence or absence of mixed steel. The spark view is based on the spark characteristics to determine the steel number of the steel to be inspected.
4. Hardness view: The Brinell hardness test should be carried out on the cold drawn annealing strip and hot rolled annealed steel. The Brinell hardness test method is carried out according to the GB231-84 specification.
5. Fracture inspection: cold-drawn annealing and hot-rolled annealed steel with a diameter of 30mm or less should be fractured. Cut a notch at one end of the steel with a hammer or cut a fracture specimen with a press. See if there is any shrinkage on the cross section with the naked eye. Holes, white spots, cracks; overheating and other shortcomings.
6. Low-order arrangement view: Annealed steel and non-annealed steel with diameter greater than 30mm are usually viewed at low magnification. That is, the sawing thickness (annealing material) and the grinding wheel cutting machine (not annealed material) are cut at a thickness of 12~15mm at one end of the steel. The sample of the sample is ground with a grinder. After hot pickling, the nakedness is used to see the segregation, the degree of porosity is not shrinkage, the white point crack, the over-burning, etc. The hot pickling is 50% Industrial hydrochloric acid aqueous solution; heated to 70 ± 5 ° C; pickling time is 30 ~ 40min; after the sample is taken out, washed with alkaline water; then washed with 80 ° C hot water.
7. Analysis of chemical composition: The chemical composition is usually sampled and analyzed according to the furnace number. The analysis method of the various elemental components of the steel number is carried out according to the national normative rules. The chemical analysis method is relatively slow; it is not suitable for the production. æŽ é¦„ 馄 Zhuo Pu У ( (17) The stalked shell Ai Hui è° æ‚´æŽ æ‚´æŽ æ‚´æŽ æ‚´æŽ é¦„ 馄 馄 馄 馄 馄 馄 馄 馄 馄 馄 黾 黾 黾 黾 黾 黾
8, high-order arrangement view: high-order arrangement with metallographic microscope to view; inspection items are: annealing arrangement (enlarged 500 times) depth of decarburization layer (enlarged 100 times); view on the cross section of steel cross section. Carbide liquid analysis, Carbide ribbon and non-metallic inclusions (100 times enlargement) are viewed in the longitudinal section of the steel; however, in order to reduce the number of samples; the mesh can be viewed in the longitudinal section with the same sample of strip and liquid analysis; Time; the results of the transverse section inspection shall prevail.
The number of inspections of each project shall be subject to the specification rules; after the sampler has sampled, it shall first be machined according to the rules. Non-metallic inclusions; strip, liquid, and mesh specimens shall be quenched and tempered; sorghum chrome The bearing steel tempering process is as follows:
Quenching temperature 820~840°C,
Tempering temperature: 150 ° C; tempering time 1 ~ 2 h,
The surface to be inspected of the sample shall be ground and polished.
View the annealing surface and the surface of the decarburized layer; observe it with 2% nitric acid solution,
Non-metallic inclusions are polished by the inspection surface;
Carbide ribbon, carbide liquid precipitation, carbide mesh surface to be inspected with 4% nitric acid solution after etching.
9. Pulling inspection: wire and steel plate should be tested for tension; wire tension test; take a sample with a length of 250mm at one end of the wire; perform a tensile test on a tensile test machine. The steel plate is taken and prepared according to the normative rules; Tensile test. The tensile test method is carried out according to the GB228-76 specification rules.
Number of inspections of bearing steel: The inspection of bearing steel is based on batch or heat as inspection unit. It is necessary for each batch of steel to be from the same steel type; the same exercise furnace number; the same heat treatment heat; the same scale of steel composition. Bearing steel The quantity of the inspection depends on the ordering skill specification of the steel; the availability status.
At present, China's high carbon chromium bearing steel bars, strips and wires have three specifications. YJ284 and YB9-68 for rod and strip steel; YB245-64 for steel wire.
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