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Bearing heat treatment method for foreign machinery industry (middle)
Source: Bearing network time: 2013-12-16
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3.1 Bainite quenching arrangement and mechanical function After high-carbon chromium bearing steel is quenched by lower bainite; its arrangement consists of lower bainite, martensite and residual carbide. Bainite is irregularly intersected. The strip is a carbon supersaturated alpha layout; it is interspersed with granular or short rod-shaped carbides 55 to 60° to the long axis of the sheet; the spatial shape is convex lenticular; the sub-layout is dislocation-entangled No; there is no tantalum sub-layout. The number and shape of bainite vary with different process conditions. With the increase of quenching temperature; bainite strips become longer, isothermal temperature rises; bainite strips change Width; carbide particles become larger; and the point of view of the intersection between the bainite strips becomes 鹎飨蛴谄 ä¿¡ ä¿¡ ä¿¡ æ æ æ æ æ å—¨ å—¨ å—¨ 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓 宓Ren Lun 涫 涫 溆 溆 æ° æ° æ° è¹‹ 蹋 蹋 蹋 蹋 蹋 蹋 蹋 蹋 蹋 蹋 蹋 辶 辶 辶 辶 辶 辶 辶 辶 辶 辶 辶
The lower bainite arrangement of high carbon chromium bearing steel can improve the steel's share limit, yield strength, flexural strength and section shortening rate; compared with quenched and tempered martensite arrangement; with higher impact resistance, fracture resistance and dimensional stability Sexual; the external stress state is compressive stress.
The high threshold value ΔKth and the low crack growth rate da/dN represent that the bainite arrangement is not susceptible to germination; existing cracks or newly germinated cracks are not easily extended [2, 19, 20].
It is generally believed that the wear resistance and touch fatigue function of the full bainite or horse/shell composite arrangement is lower than the quenched low temperature tempered martensite; near the wear resistance and touch fatigue function of the martensite arrangement of the nearby temperature tempering Or slightly higher. But under smooth conditions (such as coal slurry or water); the full BL arrangement shows significant superiority; has a higher touch life than the low temperature tempered M arrangement; such as smooth water When the full BL arrangement L10 = 168h; temper M arranged L10 = 52h [21].
3. 2 production and use
3.2.5 effect
The outstanding features of the BL arrangement are impact resistance, fracture resistance, wear resistance, and dimensional stability. The residual stress on the surface is compressive stress. Therefore, it is suitable for bearings with large interference and poor service conditions; for example, accepting large impact loads. Bearings such as railways, rolling mills and cranes; mine transportation machinery or mine loading and unloading systems with poor smooth conditions, bearings for coal mines, etc. High-carbon chromium bearing steel BL austempering process has been successfully applied in railway and rolling mill bearings; .
(1) Expanded the application scale of GCr15 steel; usually GCr15 steel M quenching useful wall thickness of 12mm or less; but BL quenching can be strong because of salt cooling; if mixing, stringing, adding water, etc.; The useful wall thickness can be extended to about 28mm.
(2) The hardness is stable and uniform: because the BL transition is a slow process; usually GCr15 steel takes 4h; GCr18Mo steel takes 5h; the ferrule is isothermally long in the nitrate salt; the outer core arrangement changes simply together; Good stability and uniformity; usually GCr15 steel BL after quenching hardness of 59 ~ 61HRC, uniformity ≤ 1HRC; unlike M quenching when the ferrule wall thickness is slightly larger, it presents low hardness, soft point, poor uniformity and other problems.
(3) Reducing quenching and grinding cracks: in the production of railway and rolling mill bearings; because the ferrule is large in scale and heavy in weight; M is brittle when oil quenching; in order to obtain high hardness after quenching, strong cooling is often adopted; Quenching microcracks, because M is quenched, the external surface is tensile stress; the superposition of grinding stress during grinding process makes the whole stress level progress; it is easy to form grinding crack; constitutes batch waste. When BL is quenched; because BL arranges tolerance than M arrangement It is much better; the external surface constitutes a compressive stress of up to -400 to -500 MPa; the tendency of quenching cracks is greatly reduced [19], and the external compressive stress compensates for some grinding stress during grinding; the overall stress level is lowered; Cut the grinding crack.
(4) Progress in bearing life: about railways and rolling mill bearings that accept large impact loads; the primary failure mode after M quenching is: the inner sleeve is cracked when the device is used; The inner ring is broken; the austempered bearing has good impact resistance and external compressive stress; no matter whether the inner sleeve is cracked during the installation; it is still used in the process of the casing to stop the block, and the tendency of the inner sleeve to be greatly reduced. Concentration and maintenance of α. Therefore; after austempering, the average life and reliability after M quenching progress.
SKF applies the high-carbon chromium bearing steel bainite austempering process to railway bearings, rolling mill bearings and bearings used under special conditions; together developed steel grades suitable for bainite quenching (SKF24, SKF25, 100Mo7) [19]. It uses a longer isothermal moment for quenching; a full lower bainite arrangement after quenching. A new steel grade 775V [22] has been developed by SKF; and it is more uniform after special austempering. Lower bainite; the hardness added after quenching together is 60% higher than the conventional austempering; the wear resistance is improved by 3 times; the wall thickness of the treated ferrule exceeds 100mm. Some functions of M/BL composite arrangement after isothermal are still Controversial; such as the content of BL is the best, etc. Even if there is an optimal content; how to control in the production practice; and the compound arrangement needs to carry out an additional tempering after isothermal; adding the production cost. FAG company first choice of shellfish Sterling quenching process; its detailed process status is unknown.
4 carburizing, nitriding and carbonitriding
1.4 Low carbon steel carburizing, nitriding and carbonitriding is a traditional surface chemical thermal treatment process; carburized steel (low carbon low alloy steel, low carbon high alloy high temperature carburizing steel) by carburizing and quenching After the surface is high-hard and wear-resistant, the heart is tough. On the one hand, the carburizing process is an improvement of the carburizing medium; for example, participating in the additive for adding the osmotic velocity; selecting the strong osmosis-dispersion replacement cycle process to improve the osmotic velocity and improving the permeable layer Arrange and so on.
Following the development of vacuum skills; vacuum low pressure carburizing and plasma carburizing. The low pressure carburizing process of acetylene developed by Ipsen and other companies [23] is under low pressure of 10 mbar; acetylene is used as carburizing medium in vacuum furnace. Conducted. It is characterized by fast permeation speed, uniform infiltration layer, less carbon black, bright light after infiltration, and other; carbonitriding or carburizing of stamped needle roller bearing parts with thinner requirements for seepage; Depth, composition control and how to improve the seepage speed is a big problem; the vacuum low pressure carburizing skills will be used to deal with these problems.
Plasma carburizing of high alloy carburized steel can improve the rate of seepage and reduce the composition of coarse carbides on the surface [24]. Select nitriding or carbonitriding for the inner and outer rings of needle bearings and low-carbon steel needle bearings; Its wear resistance and corrosion resistance, reducing friction coefficient.
4. 2 high carbon chromium bearing steel carburizing or carbonitriding high carbon chromium bearing steel is usually hardened; the residual stress after quenching is the external tensile stress; easy to form the quenching crack, the use of the bearing function. Carburizing, nitriding or carbonitriding; improving the carbon and nitrogen content of the surface layer; decreasing the Ms point of the outer surface; forming a surface compressive stress after the appearance of the quenching process; improving wear resistance and rolling touch The fatigue function [25; 26]. The study also shows that high carbon chromium bearing steel can improve the bearing fatigue life under the condition of carburization after carburizing or carbonitriding [25~27]. Usually; During quenching and heating; the carbon (nitrogen) potential of the controlled atmosphere can reach the above intention. However, if the carburizing of high carbon chromium bearing steel is carried out (carbon potential > 2%), it is necessary to increase the machining allowance; Coarse carbides on the surface layer after carburizing and quenching.
3.4 Process control Carburizing (nitriding or carbonitriding) atmosphere detection and control is the key parameter; the earliest is the use of dew point meter, CO2 infrared analyzer; currently the primary choice of oxygen probe to detect carbon potential (or nitrogen potential) The response speed is fast; real-time monitoring is possible; cooperative CO2 infrared analyzer or other measurement methods (such as the HydroNit probe [28] developed by Ipson) can accurately control the carbon potential (or nitrogen potential).
Another aspect of process control is the computerized operation of carburizing (nitriding or carbonitriding) processes. The computer simulation of carbon transfer and dispersion in steel began in the 1980s; further development of human-machine dialogue Software (Carb-o-Prof); enables everyone to calculate the carbon transfer and dispersion speed of different steels at any time during the carburizing process. The software considers the effects of process parameters such as temperature and carbon potential; The required process carbon content and the depth of the process parameters are calculated; and the process parameters such as carbon potential and carburization time can be actively adjusted according to the change of the parameters in the process or the disturbance of the process; More than; has introduced the "Carb-o-Prof-Expert" expert system. The software integrates most of the carburizing steel and carburizing quenching physical metallurgy common sense, equipment functions, workpiece skill requirements and other data; as long as the computer Input the steel grade, component, several scales, hardenability, seepage requirements and furnace type of the workpiece; the computer will output a carburizing process; and actively complete the process [29].
5 appearance modification skills
5.2 Ion writing ion injection compared with other external reinforcement skills; has the following significant advantages: (1) parts after ion injection; can adhere to the original scale accuracy and appearance roughness; Other surface processing and disposal is very suitable for the final process of producing fine parts such as aerospace bearings. (2) It is not subject to metallurgy or equilibrium phase diagram in principle; it can be based on the working conditions and skill requirements of parts; Any injection of elements; injection dose and energy; bearing appearance with special requirements such as high wear resistance or corrosion resistance; great flexibility; practicality; selection of substrate data can be properly relaxed; Valuable high-alloy steel and other precious metal materials, (3) the writing layer and the matrix data are firmly connected and have no obvious interface; no drop and peeling appearance will occur in the application; this is a firm bearing life and operation Sex is very important, (4) ion injection is a non-high temperature process; can be completed at a lower temperature; parts will not temper, deformation and appearance oxidation, (5) have Good controllability and reproducibility. Europe and other countries has been written many ion discussion [30 to 37].
The US Naval Laboratory conducted research on ion writing of bearing parts since 1979; countries such as the United Kingdom, Denmark, and Portugal began operations similar to the US Naval Laboratories in 1989. The effect indicates that writing chromium ions can significantly improve The anti-corrosion function of M50 steel; and the anti-touch fatigue function has also been improved. In addition, boron ion is injected to improve the anti-wear ability of the outer bearing. The bearing steel 52100 is subjected to nitrogen plasma ion writing (PSII) and then formed on the outer surface. Thin layer nitride; can improve the corrosion resistance of bearing steel; used to replace precious stainless steel, nitrogen and boron ion writing on SUS440C stainless steel ball bearing can reduce the micro-vibration wear of the ball bearing and the dust emission of the bearing; Others; (Ti + N) or (Ta + N) plasma immersion ion writing (PSIII) can significantly improve its microhardness, wear resistance and life.
5.2 External coating coating skills include: physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spraying (PSC), electroless plating, etc. [38~42 ]. PVD is compared with CVD; the temperature of the treated workpiece is low during the process; no heat treatment is required after plating; and the external parts of the bearing parts are widely used. The steel bearing parts such as 100Cr6 and 440C are After PVD, CVD or RF plating of TiC, TiN, TiAlN, etc.; can improve the wear resistance of the bearing parts, touch fatigue resistance; reduce the external friction coefficient.
SKF has developed two coating skills over the years: First, PVD is used to weld diamonds (Diamond-Like Carbon, DLC) with extremely high hardness in bearing rings and tumbling external surfaces; the hardness of the outer surface is higher than that of hardened bearing steel. 40~80%, the friction coefficient is similar to PTFE or MoS2; it has self-smoothing property; it has excellent connection with the substrate, no shedding; the bearing life and wear resistance are greatly improved; it can still work normally under the condition of oil cut; It is called “NoWear bearing†[38]; The second is to use PSC to spray a layer of 100μm alumina on the outer circumference of the outer ring of the bearing; the insulation of the bearing can be as high as 1000V or more; the thickness of the added alumina makes the bearing more High insulation ability. The coated alumina is firmly bonded to the substrate; the corrosion resistance of the bearing can be improved; the post-plated bearing (INSOCOATTM bearing) can be mounted like a normal bearing [39].
Low-temperature ion sulphurization is a surface modification skill presented in the late 1980s. Its basic principle is similar to that of ion nitriding; under a certain degree of vacuum; high-voltage direct current is used to ionize sulfur-containing gas; the generated sulfur ions bombard the surface of the workpiece; On the surface of the workpiece and the iron reverberation, a sulfide layer with a thickness of about 10 μm mainly composed of FeS is formed. The sulfide is an outstanding solid smoothing agent; usefully reduces the friction coefficient of the steel part touching the outer surface; and increases with the load; the friction coefficient further decreases. Therefore, the wear resistance of the bearing under heavy load can be greatly improved; the life of the bearing can be improved by about 3 times.
The effect of low-temperature phosphating is similar to that of sulphurization. After the workpiece is placed in a TAP solution (phosphoric acid tristearate) at 40 ° C for 4 h, 0. 05 ~ 0. 25 μm thick Fe2O3 and Fe4 can be obtained. P2O7)3 outer layer; reduced friction coefficient, improved wear resistance. The phosphatized M50 steel bearing does not show stuck in the short-term oil cut condition; the bearing reliability is improved [36].
Dispersed chromizing is carried out by gas method (powder method) at 850~1100 °C; time is 1~9h; according to the steel used in the parts (ШХ15, 95Х18, 55СМ5Ф.┘ target 枰∮ 枰∮ 枰∮ 嘤 嘤 奈 奈 奈 奈 褪 褪 褪洌 谥岢 谥岢 薷 薷 薷 薷 墒褂 墒褂 墒褂 墒褂 墒褂 墒褂 墒褂 墒褂 墒褂 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗 渗After chrome and conventional heat treatment, heat resistance, corrosion resistance, wear resistance and touch fatigue strength are all significantly improved [42].
6 Appearance heating quenching induction heating surface quenching is one of the more widely used methods; the former Soviet Union has carried out more research on the theory and production of this process [43~48]; its primary application is divided into two categories: It is the surface induction heating quenching of the railway bearing; the ferrule made of 新4 steel is selected after induction heating and quenching; the appearance is hard and wear-resistant martensite arrangement; the heart is the well-resistant sorbite and troostite The appearance is a compressive stress of up to 500Mpa; its service life is 1 times higher than that of the ШХ15СГ bearing; and the appearance of the brittle fracture when the ferrule is applied is completely eliminated; the bearing reliability is improved; the function and the low carbon steel carburizing and quenching Similar; but the cost is much lower than the latter. Also; the corresponding special sensors and quenching equipment are also announced; and this information and the effect of induction hardening are promoted to heavy-duty bearings such as rolling mill bearings that require wear resistance and high resistance. Another application of induction heating for external quenching is the thermal treatment of extra-large bearings; reducing the quenching deformation and hardness non-uniformity of large bearing rings; together saving equipment costs. Japan [47] has a sense of appearance The quenching should be successfully applied to the heat treatment of the constant velocity end of the car; the quenching including the stepped shaft, the inner and outer surfaces of the casing and the raceway are all heated by a special induction coil. The application of high frequency heat treatment and cold forging skills makes The cost of production has dropped significantly; the reliability of the product has also improved significantly.
High-energy beam external heat treatment such as laser is a new heat treatment method developed in these years [49~50]; more CO2 laser beams are used. After laser heating, a hardened layer of 0. 25~2.0 mm can be obtained; and other appearances Compared with the hardening method; it has the advantages of accurate depth and azimuth control of the hardened layer and no deformation. The high carbon chromium bearing steel parts are hardened by external laser hardening, the martensite of the hardened layer is very fine, the carbide dispersion is more uniform, and the retained austenite Rare; higher hardness and sliding wear resistance than usual quenching and tempering. Others; high energy beam such as laser can also be used as a heat source for the surface coating process; one can finish the surface quenching and coating process; especially these years The development of skills; this composite process will have a broad application perspective in the appearance of fine bearing parts.
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