Talking about the New Heat Treatment Technology of Stamping Needle Roller Bearings_PTJ Blog

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Talking about the New Heat Treatment Technology of Stamping Needle Roller Bearings


Since the end of the First World War, two German brothers named Schaeffler have developed the technology of punching the outer ring and cage of the bearing outer ring and cage from a thin low-carbon steel plate. The perfection and gradual improvement, thus formed the stamping needle roller bearing series, and established the INA Bearing Co., Ltd. (Schaeffler INA Bearing co.,ltd), the INA (Immer neu Aufgabe) trademark means that there are always new tasks. . The emergence of stamped needle roller bearings has rapidly changed the structure of mechanical parts in machinery, automobiles, motorcycles, aviation, aerospace, textile machinery, light industry and other industries, making it lighter and achieving the goal of energy saving.

In many industries that use stamped needle roller bearings, the application of stamped needle roller bearings represented by the automotive industry is used to illustrate that to make the car run fast, the weight of the entire car itself must be reduced, and other conditions remain unchanged. The following only discusses the bearing structure in the reducer (box) of the automobile. In addition to the installation of normal physical bearings, it is also limited by the radial size of the internal space. It is hoped that the structure of all the bearings in the reducer can be made smaller. Lightweight, with high strength, able to withstand larger loads and higher speeds, and has good wear resistance, so a certain number of HK-type stamped needle roller bearings are needed in the reducer.

1. Stamping outer ring and cage

The material for manufacturing the stamped outer ring and the protective frame is made of low carbon steel cold-rolled coils with low carbon content (wC=0.006%~0.009%), clean and bright surface, and high dimensional accuracy. , Their thickness is generally 0.63 ~ 1.5mm, the hardness of this material is about 90 ~ 120HV10, its metallographic structure is relatively uniform granular ferrite and a very small amount of tertiary cementite (free cementite), This kind of organization is very suitable for making deep-drawn hole parts, such as deep hole parts such as stamping outer ring and cage. The forming of the stamping outer ring or the cage, according to the equipment condition of the enterprise, has two kinds of working conditions: multi-distributed single-process processing or a multi-functional combined stamping machine tool. The one-time investment cost of the former equipment is relatively low, but from the point of view of the processes of blanking, multiple stretching, trimming, etc., there are more process flow and operators, and the dimensional accuracy of the parts is low; the latter’s equipment is one-time investment although Larger, but less manpower input. One person can do blanking, multiple stretching and trimming processes on a multifunctional machine tool, and easily complete the entire process of stamping the outer ring or cage, and its products have high precision , Large output and other characteristics are worthy of popularization and use.

1. Heat treatment requirements for stamping outer ring and cage

The materials of the stamped outer ring and cage are the same as those mentioned above. The material grades are generally DC03M, DC04M, C15Mn, 16MnCr5, etc. The carburized (carbonitriding) layer depth of the stamped outer ring is generally 0.08~0.33mm (measure the effective hardened layer) Depth: should be measured vertically from the surface to 550HV. The hardness after carburizing and quenching is 840~900HV; the carburizing (carbonitriding) layer depth of the cage is generally 0.02~0.12mm (measure the depth of the total hardened layer: from the part The surface is measured perpendicular to the metal matrix structure in the microhardness or the distance of the hardened layer that has no obvious change in the microstructure), and the hardness after carburizing and quenching is 410~550HV. The metallographic structure is fine needle-like martensite, dispersed with fine carbides and a small amount of retained austenite.

Talking about the New Heat Treatment Technology of Stamping Needle Roller Bearings

For the carburizing and quenching of the above two products, it is more appropriate to choose a mesh belt furnace. Among them, the mesh belt furnace with a circulating fan and a muffle tank has the best product quality. Because the carburized layer is very thin, its carburizing (carbonitriding) time is very short, which requires a uniform atmosphere in the furnace. For this reason, a circulating fan is needed to stir the atmosphere to make the furnace carburized (carbonitriding). ) The atmosphere is homogenized to ensure the stability of product quality. However, domestic mesh belt furnaces with circulating fans basically do not have muffle tanks. Mesh belt furnaces with muffle tanks have very small furnace chambers and no fans. The structure of circulating fans at home and abroad is also different. Domestic circulating fans must be ensured. The continuous stability of product quality is somewhat difficult. The muffle tank mesh belt furnace with circulating fan made by Safed of Switzerland can already achieve a furnace with a mesh belt width of about 1000mm, and the width of the muffle tank reaches 1200mm. The furnace has 4 to 6 circulating fans according to the output size. It is specialized It is used for products with thin carburizing (carbonitriding) layers and high quality requirements. The heat treatment of stamped needle roller bearings belongs to this range.

2. Atmosphere control in mesh belt furnace

First of all, it is required that all product parts entering the furnace must be cleaned and dried. The quality of various gas sources must be guaranteed. Among them, their purity should be methanol ≥ 99.9%, propane ≥ 95%, and liquid nitrogen ≥ 99.999%. For liquid nitrogen, one liter of liquid nitrogen can be gasified into 647L of nitrogen, so Using liquid nitrogen to produce nitrogen is an ideal method, and its equipment is easy to maintain and operate.

For the new furnace, it is necessary to slowly oven and heat up according to the equipment manufacturer's requirements before putting it into production, and all bolts must be tightened again, and all the seals must be checked. When the furnace is restarted after the furnace is shut down, the new and old equipment must be Exclude the air in the furnace, that is, there should be a nitrogen inlet at the lower end of the blanking port. When the furnace temperature rises to 650℃, open the nitrogen ball valve at the tail, and the nitrogen enters the furnace from the blanking port and removes the CO2 and H2O in the furnace. Wait for the harmful atmosphere to gradually drive away from the inside of the furnace, which takes 5-10 hours. When the furnace temperature rises to 780℃, pass in the dripping methanol or low-temperature methanol cracking gas, continue to raise the temperature to 850℃ and keep it warm for a period of time, then pass in propane and At the same time, close the nitrogen valve at the end of the furnace. The gas production time after methanol propane nitrogen enters the furnace is a little longer, and it is better to make the atmosphere in the furnace more uniform under the action of the circulating fan.

When the new furnace is started, it is necessary to pre-carburize the mesh belt and the furnace. Sometimes the carbon potential meter shows that the technical requirements have been met, but when the flame color turns blue, it means that there is still air in the atmosphere in the furnace, and the presence of oxygen in the furnace will make the workpiece There are mottling and decarburization on the surface, the hardness is reduced, and the black structure can be seen in the metallographic structure inspection. For furnaces without muffle tanks, the air and moisture in the small holes on the lightweight refractory bricks and aluminum silicate refractory fibers have to be replaced. If the new furnace is not dried very dry, the furnace cover should be used. Loosen the plate, leave a gap of about 10mm to allow water vapor to escape, then cover the furnace cover and tighten the screws, or you can use other methods to remove it. In order to reduce the amount of air entering the furnace, in addition to controlling the height of the furnace door, the oil fume above and below the oil curtain at the discharge port must be pumped out with an air pump. The amount of extraction is adjusted by a ball valve, and the extracted oil fume is passed through a steam-water separator. It is controlled by a flow meter and then sent to the furnace mouth to merge with methanol gas for flame sealing of the furnace mouth.

In the process of adjusting the atmosphere in the furnace, no carbon black should appear on the surface of the mesh belt. When the flame color of the furnace mouth is orange, the atmosphere in the furnace should be adjusted, and the workpiece can be formally entered at this time. The furnace life of the muffle-free mesh belt furnace has a certain number of years. The life of imported equipment is about ten years. At this time, the equipment has normal conditions of process materials, instrumentation, furnace atmosphere and operation, but The layer depth and hardness of the carburized (carbonitriding) parts made are not up to the technical requirements, and it is very difficult to check the leakage point of the furnace. I have to increase the temperature and extend the time to deal with it. In fact, the furnace lining has cracks. The atmosphere consumption in the furnace increases, and the furnace needs to be rebuilt. For a mesh belt furnace with a muffle tank, the service life of its muffle tank is about 5 years. Once the muffle tank cracks, it can be quickly detected. Repair or update the muffle tank in time and use the furnace. The life span can be as long as about 30 years.

The calibration of the carbon potential in the furnace can be calibrated with a low-carbon steel foil (fixed carbon sheet) with a carbon content of 0.06% and a thickness of 0.05mm. There are generally two ways to calibrate the carbon potential of the oxygen probe:

One is the weighing method, using an analytical balance with an accuracy of one ten thousandth as the weighing measurement standard. The steel foil that has been derusted, degreasing, cleaned and dried is weighed, and then enters the furnace for process carburization , The steel foil should not touch the dirt when it enters and exits the furnace. The steel foil needs to be pre-cooled before it can be discharged. The formula is calculated to be the carbon potential at the position of the oxygen probe in the furnace.

The other is to use the combustion method to measure the carbon potential of the steel foil. This method is simple, accurate and fast. Its sample preparation method is the same as before. After the steel foil is taken out of the sample hole, it is placed in the C230 carbon and sulfur analyzer. The carbon-sulfur analyzer is connected to the balance. Nitrogen and oxygen are introduced into the instrument, and the switch is turned on. The steel foil burns instantaneously. At this time, carbon monoxide and carbon dioxide are produced. The carbon potential of the steel foil is displayed within one minute. Out.

The intake pipe of the drip-type mesh belt furnace is often blocked due to the low temperature of the insulation layer on the top of the furnace, which causes the controllable atmosphere to fail to meet the technical requirements, which affects the product quality. If special modifications are made to the drip-injection methanol and propane pipes, it will not frequently block the atmosphere entering the pipes, and the pipes can be cleaned for more than a year, so that the furnace atmosphere can remain stable for a long time. In addition, there must be ∮150mm peepholes on the upper part of the oil tank, that is, on the two sides of the blanking port, to facilitate observation and adjustment of the size of the oil curtain (curtain), so as to reduce the oil smoke entering the furnace, causing the phenomenon of false high carbon potential. . When necessary, after setting the furnace, you can open the peepholes on both sides to clean the notches of the oil curtain (curtain) to make the oil curtain unblocked. The oil circulation pump, oil curtain pump, oil cooling pump, etc. inside the oil tank must be of high quality, otherwise it will affect the bright color of the quenched parts.

2. Rolling needle

The rolling elements of stamped needle roller bearings need to bear a relatively large load. For this reason, they can only be made into needle-shaped forms with a large difference in length-to-diameter ratio. Because the structure of general stamped needle roller bearings cannot be too large, the diameter of needle rollers is generally ≤ 5mm, its length is 3 to 10 times the diameter.

The material of the needle roller is GCr15 or 100Cr6, the original structure needs to reach the level 2 spheroidization in the heat treatment technical standard of JB/T 1255—2014 high carbon chromium bearing steel parts, the hardness is 179~207HBW, and the high carbon chromium bearing steel wire is cut at high speed The machine is cut to the required specifications, and then placed in an octagonal drum inclined at a certain angle. A certain amount of needle wool, abrasives and solvents are placed in the barrel, and it is circulated at a certain speed for tens of hours. After polishing, both ends of the needle roller showed smooth arcs, which met the technical requirements.

There are two methods for quenching needle rollers: For quenching with more varieties and specifications but lower output, a small roller furnace is used. The feeding and discharging methods are all from the front furnace mouth, and the rear furnace mouth is introduced into a controlled atmosphere. The structure of the furnace Simple operation and convenient operation. Its furnace (muffle tank) is an octagonal cylindrical structure with taper at both ends. The needles can roll naturally during cyclic rotation, so that each needle can be heated evenly. There are three thermocouples in the heating furnace. , Namely, one in the furnace, used to control the temperature; one in the furnace (muffle tank) indicates the actual temperature, and the other is used for over-temperature alarm. Nitrogen, methanol, and propane are introduced into the furnace to protect and heat the needle rollers. There are circulating stirring pumps and baffles in the quenching tank to make the needle rollers quenched into the oil uniform in hardness and obtain the quenching hardness (65~66HRC). Small advantages. It is recommended not to use mesh belt furnace for heating. For a large batch of needle roller heating and quenching, a large roller furnace is used. The feeding method is the rear feeding and the controlled atmosphere, and the front discharge. The furnace structure is made of heat-resistant steel plate rolled into a circular cylinder. Welded with continuous and smooth inner-rotating heat-resistant steel plate, the needle roller can move forward and flip naturally in the drum, so that the needle roller can be heated evenly. In the quenching tank, there is a circulation guiding device and 4 round stainless steel receiving baskets with many small holes that can be transferred regularly. Other material receiving methods can also use conveyor belts to lift the structure and so on.

3, the choice of quenching cooling medium

The first quenching of the needle roller is heated in a large (small) drum furnace. Its quenching method is to continuously discharge the material while the drum rotates, and at the same time successively drop into the quenching oil tank to obtain a hard and uniform hardness. Its quenching oil uses fast bright quenching oil, because its vapor film stage is short, high and low temperature cooling speed is fast, thermal oxidation stability is good, the oil consumption is small and easy to clean, the surface smoothness is good, and the service life is long, which can make the needle roller After quenching, the hardness dispersion is small and uniform.

For the assembled integral stamping needle roller bearing, its quenching cooling medium should be quenching oil with saturated vapor pressure, brightness, cooling, good oxidation resistance, easy to clean and long service life, and it is in the gas permeability. The quenching process after carbon (carbonitriding) has good hardening performance and very little quenching distortion. Because the effective wall thickness of the product is thin, the graded quenching oil can be used; the graded quenching oil can be used at high temperatures. High flash point and ignition point, fast cooling rate at high temperature, slow cooling rate at low temperature, so that the stamped outer ring and cage can be carburized (carbonitriding) and hardened, and can be made of high-carbon chromium bearing steel The needle roller obtains a hard and uniform hardness, and can significantly reduce the distortion and meet the technical requirements. The new oil needs to be degassed before it is put into use. The oil in the working process is usually equipped with a high-speed centrifuge, which does not damage the physical and chemical properties of the oil, and can also remove the dirt, so that the quenching oil can maintain a long-term The stability of cleaning and quenching performance also prolongs the service life of quenching oil to more than ten years.

4. Heat treatment of integral stamping needle roller bearings

The first step in the heat treatment of integral stamping needle roller bearings must first process the needle rollers, that is, the needle rollers are heated, quenched and tempered in the roller furnace. During heating, the carbon potential CP in the furnace is 1.0~1.1%, and the heating temperature is (845 ~870)℃±5℃, use fast bright quenching oil, oil temperature 80℃, core quenching hardness 760+80HV, surface hardness 780+160HV, tempering temperature 160~180℃, hardness after tempering ≥810HV , The metallographic structure is tempered cryptocrystals, fine crystals or small needle-shaped martensite, evenly distributed fine residual carbides and a small amount of retained austenite.

After the heat treatment, the needle rollers are ground, polished, etc., and then the shape and size are sorted by a fully automatic high-speed sorting machine. Then, according to the technical requirements, the needle rollers that have been heat-treated and those that have not been heat-treated can be kept in one machine. The frame and the stamped outer ring are assembled on a fully automatic assembly and sealing machine to become an integral stamped needle roller bearing.

Finally, they are carburized (carbonitriding) quenched in a mesh belt furnace. The quenching cooling medium is graded quenching oil, the oil temperature is 80°C, the carburizing temperature is 820-870°C, and the atmosphere in the furnace is methanol, propane, nitrogen, and carbon. Potential CP =0.9%~1.2%, carburizing time is generally 30~100min according to technical requirements, the carburizing layer depth of the stamped outer ring is 0.08~0.33mm, the carburizing quenching hardness is 840~900HV; the carburizing layer depth of the cage is 0.02 ~0.12mm, carburizing and quenching hardness 410~550HV.
For roller needles, the metallographic structure before carburizing and quenching is tempered hidden needles, fine crystals or small needle-like martensite, evenly distributed fine residual carbides and a small amount of retained austenite, which are in the second carburizing During quenching, because the time is short, the structure will not be coarse for high carbon chromium bearing steel, but it can make the structure finer and more uniform, and improve the contact fatigue strength. At this time, the quenching hardness is ≥910HV, and the hardness after tempering ≥810HV.

After the heat treatment, cleaning and drying of the integral stamped needle roller bearing is completed, its size will be reduced by 0.01~0.05mm. For this reason, 100% enveloping circle inspection is required for the integral stamped bearing. For a very small number of out-of-tolerance, extrusion is required. It can be put into storage after correction. This technology has been used in foreign countries for decades, and it is also being developed in China, hoping to achieve good results.

Link to this article: Talking about the New Heat Treatment Technology of Stamping Needle Roller Bearings

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