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Research on Turning of Austenitic Stainless Steel Thin Wall Sleeve

The austenitic stainless steel material has high plasticity and toughness, high cutting temperature, serious work hardening, large cutting force, easy chip adhesion, and machining accuracy is difficult to guarantee. Thin-walled sleeves are prone to vibration and deformation under the action of cutting force. It is necessary to improve the process route, form a more rigid process system, appropriately select the tool angle and appropriate cutting amount, and change the radial clamping to axial tightening. To use the inner circle as the positioning reference, an elastic mandrel can be used for clamping.

Austenitic stainless steel material such as 1Cr18Ni9Ti is a relatively excellent mechanical material, which is widely used, but its processing performance is poor, and the relative machinability is only 0.3~0.5. Thin-walled stainless steel pipe sleeve parts are more difficult to process. This article focuses on Analyze the technical difficulty of turning such parts and propose corresponding solutions.

Difficulty analysis of turning austenitic stainless steel thin-walled sleeve

Poor processing performance of materials

Although the strength and hardness of 1Cr18Ni9Ti material are not high, it has high plasticity and toughness. Its elongation is 40%, which is 2.5 times that of 45# steel, and its impact value is 3 times that of 45# steel. It takes a lot of work to remove the material. Most of the work is converted into heat. However, the thermal conductivity of stainless steel is small, and the heat carried away by the chips is small, so the cutting temperature is high. Due to the high cutting temperature and the effect of cutting stress, austenite will be transformed into martensite with high hardness and work hardening will be serious. Due to the toughness of stainless steel, the chips are not easy to break, and they will be clamped between the workpiece and the tool to squeeze the machined surface, which will form your adhesion. When the adhesion is serious, it will form a built-up edge, which directly affects the processing quality of the machined surface. Increase tool wear.

Poor rigidity of thin-walled sleeve processing

As shown in the picture, the bearing sleeve produced by a beer machine company has high outer diameter requirements and certain shape and position accuracy. It is planned to be processed on a CA6140 horizontal lathe. Since the thinnest part of the wall is only 2mm, Vibration and deformation are prone to occur under the action of radial clamping force. The cutting heat generated during stainless steel cutting will also deform the workpiece. Therefore, improve the process route and change the clamping method to form a more rigid process system, and appropriately select the tool angle And suitable cutting amount.

20200728142635 24591 - Research on Turning of Austenitic Stainless Steel Thin Wall Sleeve

Process route

Machining is divided into multiple stages of rough, semi-finish and finishing, and can be repaired in the semi-finishing process.
Deformation of the workpiece due to rough machining. The inner hole first and then outer circle processing method can be adopted, and the clamping force and cutting allowance can be larger during rough processing to improve the processing efficiency. When machining the outer circle, the inner hole is used as the positioning reference to ensure the position accuracy. Select appropriate cutting amount for finishing to ensure machining accuracy.

Selection and design of fixture

The inner hole and end face are processed first, leaving a larger margin, using a slotted pipe sleeve and special soft jaws to increase the contact area, and the clamping force is evenly distributed on the surface of the workpiece to reduce deformation. After the outer circle is processed, the wall thickness will become smaller and smaller. The radial clamping must be changed to axial tightening. At the same time, the inner circle should be used as the positioning reference, and the elastic mandrel can be used for clamping. Three rigid lobes are used, the inner hole size tolerance is the same as the outer circle size, and the radius of curvature is also the same. A taper sleeve is installed on the mandrel, and the right nut is screwed to make it move inward. The taper sleeve gives a radial expansion force to the expansion valve to clamp the workpiece. When it is screwed in the opposite direction, the workpiece is loosened. The fixture structure can improve the positioning accuracy because the radial gap is eliminated.

Tool selection

Tool material

In view of the fact that cutting heat is not easy to dissipate when cutting austenitic stainless steel, tool wear is large, and the material has affinity, the tool material should have sufficient strength, hardness, toughness, high thermal hardness, chemical stability and wear resistance . Among the tool materials, the hardness and wear resistance of cemented carbide are higher than that of high-speed steel. It is a cheap and practical tool material. Among them, YG8 tools have strong thermal conductivity and can be selected. The disadvantage is the adaptation of the front and rear angles. The range is small.

Sharpening of the geometric angle of the tool

In order to reduce the turning force and turning temperature of the austenitic stainless steel thin-walled sleeve, reduce vibration, reduce thermal deformation and surface roughness, the selection of tool geometric angles, especially rake angle, relief angle, entering angle, and blade inclination is very important.
The rake angle γ0 determines the sharpness of the tool. Stainless steel has large plastic toughness. γ0 should be appropriately large to reduce cutting deformation and friction, but γ0 should not be too large, otherwise it will reduce the strength of the cutter head, cause chipping, and also cause heat dissipation conditions. Getting worse. The rough turning γ0 is selected as 5°, and the fine turning is 20°.
The clearance angle α0 affects the contact area between the machined surface and the back and the friction between the two. Because the elasticity and plasticity of stainless steel are higher than that of carbon steel, the cutting vibration will increase the contact between the two, so α0 cannot be small , Otherwise the high temperature generated by friction will be concentrated on the flank surface and shorten the tool life. But it can’t be too large, which will reduce the strength of the cutter head and reduce the heat dissipation volume. Select 4° for rough turning α0 and 12° for fine turning.
The entering angle Kr changes the force distribution of the main cutting edge, the thermal conductivity, the thickness of the cutting layer, etc. Reducing the entering angle will increase the back force and increase the vibration when cutting stainless steel, so the entering angle should be greater than 75° .

Selection of cutting amount

There are three main parameters: cutting speed, feed rate, and back-grabbing amount. We must comprehensively consider their influence on the cutting process, productivity, machining accuracy and tool life. For cutting stainless steel pipe sleeve parts, you can choose a larger amount of back-cutting and a lower speed when roughing, cutting speed VC (10~13m/min), feed rate f (0.25~0.36mm/r), back-cutting Knife amount ap (3-4mm); when finishing turning, choose a higher speed and a smaller feed rate, and the amount of back knife, VC (40~50m/min), f (0.05~0.1mm/r) , To ensure processing accuracy.

Selection of cutting fluid

In order to reduce the cutting heat, cutting deformation and the formation of chip buildup during the processing of austenitic stainless steel thin-walled pipe sleeves, oil-soluble cutting fluids should be selected, with lubrication as the main purpose, and cooling effects. Generally choose the cutting fluids containing S, Cl and other extreme pressure additives, sulfurized oil, kerosene, CCl4 and oleic acid synthetic cutting fluid. The cutting fluid should be supplied adequately and timely, supplemented by spray cooling and high pressure cooling to achieve better results. .
In short, the processing of austenitic stainless steel thin-walled sleeves is difficult. It is possible to improve the machinability and achieve the required processing accuracy through the selection of the tool sharpening angle, the ingenious design of the fixture, and the reasonable selection of the process flow.

Source: China Pipe Sleeve Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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