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Turning processing of 304 stainless steel thin-walled ring parts

Introduce the characteristics of the stainless steel machining of thin-walled ring parts, summarize the undesirable phenomena in the actual machining and analyze the reasons, aiming at the high toughness, poor thermal conductivity, high temperature hardness, high cutting adhesion, and easy work hardening of such parts And cutting deformation and other unfavorable factors, based on workpiece clamping, tool materials and parameters, cutting amount, cooling and lubrication, etc., a set of processing plans are summarized. Practice has shown that this solution can prolong the service life of tools, ensure the processing quality of parts, improve production efficiency, reduce tool loss, and save energy and reduce emissions. The cooling method adopted is environmentally friendly and close to clean production.

06Crl9Nil0 (304) stainless steel has excellent stainless corrosion resistance and good intergranular corrosion resistance. It also has good corrosion resistance to alkaline solutions and most organic and inorganic acids. It can withstand high temperatures of 1000-1200℃ and is widely used Used in aviation, chemical, petroleum, construction, food and other industries. But its cutting performance is extremely poor, and the machining accuracy of thin-walled ring parts is required. This article analyzes and summarizes such workpieces and their practical processing schemes that are encountered in daily work.

The characteristics of stainless steel thin-walled ring parts cutting

The main problems in cutting stainless steel materials

Stainless steel belongs to the category of difficult-to-process materials, which is determined by its metallographic structure and physical properties. 304 is austenitic stainless steel, and its chemical composition is shown in Table 1.
Table.1 Chemical composition of 304 stainless steel (mass fraction) (%)

Item C% Si% Mn% P% S% Ni% Cr% N% Others
Min 8.00 18.00
Max 0.08 1.00 2.00 0.045 0.030 10.50 20.00

Problems in processing:

  • 1) The plasticity is large, and the work hardening is serious, which intensifies the wear of the tool, especially the boundary wear of the tool. It is easy to form a built-up edge and deteriorate the surface quality of the workpiece. There are also chip breaking and chip curling problems. The cutting force is higher than that of low carbon steel About 50% higher, the depth of the hardened layer is large after processing, which brings difficulties to the processing of subsequent procedures;
  • 2) The thermal conductivity is small, only 1/3 of the 45# steel, and the cutting force is large, and the power consumed for separating the chips is also large. The cutting heat generated is also more, and the heat transferred to the tool can reach 20% (while processing carbon steel only accounts for 9%), resulting in high local (cutting edge) cutting temperature, the tool is easy to burn, and the durability is reduced;
  • 3) High cutting temperature causes severe work hardening, and some carbides (TiC, etc.) in the steel form hard inclusions, which accelerate tool wear.

Factors affecting the machining accuracy of stainless steel thin-walled parts

The machining accuracy of parts is mainly affected by the geometric errors of the process system composed of machine tools, fixtures, cutting tools and workpieces, stress and thermal deformation, and errors caused by internal stress of the workpiece. For stainless steel thin-walled parts, the main factors affecting machining accuracy are:

  • 1) The rigidity of the workpiece is poor, and the parts are prone to elastic deformation during clamping and cutting, which seriously affects the geometric accuracy and position accuracy of the machined surface;
  • 2) The rough and semi-finished products themselves have shape errors, which are reflected on the processed parts according to the error re-mapping law;
  • 3) The accuracy of the machine tool, accessories, and fixture itself;
  • 4) Thermal deformation during processing increases the shape and position error of parts;
  • 5) Large cutting force produces cutting vibration;
  • 6) In the process of storage, handling, assembly, and debugging of parts, the residual stress and use environment of the parts are affected. Overall analysis, in order to obtain thin-walled parts with stable size, shape, and position accuracy, the main factor of elastic deformation must be overcome first, and the second is to reduce the thermal deformation of the process system and the residual stress of the parts.

Due to the above-mentioned processing difficulties of stainless steel thin-walled ring parts, the selection of tools, clamping methods, and cooling methods during the processing process is very important.

Fixing method

Based on the above-mentioned processing characteristics of thin-walled stainless steel ring parts, if the clamping is unreasonable, under the action of clamping force, radial cutting force and cutting heat, it is extremely easy to deform and cause vibration, especially due to the change of cutting angle of the tool and the dressing of the grinding wheel. The effect and the clamping form of the tooling and other factors make it difficult to guarantee the size, shape, position accuracy and surface roughness of the parts.
When a self-centering chuck is used to clamp the workpiece during processing, obvious elastic deformation will occur at the clamping position of the jaws, resulting in unqualified workpieces after processing. That is, the elastic deformation part is cut away, the jaws are released to remove the workpiece, the elastic deformation part is restored, and the geometric shape of the part becomes a triangle or polygon. Use slotted sleeves or soft jaws (that is, the equipped sleeve is divided into three parts and installed on the chuck jaws, and the dimensional accuracy can be corrected). Special attention should be paid to the matching accuracy between each positioning surface. , Otherwise it will cause positioning failure and even damage the parts.
For the machining of general thin-walled sleeve workpieces, the repeated positioning of the inner and outer diameters will inevitably bring trouble to the fixture and make it difficult to guarantee the product quality. The processing method of positioning the outer circle and the inner hole after the outer circle, that is, turning the end face, outer circle-finishing end face, outer circle-boring and fine boring, can effectively overcome the easy occurrence of thin-walled ring workpieces in the processing process. Disadvantages, reduce the amount of deformation of parts during processing, and improve product quality.

Select tool

The cutting tool is the key to ensuring the surface quality of austenitic stainless steel parts. The material, performance, quality and geometric parameters of the cutting tool will directly affect the processing efficiency and accuracy.

Selection of tool materials

According to the cutting characteristics of stainless steel, the tool material is required to have good heat resistance, wear resistance, and low affinity with stainless steel. At present, the commonly used tool materials of this type are divided into the following two categories.

  • 1) Cemented carbide. YG or Yw type tools are usually used for processing, especially for processing austenitic stainless steel. YT type cemented carbide should be avoided, because titanium Ti in stainless steel and Ti in YT type cemented carbide produce affinity, leading to cold welding. Chips take away the Ti in the alloy, which is prone to severe diffusion wear at high temperatures. YG cemented carbide has good toughness, high wear resistance and red hardness, and is more suitable for the processing of stainless steel materials; its thermal conductivity is good, which is beneficial to reduce the cutting temperature, and the chips and tools are not easy to stick.
  • 2) High-speed steel. When the shape, size and structure of the workpiece determine that it is not suitable to use cemented carbide tools or the use of cemented carbide tools is easy to damage, high-speed steel tools should be used. Such as cobalt-containing high-speed steel (W6Mo5Cr4V2AI), nitrogen-containing high-speed steel (w12M03Cr4V3N) and so on.

For 304 stainless steel sleeve, YGSN tool can be used. Nb is added to this tool material, which increases the cutting performance by 1 to 2 times compared with YG8. It has a good effect in rough machining and semi-finish machining.

Selection of tool geometry parameters

The geometric angle of the tool has a great influence on the productivity, tool durability, surface roughness, cutting force and work hardening of the cutting process. Reasonable selection and improvement of tool geometric parameters is an effective way to ensure processing quality, improve efficiency, extend tool life and reduce costs. Through practice, the selection of tool geometry parameters for machining such workpieces is summarized as follows.

  • 1) Front corner. The rake angle of the tool determines the sharpness and strength of the cutting edge. Under the condition of not reducing its strength, a larger rake angle can reduce the deformation of the chip, thereby reducing the cutting force, lowering the cutting temperature, and reducing the work hardening trend. Improve tool durability. Generally take 12 ~18.
  • 2) Rear corner. Since the elasticity and plasticity of stainless steel are larger than that of ordinary carbon steel, if the clearance angle of the tool is too small, the contact area between the cut surface and the clearance corner of the turning tool will increase, and the high temperature area formed by cutting friction will be concentrated in the clearance corner of the turning tool. Speed up the tool wear and increase the roughness of the machined surface. Therefore, when processing such parts, the back angle of the turning tool should be sharpened slightly, but too large will reduce the strength of the blade, generally 8 ~1O .
  • 3) Blade inclination. The blade angle of the tool mainly affects the strength of the tool tip and the direction of chip flow. Since the tool tip strength of turning tools for processing such parts is weak, in order to enhance the tool tip strength without increasing the back component force too much, generally -15~-5 . The larger value is used for continuous cutting, and the smaller value is used for interrupted cutting.
  • 4) The main and secondary deflection angles. Generally the entering angle is 45~75, The secondary declination angle is 8~15 , Under the condition that the rigidity of the process system allows, choose a smaller value as much as possible.
  • 5) Chip flutes. Due to the good toughness and plasticity of stainless steel, it is difficult to break chips during cutting. If chip removal is not good, chip splashing will easily hurt people; if it is wrapped around the workpiece, it will easily damage the tool and increase the surface roughness of the workpiece. In production, the arc-shaped chip flutes are ground on the rake face of the tool, so that the chips are discharged in a certain direction after curling. The arc radius and groove width of the chip flute increase with the increase of the workpiece diameter. Practice has proved that the arc radius is generally 2 to 5 mm, and the groove width is preferably 3 to 6.5 mm. It can also be forced to deform, that is, to grind the double-edged inclination on the basis of a reasonable selection of cutting parameters, so that the chip section is prismatic, forming an externally inclined chip flute, and the chip guide curls into a pagoda or short tight spiral chip. .
  • 6) Sharpen the negative chamfer to improve the strength of the tool. Disperse the heat generated during turning to the front and back of the tool, reduce the wear of the blade part, and improve the durability of the tool. In production, the machining volume of rough turning and semi-finishing turning is 0.2~0.4mm, and the negative rake angle of the tool is -3~-5, The effect is better. In addition, stainless steel chips have strong adhesion to tools and are prone to build-up edge. Reducing the roughness of the tool working surface can effectively reduce cutting resistance, reduce chip adhesion, and increase tool life.

In short, for the turning of this type of workpiece, the tool material should be hard alloy with good toughness and high strength, and the tool parameters should be reasonable, and the tool edge should be sharpened as much as possible to facilitate the curling and breaking of the chips to ensure such the processing quality and production efficiency of the parts.

Select cutting amount

The choice of cutting amount is directly related to the surface quality of the workpiece, production efficiency and tool durability, and plays an important role in the entire processing process.

  • 1) Cutting speed. In order to improve the durability of the tool and prevent the cutting temperature from being too high, the cutting speed should not be too high, generally only 40% to 60% of the cutting speed of ordinary carbon steel, that is, 50 to 75m/min.
  • 2) Feed amount. Too much feed will increase the cutting force and directly affect the surface roughness, but it should not be too small or no feed to prevent the cutting edge from working in the chilled layer formed by the last cutting, generally 0.15~0.4mm/r Appropriate.
  • 3) The depth of the knife. Choosing the largest possible cutting depth will not only have the least impact on tool durability and cutting heat, but also help improve production efficiency. Take care to avoid cutting on the rough skin or in the hardened layer left by the previous process. Take 2~4mm for rough machining and 0.1~0.4mm for finishing.

Cutting lubrication cooling method

In order to achieve the purpose of cooling, lubrication and chip discharge, cutting fluid is usually used in the cutting process. Cutting fluids can generally be divided into two types: the first type is water-insoluble mineral cutting fluid; the second type is water-soluble cutting fluid, also known as emulsion. The second type has good cooling performance and low price, and is widely used in production and processing; however, because the lubricating performance is worse than the first type, additives are often added to the second type in order to improve the lubricating performance during production. For processing stainless steel parts, emulsions with good lubricating and cooling properties containing sulfur and chlorine additives are generally selected. Most of these additives are harmful ingredients, which volatilize into the air during use, which will bring great harm to the health of the operators and the surrounding environment.
After many trials and adjustments, we chose to use -30~-10℃ cold air to cool the cutting points. At the same time, the effect of supplying a small amount of vegetable oil cutting fluid to the front and rear face of the tool is very good, which greatly improves the machining accuracy and extends the tool life. Moreover, this vegetable oil is a natural organic matter, and its natural decomposition rate is higher than that of mineral cutting fluid. It is not harmful to the environment and not toxic to the human body.

Conclusion

Combining the cutting characteristics of stainless steel and the size characteristics of thin-walled pipe sleeve, reasonable selection of tool materials, angles, cutting parameters and cooling methods, and selection and design of corresponding fixtures can greatly improve the production efficiency and processing quality of such workpieces in traditional processing.

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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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