I. Overview
Although rough machining, semi-finishing and finishing of stainless steel materials on general-purpose machine tools are not too difficult. However, in the case of high productivity dedicated automatic lathes, how to solve the problems of large cutting force, high temperature, severe tool wear, low durability, poor machining surface quality, low productivity, etc. in the cutting of stainless steel materials can be achieved in one cutting process. It is not easy to achieve the requirements of the design. In the processing of martensitic stainless steel materials, our factory conducted trial and error from the aspects of tool material selection, tool geometry, structure determination, selection of cutting amount, blank supply status, lubrication, and selection of coolant. A certain amount of successful experience. Take 3Cr13 stainless iron as an example.
Second, the difficulties in the cutting process and the reasons
In the trial production of parts, our factory turned the 3Crl3 steel according to the turning method of ordinary carbon steel. As a result, the tool wear is serious, the productivity is low, and the surface quality of the parts cannot meet the requirements.
Compared with the mechanical properties of 3Crl3 steel and 40 steel, 45 steel and other carbon structural steel, 3Crl3 steel strength, elongation, reduction of area, impact performance and other indicators are higher than 40 steel, 45 steel, is a high strength, plasticity Good medium carbon martensitic stainless steel. Due to severe work hardening during cutting, large cutting resistance, high cutting temperature, severe tool wear, increased number of sharpening, increased downtime and machine adjustment time, and reduced productivity. Because it is easy to stick the knife and generate built-up edge, it causes the change of the workpiece size and affects the surface roughness, and the chip is not easy to curl and break, and it also damages the processed surface of the workpiece and directly affects the quality of the part. Therefore, 3Crl3 cannot be cut by a 45-gauge cutting process. It is also not possible to copy the processing method on a universal lathe onto an automatic lathe. Because the automatic lathe has less knives, it is required that the machining surface can reach the required size and surface roughness in one pass to ensure higher productivity.
In response to the above problems, our factory has taken the following measures.
Third, the main technical measures taken
1) Use heat treatment to change the hardness of the material
The hardness of martensitic stainless steel after heat treatment has a great influence on turning. Table 1 shows the turning of different hardness 3Cr13 steels after heat treatment using a turning tool of YW2 material. It can be seen that although martensitic stainless steels in the annealed state have low hardness, they have poor turning performance. This is due to the material's plasticity and toughness, non-uniformity of the structure, adhesion, and strong fusion, and it is easy to produce turbidity in the cutting process, and it is not easy to obtain better results. The surface quality. The 3Cr13 material with a hardness below HRC30 after quenching and tempering process has good processability and easy to achieve a good surface quality. However, if the hardness is greater than that of HRC30, the surface quality is better, but the tool is easy to wear. Therefore, after the material enters the factory, it is first quenched and tempered, the hardness reaches HRC 25-30, and then the cutting process is performed.
2) Selection of tool materials
Under the condition of the same cutting parameters, our company carried out comparative turning tests on the cutting tools of several materials. The external turning tool with TiC-TiCN-TiN composite coated inserts has a high durability, good workpiece surface quality and high productivity. This is because the coated hard alloy blade has better strength and toughness, and because the surface has higher hardness and wear resistance, smaller friction coefficient and higher heat resistance, and It is a good tool material for turning stainless steel on automatic lathes. It is the material of choice for processing the 3Cr13 stainless steel external return tool. Since there is no cutting blade of this material, the comparison test of Table 2 shows that the cutting performance of YW2 cemented carbide is also good, so the blade of YW2 material can be selected as the cutting blade.
3) Selection of tool geometry and structure
For good tool materials, choosing a reasonable geometric angle is especially important.
The rake angle g0 directly affects the strength and thermal conductivity of the tool. Generally, when the martensitic stainless steel is turned, the rake angle of the tool is 10° to 20°. The posterior angle a0 is preferably 5° to 8°, and the maximum is not more than 10°.
The blade angle ls, negative blade angle can protect the blade tip and increase the blade strength. Generally, g0 is selected to be -10° to 30°.
The main declination angle Kr should be selected according to the shape of the workpiece, the machining location and the loading conditions.
The edge surface roughness should be Ra0.4-0.2μm.
In the structure of the tool, the external turning tool of our factory adopts external oblique arc cutting flutes, and the chip curl radius is large at the tip of the chip, and the curl radius of the chip at the outer edge is small. The chip is turned to the surface to be machined and broken. The situation is good. For the cutting knife, our factory will control the declination angle within 1°, which can improve the chip removal conditions and prolong the service life of the tools.