1 Analysis of problems and reasons in the cutting process
In the part of trial production we took turning test of stainless steel 420 stainless steel according to the process of turning ordinary carbon steel. The tool wear is very serious, productivity is extremely low and the surface quality of parts is not up to the requirements.
Comparison of mechanical properties of stainless steel 420 steel with 40# steel, 45# steel of carbon structural steel, stainless steel 420 steel strength is higher than 40# steel and 45# steel. It is a kind of high strength, good plasticity of medium carbon martensitic stainless steel. Because of serious work hardening, great cutting force, high cutting temperature, all lead to serious tool wear, knife replace increase and downtime and machine adjustment time increase. Finally productivity reduced. At the same time, tools is easy to stick and generate steel scrap bue which caused the change of size and surface roughness. And the chip is not easy curling and easy breaking which damage the machined surface and affect part quality.
Consequently we can not use the process of cutting 45 steel, nor the general lathe machining method used. Because the general automatic lathe has less tool which demand the best to have one pass making the surface to be machined to meet the requirements of the size and the surface roughness to ensure high productivity.
2 main technical measures
1) Change the hardness of the material by heat treatment
Martensitic stainless steel has different hardness after heat treatment which has great impact on the turning processing. Table 1 is the turning result of stainless steel 420 steel with different hardness after heat treatment machined by tool YW2 materials. Obviously annealed martensitic stainless steel like 0.10.10.1 state has poor turning performance even if the hardness is low. This is because the material has great plasticity and toughness, inhomogeneous microstructure, adhesion, strong weld property, easy to produce scrap bue which all not easy to get a good surface quality.
However after quenching and tempering treatment stainless steel 420 material with hardness below HRC30 can has good machining performance and can be easy to reach the better surface quality. Use the material with hardness above HRC30 also can has good quality but the tool easy to be wear. So in the conditions allow the material hardness should be HRC25 ～ HRC30 by quenching and tempering treatment before machining.
2) Tool material selection
Commonly In the automatic lathe for turning stainless steel the hard alloy tool materials used are: YG6, YG8, YT15, YT30, YW1, YW2 and others.
High speed steel tool materials commonly used are: W18Cr4V, W6M05Cr4V2AL and others. We had the turning test for tools of several materials with the same conditions of cutting parameters.
We can see from table 2: the cylindrical turning tool with TiC-TiCN-TiN composite coating blade has high durability, high cutting speed is desirable, high surface quality and high productivity. However, we have no cutting blade of this material. Through the contrast test of table 2, the cutting performance of YW2 cemented carbide is good and it is used for cutter blade.
Table 2 Different tool cutting contrast test
Material The geometric angle of tool Cutting consumption Tool durability
Min Surface roughnessμ
Rake angle γ Back angle α main angle Kr edge angle λ
YG8 15 ° 8 ° 90 ° -10 45 ~ 550.172 ~ 82 Ra3.2
YT14 15 ° 8 ° 90 ° -10 45 ~ 550.180 ~ 95 Ra3.2
YW2 15 ° 8 ° 90 ° -10 45 ~ 550.190 ~ 110 Ra3.2
Composite coating blade 12 ° 8 ° 90 ° -4 45 ~ 550.1128 ~ 185 Ra6.3 (off)
Ra ～ Ra1.6
3) Selection of the tool geometry and structure
For good tool materials it is very important to choose reasonable geometric angle.
1) Rake angle: For martensitic stainless steel the turning tool rake angle 10 °～20 °is suitable.
2): Back angle 5 °～ 8 ° is more appropriate, but 10 degrees maximum.
3) Cutting edge inclination angle λ: selecting -10 °～ 30 °.
4) The main deflection angle of Kr should be selected according to the shape of parts, machining position and tool condition.
5) The edge surface roughness should be no more than Ra0.4～ Ra0.2.
In structure, our foreign circular tool using external oblique arc chip breaker groove, on the tool tip chip curling radius is big, on the outer edge of chip curling radius is small, the chip will be turned to the surface to be machined and broken. The cutting knife, we will associate the angle control within 1o, this can improve chip removal condition, prolong the service life of the cutter.
1) Selection of the cutting
The cutting theory is that the cutting speed of V has the greatest impact on cutting temperature and tool wear. The second is cutting amount s and the third is cutting depth t. For machining surface in the automatic lathe the cutting depth of t is decided by parts size and blank size. Generally it is 0 ～ 3mm. The cutting speed of hard processing material is much lower than ordinary steel cutting speed.
Cutting feed s has low influence to the durability of the tool than the cutting speed. But it will affect the chip removal and breaking, pull or scratch the surface of the parts, affects the surface quality. Generally believed that if the depth is large, the cutting feed should be reduced and the cutting feed is smaller than outside the circle. For the processed surface roughness requirements is not high the s can be 0.1mm/r ～ 0.2mm/r.
In short, for difficult materials to machine general selection is of slower cutting speed and medium cutting feed.
2) The appropriate cooling and lubricating liquid
Cooling and lubricating liquid for cutting stainless steel should have the following characteristics:
A. The high cooling performance which can take a large amount of cutting heat.
B. Stainless steel is toughness, easy to produce the knife cutting tumor, deterioration of machining surface. So it requires cooling and lubricating liquid has high performance of lubrication.
C. Good permeability which can play a better role of wedging, diffusion and internal lubrication for that not easy to chip away.
The commonly used cooling and lubricating liquid for stainless steel are as follows: the vulcanized oil, sulfide oil, acid kerosene oil or plant oil, carbon tetrachloride with mineral oil, emulsion etc..
Considering the sulfur corrosion effects on the machine, vegetable oil (such as oil) easily attached to scab in machine tools and deteriorating, we choose with oil according to the weight ratio of 1:9. The permeability of carbon tetrachloride is good and oil has good lubricating ability. Test shows that this kind of lubricant oil is suitable for the small surface roughness requirements of stainless steel parts of semi finish machining and finish machining process, particularly suitable for martensitic stainless steel parts.
Due to adopting a series of measures the stainless steel 420 machining process has been completely in the normal state. Sharpening frequency has been 1/3 of the trial production period. The production efficiency improved and the product quality fully meets the design requirements.