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Cutting Processing of Quenched Steel

Cutting Processing of Quenched Steel

In the past, grinding was used to machining quenched steel, which had low production efficiency, high processing costs, and often caused surface burns or microcracks due to the large grinding force. With the development of the hard alloy industry, traditional and backward grinding processes have been replaced by advanced turning, resulting in efficiency improvements of tens or even tens of times. In order to better solve the cutting problems of quenched steel, we will discuss the following issues together. 

1. characteristics of cutting quenched steel

High hardness, poor thermal conductivity: The microstructure of quenched steel is tempered martensite. The hardness can reach HRC60 or above, and the strength can reach 260kg/mm2. And its thermal conductivity is poor, with some quenched steels having a thermal conductivity as low as 0.017 kcal/cm, s, degrees (unquenched 45 # steel λ=0.162 kcal/cm, s, degrees). Therefore, when cutting quenched steels, the unit cutting force is large, up to 450kg/mm2, the cutting temperature is high, and the cutting heat is easily concentrated at the edge. According to the classification table for machinability of the processed material, the hardness and strength of quenched steel are both 9a, which belongs to the category of the most difficult cutting materials.

The cutting force is large, and the cutting force FY is close to or even greater than the main cutting force. This is to increase the strength of the blade tip, increase the heat dissipation area, and choose a smaller main angle. When the rigidity of the machine tool fixture workpiece system is poor, it is prone to vibration and tool cutting due to the large FY.

The short contact length of a blade means that the cutting and the large amount of cutting heat caused by the high cutting force are concentrated near the cutting edge. If the strength of the tool material is not high, it is easy to cause the cutting edge to shatter and break. 

2. Selection of cutting tool materials

In low-speed and intermittent cutting of quenched steel, M-type alloys with TaC, NbC, and appropriate amounts of TiC are generally used because these alloys have good comprehensive properties and are also suitable for variable speed end face cutting and intermittent cutting conditions. Because using alloys with excessive TiC content, even if they have good hardness, their toughness and strength are insufficient, which can easily cause tool chipping and wear. So we choose ultrafine particle alloys with high strength, good toughness, heat resistance, and wear resistance. After practical experience, YS8 has been proven to have better results.

If YS8 is used to cut T11 tool steel with HRC=60 using V=14.3m/min, f=0.3mm/rev, ap=2mm, the effect is very good. However, using ordinary hard alloy under the same conditions, the effect is quite poor and even cannot be cut. Using YS2 to cut W18Cr4V white steel square bars, with a hardness HRC of 63-65, cutting parameters of V=10m/min, f=0.25mm/rev, ap=1.2mm, the use effect is quite good. Under the same conditions, we tested with other grades of alloys, and the results were not satisfactory.

When cutting quenched steel continuously at high speeds, it is advisable to use P-type alloys with high titanium carbide content and excellent high-temperature hardness. Because P-type alloys experience bonding wear, while M and K alloys begin to experience bonding wear at 6000C and diffusion wear at 9000C. Especially after adding an appropriate amount of TaC and NbC to the alloy, its high-temperature performance is significantly improved. When the cutting speed is below 50m/min, it is recommended to choose:TY05 YC12

If YT05 cutting speed is further increased, or for processing large workpieces and high-precision products, ceramic blades and cubic boron nitride(CBN) will demonstrate their unique superiority. uses ceramic blade AT6 to cut T8 carbon tool steel with HRC=60 at f=0.15mm/rev, ap=0.15mm, the cutting speed can reach 100m/min, and the tool life is high with good workpiece quality. Especially cubic boron nitride, due to its hardness and good thermal stability, has higher durability and cutting efficiency than ceramic blades when processing quenched steel, and the processed workpieces have high dimensional accuracy and good surface smoothness.

3. Geometric angles selection for cutting tools

In metal cutting, the geometric angle of the cutting tool is very important, and this is particularly prominent in difficult to machine materials. Quenched steel has a large cutting force in cutting, so it is often prone to tool breakage. This largely depends on whether the tool angle is selected reasonably or not. Improper selection can also cause cutting in high-strength tool materials. If the tool angle is reasonable, brittle tool materials can also perform intermittent cutting. Like ceramic alloy, it is an extremely brittle tool material, but experiments have shown that as long as the angle is reasonable, it can intermittently cut hardened steel. In practice, we believe that the principle for selecting geometric angles is:

Due to the high hardness and strength of quenched steel, it exhibits significant cutting forces during the cutting process. Therefore, when selecting geometric angles, we focus on protecting the tool tip and choose 0-degree or negative rake angles with small back angles. However, considering that the vast majority of quenched steel is precision machined with small allowances and thin chips, the principle for selecting the diaphragm angle is to ensure the strength of the cutting edge, and the back angle can be slightly larger. The inclination angle of the blade is generally negative. To balance the pros and cons of various aspects, generally choose:

λ0=0 ao=6～80 λ0=-40

λ01=-60 λ§=0.5mm

In practice, it has been proven that when turning quenched steel with HRC60 or higher, the rake angle can be smaller. For example, when turning W18Cr4V with HRC63-65, using        λ01=-60 and ao=80,λ§=-50 yields good results.

When cutting quenched steel with a ceramic knife, the rake angle λ 0 can also be smaller. When using SG4 to turn 6W6M05Cr4V (HRC66), the tool angle should be selected as follows:

λ0=-80 ao=80 λ§=0.5mm

Br=0.3mm λ01=-200 (V=103m/min ap=0.25mm f=0.1mm/rev) is more suitable 

4. Selection of cutting parameters

Cutting parameters generally include cutting speed, cutting depth, and feed rate. In cutting, it is as important as the geometric angle of the tool and is also an important factor affecting cutting. 

Cutting speed:

In production practice, cutting speed directly affects work efficiency, so high-speed cutting is desirable. However, as the cutting speed increases, the cutting temperature rises linearly. The high cutting temperature will inevitably affect the durability of the tool, so the appropriate cutting speed must be determined based on the thermal stability temperature of the tool material. 

Cutting depth:

The cutting depth has a significant impact on the cutting force. Generally speaking, the cutting depth should be determined based on the hardness of the processed material and the strength of the tool material. However, for the cutting of quenched parts, the cutting allowance is generally small, and it is recommended to achieve one-time cutting to reduce tool wear and improve work efficiency. 

Cutting feeding speed:

The cutting of quenched parts is mostly precision machining. Therefore, the basic principle for selecting cutting amount is to ensure the dimensional accuracy and surface smoothness of the workpiece. Therefore, the cutting amount tends to be chosen smaller. Through practical exploration, deduce the following data.

V=10～50m/min

Ap=0.05～1mm f=0.05～0.25mm/rev

Ceramic blade cutting parameters

V=80～150m/min

Ap=0.1～0.5mm f=0.05～0.3mm/rev