Hard steel has the characteristics of high hardness, high tensile strength, low thermal conductivity and high brittleness. According to the machinability of the workpiece material, it is classified as 9A, which is a difficult-to-cut material and has become a major cutting problem in production. However, as long as the material, geometric parameters, cutting parameters and operating technology of the cutting tool are reasonably selected, the cutting of hardened steel can be successfully completed to achieve the required machining accuracy and surface quality.

Part 1 Characteristics of Hardened Steel

Hardened steel refers to the steel that is transformed into martensite after quenching, and its hardness is greater than 50hrc, which accounts for a large proportion of difficult materials. The traditional processing method of hardened steel is grinding, but in order to improve the processing efficiency, it is impossible to grind the complicated shape of the workpiece, as well as the large shape and position error caused by the deformation after quenching, turning and milling. , Planing, drilling, hinges often use cutting methods such as cutting.

Part 2 Characteristics of Hardened Steel

(1) The main characteristics of hardened steel are high hardness and strength, with almost no plasticity. When the hardness of the quenched steel reaches 50-60hrc, the tensile strength m is 2100-2600mpa, which is a difficult-to-cut material.

(2) High cutting force and high cutting temperature. In order to cut from high-hardness, high-strength workpieces, the unit cutting force C can reach 4500mpa, and the cutting temperature is more than 50% higher than that of ordinary steel.

(3) Chip tumors are not easy to produce. Due to the higher hardness and brittleness of hardened steel, the cutting temperature is higher than the conditions for generating chips, so there will be no chips and lower surface roughness can be obtained.

(4) Tools are easy to crack and wear. Due to the characteristics of hardened steel, cutting force and cutting heat are concentrated near the cutting edge, which can easily lead to tool breakage and wear.

The third part of the choice of hardened steel processing tools

Reasonable selection of tool materials is an important condition for cutting hardened steel. According to the performance and cutting characteristics of hardened steel, select tool materials with high hardness, good wear resistance, high heat resistance, certain bending strength and good thermal conductivity.

(1) Carbide tools. Fine-grained cemented carbide or ultra-fine-grained cemented carbide should preferably be added to TAC or NBC. Adding TAC or NBC to the cemented carbide can improve the hardness and bending strength of the cemented carbide at room temperature and high temperature. After refining the cemented carbide, the bending strength BB increases by 600-800mpa, thereby reducing the cutting wear of the tool. ys8, yg600, yg610, yg726, yg758 and YG813 are usually used for cutting hardened steel. If there is no such brand, as long as the cemented carbide is a cemented carbide, it can be cut and processed, but the tool is easy to wear and has low durability.

(2) Ceramic blades. Add tic and other metal elements to alumina, and use a hot pressing process to improve the density and performance of ceramics. The hardness of the ceramic is 95.5hra, the bending strength is 800-1200mpa, and the heat resistance is 1200°C. It can be used for turning, milling, planing and boring hardened steel of ceramic tools. Cutting speed and tool durability are higher than cemented carbide.

(3) Cubic boron nitride blade (PCBN). Its hardness is 8000-9000hv, heat resistance is 1400-1500℃, and the bending strength after being compounded with cemented carbide is 1500MPa, which is most suitable for semi-finishing and finishing of hardened steel.

The commonly used tool grades of CBN tools for working hardened steel are as follows:

The third part of the recommended cutting parameters of hardened steel

The cutting parameters for cutting hardened steel are mainly selected according to the physical and mechanical properties of the tool material and the workpiece material, the shape of the workpiece, the rigidity of the processing system and the machining allowance. First determine the reasonable cutting speed, then determine the depth of cut, and finally determine the feed rate.

(1) Cutting linear speed. The heat resistance of quenched steel is generally 200-400°C. When the temperature is higher than this temperature, the hardness of the material will decrease. The heat resistance of cemented carbide is 800-1000°C, the heat resistance of ceramic tool materials is 1200°C, and the heat resistance of cubic boron nitride is 1400-1500°C. Therefore, it is necessary to make full use of this function when cutting hardened steel, and the cutting speed Vc should not be too low. Carbide tool Vc is 30-75m/min; ceramic tool Vc is 60-120m/min; PCBN tool Vc is 100-200m/min; when intermittent cutting, Vc is about 1/2 of the above.

(2) Cutting depth. It is usually selected according to the machining allowance and the rigidity of the machining system. Usually ap is 0.1-3mm.

(3) Feed amount. Usually Fr is 0.05-0.3mm/r. When the hardness of the workpiece material is high or the cutting is interrupted, the feed speed should be reduced to prevent the tool from collapsing.

Part 4 Issues that should be paid attention to when cutting hardened steel

①When the above tools are used for continuous quenching, the chips are dark red, indicating that the Vc value is reasonable. When the Vc value is too high, the durability of the tool is low. On the contrary, the cutting surface of the workpiece is not softened, and the durability of the tool is also low.

② During intermittent cutting, the Vc value should be low.

③When drilling small hardened steel holes with a cemented carbide drill, the Vc value should be reasonable (Vc = 15-30m/min), and the drill should be pulled out frequently to remove the chips to prevent the drill from being clamped. Thermal expansion and cold contraction of the workpiece. When the hardness of hardened steel is less than 50hrc, high-speed steel drills can also be used for drilling. At this time, Vc≤5m/min, which can appropriately reduce the rake angle of the outer edge of the drill.

④When the hardened steel wire is rotated, the angle of 30°-45° higher than the tooth should be cut in and out first, so as to cut in and out smoothly to prevent cutting, and the cutting depth should be small.

Conclusion of Part V

(1) Hardened steel can be cut with cemented carbide, ceramic and PCBN tool materials.

(2) When cutting hardened steel, the geometric parameters and cutting parameters of the tool should be reasonably selected according to the specific conditions, and the precautions for use should be mastered.

(3) Cutting hardened steel with PCBN tools can realize turning instead of grinding, which improves processing efficiency.