Cutting conditions for hardened steel
Hardened steel refers to steel that reaches a hardness of 50HRC or above after quenching. It has excellent properties such as high strength, high hardness, and high wear resistance, and is widely used in machinery manufacturing, mold processing, aerospace, and other fields. However, hardened steel has extremely poor machinability and is a typical difficult-to-machine material. Its cutting process is characterized by high cutting forces, high cutting temperatures, rapid tool wear, and difficulty in ensuring the quality of the machined surface. Therefore, the reasonable determination of the cutting conditions for hardened steel is of great significance for improving processing efficiency, reducing production costs, and ensuring processing quality. The cutting conditions for hardened steel mainly include the selection of tool materials, the design of tool geometry parameters, the optimization of cutting parameters, and the determination of cooling and lubrication methods.
The selection of tool material is paramount in hardened steel cutting, directly impacting cutting efficiency and quality. Due to the high hardness and excellent wear resistance of hardened steel, ordinary high-speed steel and carbide tools are no longer sufficient, necessitating the use of ultra-hard tool materials. Currently, the main tool materials suitable for hardened steel cutting include cubic boron nitride (CBN), ceramic, and diamond tools. CBN tools offer exceptional hardness (8,000-9,000 HV) and wear resistance, and can withstand cutting temperatures up to 1,300°C. They are the preferred tool material for hardened steel cutting, particularly for high-speed cutting and finishing, achieving high dimensional accuracy and surface quality. Ceramic tools also offer high hardness and wear resistance at a relatively low price, making them suitable for semi-finishing and finishing of hardened steel. However, their brittleness and poor impact resistance make them unsuitable for intermittent cutting and applications subject to impact loads. Diamond tools are even harder, but because they react chemically with iron-group elements at high temperatures, they are only suitable for cutting hardened non-ferrous metals and not hardened steel.
Proper design of tool geometry has a significant impact on the cutting performance of hardened steel. Due to the high hardness and brittleness of hardened steel, the tool should adopt a small rake angle (generally -10°-0°), or even a negative rake angle, to enhance tool strength and impact resistance and prevent edge chipping. The clearance angle should be appropriately increased (generally 8°-15°) to reduce friction between the tool flank and the machined workpiece surface, thereby lowering cutting temperatures and tool wear. The choice of the lead angle should be determined based on the rigidity of the workpiece and the machining requirements. For workpieces with less rigidity, a larger lead angle (75°-90°) should be used to reduce radial cutting forces and avoid workpiece vibration and deformation. For workpieces with greater rigidity, the lead angle can be appropriately reduced (45°-60°) to improve tool heat dissipation. The secondary rake angle is generally 5°-10° to reduce the residual area of the machined surface and improve surface quality. The tool tip radius should be small (0.2-0.8mm) to reduce extrusion and friction during cutting, reduce cutting force and cutting temperature, and also help improve the heat dissipation conditions of the tool.
Optimizing cutting parameters is key to improving the efficiency and quality of hardened steel cutting. The cutting speed should be determined based on the tool material and the hardness of the hardened steel. When using CBN tools on hardened steel, the cutting speed is generally 80-300 m/min. This value can be adjusted based on the workpiece hardness and machining requirements. The higher the hardness, the lower the cutting speed. Ceramic tools typically have a cutting speed of 50-200 m/min, lower than CBN tools. The feed rate should be kept low (typically 0.05-0.2 mm/r) to reduce cutting forces and surface roughness. However, a feed rate that is too low will increase cutting time and reduce machining efficiency. Therefore, it is important to select an appropriate feed rate while ensuring machining quality. The depth of cut should be determined based on the machining stage and workpiece allowance. For roughing, a depth of 0.1-1 mm can be used to quickly remove excess material. For finishing, a smaller depth of cut (0.05-0.3 mm) should be used to ensure machining accuracy and surface quality. In the actual cutting process, the cutting parameters should be adjusted in time according to the tool wear and the processing surface quality to obtain the best cutting effect.
The proper selection of cooling and lubrication methods can effectively lower cutting temperatures, reduce tool wear, and improve machined surface quality. The large amount of cutting heat generated during the cutting of hardened steel can rapidly wear the tool and cause thermal deformation and burns on the workpiece surface, necessitating the use of effective cooling and lubrication measures. Due to the high temperatures and pressures in the cutting zone of hardened steel, ordinary emulsions are insufficient. Extreme-pressure cutting oils or specialized hardened steel cutting fluids should be selected. These cutting fluids contain extreme-pressure additives such as sulfur, phosphorus, and chlorine. They form a strong chemical lubricating film under high-temperature and high-pressure conditions, effectively reducing friction between the tool and the workpiece, and between the tool and the chips, and lowering cutting temperatures. High-pressure jet cooling should be employed, spraying the cutting fluid directly into the cutting zone at a high pressure (typically 5-20 MPa), ensuring smooth entry and maximum cooling and lubrication. For high-speed cutting, oil mist lubrication or cryogenic cooling techniques can also be used to further enhance the cooling effect.
During the cutting of hardened steel, attention must also be paid to aspects such as workpiece clamping, machining process planning, and tool wear monitoring. Workpiece clamping should be secure and reliable to avoid vibration that can affect machining accuracy and surface quality. For thin-walled parts and workpieces with poor rigidity, specialized fixtures or auxiliary supports should be used to increase workpiece rigidity. The machining process should adhere to the principle of “roughing first, then finishing, and gradually removing excess stock” to avoid excessive stock removal at once, which can lead to increased tool wear or workpiece deformation. During the cutting process, tool wear should be closely monitored. The degree of tool wear can be assessed by observing the color, shape, and surface quality of the chips. Severe tool wear should be promptly replaced to prevent degradation of machining quality and tool breakage. Furthermore, the cutting environment should be kept clean to prevent chip accumulation and impurities from entering the cutting area to ensure a stable cutting process. Properly controlling the cutting conditions of hardened steel can effectively improve its machinability and achieve efficient, high-quality machining.
