Practical process parameters and tools for turning hardened steel
Hardened steel typically refers to steel with a hardness exceeding HRC50, such as bearing steel (GCr15) and die steel (Cr12). This material offers high strength and wear resistance, but also presents significant challenges in turning. The cutting resistance of hardened steel is 2-3 times that of ordinary steel, and cutting temperatures can reach 800-1000°C, resulting in severe tool wear. Therefore, systematic optimization is required across three aspects: tool material, geometry, and process parameters. The proper matching of practical process parameters with the tool is key to achieving efficient, high-quality turning of hardened steel, ensuring both machining accuracy (IT6-IT7) and controlling production costs.
The selection of tool material is paramount in hardened steel turning. It must meet the requirements of high hardness, high wear resistance, and high temperature resistance. Cubic boron nitride ( CBN ) tools are the preferred choice for hardened steel turning. With a hardness of up to HV3000-4000 and the ability to withstand temperatures exceeding 1200 °C, they are suitable for machining hardened steels with a hardness of HRC50-65 . Solid CBN tools are suitable for continuous cutting and finish turning, while welded CBN tools ( with a CBN content of 70%-90% ) are suitable for interrupted cutting and rough turning, and can withstand significant impact loads. For hardened steels with a hardness of HRC50-55 , ceramic tools (such as Al₂O₃-TiC composite ceramics) can also be used. These tools are less expensive than CBN tools, but offer slightly lower wear resistance and are suitable for small to medium-volume production. Coated carbide tools (such as WC-Co alloys with TiAlN coatings) are only suitable for hardened steels below HRC50, and cutting speeds must be strictly controlled to prevent coating loss.
Tool geometry must be designed to balance cutting edge strength and cutting performance. When turning hardened steel, the tool rake angle is generally between -5° and 0°. A smaller rake angle, or even a negative rake angle, strengthens the cutting edge and prevents chipping. The clearance angle is 6° to 8° to reduce friction between the flank and the workpiece while ensuring tool rigidity. The lead angle should be between 45° and 75°. A 45° lead angle is suitable for rough turning and distributes cutting forces, while a 75° lead angle is suitable for fine turning and reduces the effects of radial forces on the workpiece. The rake angle should be between -3° and 5° to transfer cutting forces to the toolholder and enhance the tool’s impact resistance. The tool tip radius should be between 0.4 and 0.8 mm. A larger radius can increase tool tip strength, but avoid excessive vibration. Furthermore, the tool’s rake and flank faces should be honed (surface roughness Ra 0.02-0.05 μm) to reduce friction and adhesive wear.
Cutting parameters must balance machining efficiency and tool life. Cutting speed is a key parameter affecting hardened steel turning. When using CBN tools to machine HRC55-60 steel, the typical cutting speed is 80-120 m/min. A speed too high can cause CBN particles to break off, while a speed too low can easily cause adhesive wear. The feed rate is typically 0.08-0.15 mm/r. For finish turning, a lower value (0.08-0.1 mm/r) is used to ensure surface quality, while a higher value (0.12-0.15 mm/r) is used for rough turning to improve efficiency. The depth of cut is determined by the machining allowance: 0.3-0.5 mm for rough turning and 0.1-0.2 mm for finish turning. The depth of cut should be kept to a minimum to prevent tool overload. For example, when turning a GCr15 bearing sleeve with HRC60, a CBN tool is used with a cutting speed of 100m/min, a feed rate of 0.1mm/r, and a cutting depth of 0.3mm. A surface roughness of Ra0.8μm can be obtained, and the tool life is 30-40 minutes.
The coordination of cooling, lubrication, and machining processes can further enhance turning results. Extreme-pressure emulsions or specialized cutting oils (containing sulfur and phosphorus additives) are required for hardened steel turning to enhance lubrication and cooling. The cutting fluid pressure should reach 5-10 MPa, with a flow rate of at least 20 L/min. It should be sprayed directly onto the cutting zone to reduce tool-chip adhesion. For hardened steel parts requiring high precision (such as mold cavities), a combined process of “rough grinding, semi-finishing turning, and fine grinding” can be employed. Semi-finishing turning uses CBN tools to remove the majority of the excess stock, while fine grinding ensures final accuracy, improving efficiency while reducing grinding costs. Tool wear should be monitored in real time during machining. When flank wear reaches 0.3-0.5 mm, the tool should be replaced promptly to avoid excessive tool wear and deterioration of the workpiece surface quality. These measures ensure stable turning of hardened steel, meeting the machining requirements of high-strength parts in mechanical manufacturing.
