Practical process parameters and tools for turning high-strength steel
High-strength steel, defined as steel with a yield strength greater than 345 MPa, is widely used in aerospace, bridges, pressure vessels, and other fields. However, its turning process presents numerous challenges due to its high strength, hardness, and low plasticity. High-strength steel has a complex chemical composition, containing alloying elements such as chromium, nickel, molybdenum, and vanadium. After heat treatments such as tempering and quenching, its hardness can reach HRC30-50. However, high cutting forces and temperatures during cutting result in severe tool wear, making it difficult to maintain surface quality. Therefore, turning high-strength steel requires the selection of appropriate tool materials and process parameters to improve efficiency and quality while reducing production costs.
The choice of tool material for high-strength steel turning is crucial for ensuring effective machining. Tool materials must possess high hardness, high wear resistance, and good heat resistance. Commonly used tool materials include ultrafine-grained carbide, ceramic tools, and cubic boron nitride (CBN) tools. Ultrafine-grained carbide (such as WC-Co alloy, with a grain size of 0.5-1μm) offers high hardness (HV 1800-2000) and toughness, making it suitable for rough and semi-finish turning of high-strength steel and capable of withstanding high cutting forces. Common grades include WC-Co8 and WC-TiC-Co. Ceramic tools (such as Al2O3-based ceramics and Si3N4-based ceramics) offer high hardness (HV 2000-2500) and excellent heat resistance, maintaining cutting performance at temperatures of 800-1000°C. They are suitable for finish turning of high-strength steel, especially in continuous cutting applications. CBN tools have the highest hardness (HV3000~5000) and excellent wear resistance. They are suitable for high-speed finishing and hard turning of high-strength steel (hardness above HRC45). However, they are brittle and expensive, making them unsuitable for intermittent cutting.
Tool geometry significantly impacts cutting forces, cutting temperatures, and tool life in high-strength steel turning. The goal should be to enhance tool strength and reduce cutting forces. The rake angle is generally between -5° and 5°. A smaller rake angle, or even a negative rake angle, can enhance tool edge strength and prevent chipping. The clearance angle is 5° to 8° to reduce friction between the flank and the workpiece while ensuring adequate heat dissipation. The lead angle should range from 45° to 90°. A 90° lead angle reduces radial cutting forces, making it suitable for machining thin-walled, high-strength steel parts and preventing workpiece deformation. A 45° lead angle provides improved heat dissipation and is suitable for rough turning. The tool nose radius should range from 0.4 to 1.2 mm. A larger nose radius increases tool strength but increases cutting forces, so the tool should be appropriately selected based on the depth of cut and feed rate. Furthermore, the tool edge should be passivated, with a radius of 0.02 to 0.05 mm, to enhance impact resistance and reduce the risk of chipping.
Cutting parameters for high-strength steel turning must be optimized based on the tool material and machining requirements to maximize efficiency while ensuring tool life. Regarding cutting speeds, ultrafine-grain carbide tools for high-strength steel machining should maintain a cutting speed of 80-150 m/min; ceramic tools can achieve 150-300 m/min; and CBN tools can reach 300-600 m/min. The feed rate should be kept low, generally 0.1-0.2 mm/min. A low feed rate reduces cutting forces and surface roughness, helping to ensure machining accuracy. However, a too low feed rate can increase machining time and reduce efficiency. The depth of cut should be determined based on the machining allowance, ranging from 1-3 mm for rough turning and 0.1-0.5 mm for finish turning. Avoid excessive depth of cut, which can lead to increased tool wear or workpiece deformation. During machining, adequate cooling and lubrication are essential. Use extreme-pressure emulsions or extreme-pressure cutting oils. A high-pressure cooling system sprays the cutting fluid directly into the cutting area to lower cutting temperatures, minimize tool wear, and prevent burns on the workpiece surface.
Practical process measures and operating techniques for turning high-strength steel are equally important for ensuring machining quality. Before machining, the workpiece must be inspected to remove surface scale and burrs. Workpieces with cracks or defects should be removed in advance. When clamping the workpiece, use a rigid fixture, such as a three-jaw self-centering chuck with a center. For large or complex workpieces, specialized fixtures are required to ensure secure clamping and reduce vibration. The machining sequence should follow the principle of “roughing first, finishing second, exterior first, interior second.” After rough turning, perform an aging treatment to eliminate machining stress before proceeding to finish turning. During finish turning, all surfaces should be machined in a single clamping to minimize clamping errors. During machining, closely monitor tool wear and regularly measure workpiece dimensions. If flank wear reaches 0.3-0.5mm, replace the tool promptly to avoid compromising machining quality. For interrupted cutting or machining high-strength steel parts with keyways or notches, reduce cutting speeds and feeds to minimize impact loads on the tool. Through reasonable tool selection, cutting parameter setting and process control, efficient and high-quality turning of high-strength steel can be achieved.
