Processing characteristics of high-strength steel
High-strength steel, defined as steel with a yield strength greater than 345 MPa, is widely used in aerospace, marine engineering, high-rise buildings, and other fields. Its excellent mechanical properties (such as high strength, high toughness, and good fatigue strength) are derived from a well-designed alloy composition (such as the addition of elements such as chromium, nickel, molybdenum, and vanadium) and advanced heat treatment processes (such as quenching and tempering, and controlled rolling and cooling). However, these characteristics also make it difficult to machine, resulting in high cutting forces, high cutting temperatures, and rapid tool wear. Therefore, a thorough understanding of its processing characteristics is essential for developing a sound machining process.
The high strength and hardness of high-strength steel directly lead to a significant increase in cutting forces, which are typically 1.5 to 2 times that of ordinary low-carbon steel. The increase in radial cutting forces is particularly significant. This is because the alloying elements within the material form a large number of strengthening phases (such as carbides and intermetallic compounds), which increase the material’s resistance to deformation, and greater resistance to plastic deformation needs to be overcome during cutting. High cutting forces not only increase tool wear and the risk of chipping, but also place greater loads on the machine tool, which can easily cause vibration and affect machining accuracy. To address this issue, when machining high-strength steel, it is necessary to use machine tools with good rigidity and high power. At the same time, the tool material must have high compressive strength and wear resistance, such as ultrafine-grained cemented carbide (WC-Co alloy) or coated tools (such as AlTiN coating) to withstand the high cutting forces.
High cutting temperatures are another significant characteristic of high-strength steel machining. Due to its low thermal conductivity (approximately 50%-70% of 45 steel), the heat generated during cutting is difficult to dissipate quickly through the workpiece and chips, resulting in a sharp increase in cutting zone temperature, typically reaching 800-1200°C. High temperatures reduce the hardness and wear resistance of the tool material, accelerating tool wear and significantly shortening tool life, especially during continuous cutting. High temperatures can also cause thermal damage to the workpiece surface, such as burns and oxidative discoloration, affecting surface quality and fatigue performance. Therefore, when machining high-strength steel, enhanced cooling and lubrication are essential. High-pressure, high-flow cutting fluids (such as extreme pressure emulsions) should be used, spraying the cutting fluid directly into the cutting zone to forcefully cool the tool and workpiece. Cutting speeds should also be appropriately controlled, generally ranging from 80-150 m/min (for carbide tools), to avoid temperature spikes caused by excessive speed.
High-strength steels are subject to severe work hardening. During cutting, the workpiece surface metal undergoes intense plastic deformation due to the extrusion and friction of the tool, significantly increasing the surface hardness. The hardened layer can reach depths of 0.1-0.5 mm, with a hardness increase of 20%-50%. Work hardening can make subsequent cutting more difficult, as the high hardness of the hardened layer exacerbates abrasive wear on the tool and can even lead to tool chipping. Furthermore, significant residual stresses within the hardened layer can cause deformation or cracking of the workpiece during use if not properly controlled. To mitigate work hardening, sharp tools should be used to minimize friction between the tool rake face and the chip, reducing the degree of plastic deformation. The feed rate and depth of cut should also be appropriately selected. A low feed rate increases the friction time between the tool and the workpiece, exacerbating hardening, while a high feed rate increases plastic deformation. A typical feed rate is 0.1-0.3 mm/min, with a depth of cut of 1-3 mm, to remove most of the hardened layer in a single pass.
High-strength steel is difficult to break and remove chips from. Due to its high toughness, continuous ribbon-like chips are easily generated during cutting. These chips can entangle the tool or workpiece, not only affecting the continued processing, but also scratching the processed surface and even causing safety accidents. In addition, the large contact area between the continuous chips and the tool rake face generates more heat and exacerbates tool wear. To improve chip breaking and chip removal performance, the tool rake face should have a suitable chip breaker groove. The shape and size of the chip breaker groove should be adjusted according to the cutting parameters and workpiece material. Generally, the groove width is 3-5mm and the groove depth is 0.5-1mm. This allows the chips to be sufficiently bent and broken during the flow process. For high feed rate processing, negative chamfered tools can be used to enhance the cutting edge strength and promote chip breaking. In addition, chips should be cleaned promptly during the processing to avoid excessive accumulation that affects processing quality and safety.
