**1. What is quenched steel? What cutting characteristics does it have?**
Quenched steel refers to a type of steel that has been heat-treated and rapidly cooled (quenched) to achieve a hardness greater than HRC 50. It is a significant portion of hard-to-machine materials. Traditionally, grinding was the main method for machining such materials. However, due to the complexity of workpiece shapes and the limitations of grinding, cutting methods like turning, milling, boring, drilling, and reaming are often used instead.
Quenched steel exhibits several distinct cutting characteristics:
(1) High hardness, high strength, and almost no plasticity: When the hardness reaches HRC 50–60, its tensile strength can reach σb = 2100–2600 MPa. According to material machinability standards, quenched steel falls into the 9a grade, making it one of the most difficult materials to cut.
(2) High cutting forces and high cutting temperatures: The unit cutting force can be as high as 4500 MPa. To improve cutting conditions, tools typically use smaller angles, which may cause vibration and require a rigid process system.
(3) Difficult to form built-up edge: Due to its high hardness and brittleness, quenched steel rarely forms a built-up edge, resulting in a low surface roughness.
(4) Blade wear and chipping: The short contact between chip and tool causes concentrated cutting force and heat near the cutting edge, leading to rapid blade wear.
(5) Low thermal conductivity: With a thermal conductivity of about 7.12 W/(m·K), which is roughly 1/7 of that of No. 45 steel, quenched steel retains heat, increasing cutting temperature and accelerating tool wear.
**2. How to choose cutting hardened steel tool material?**
Selecting the right tool material is crucial when machining quenched steel. The material must possess high hardness, wear resistance, heat resistance, and sufficient strength and thermal conductivity.
(1) Cemented Carbide: Adding ultra-fine particles like TaC or NbC improves the performance of WC-Co cemented carbides. These elements enhance high-temperature strength and hardness while reducing friction and thermal cracking. Common grades include YM051, YM052, YN05, and others.
(2) Hot-pressed ceramics and silicon nitride: These materials offer high hardness and heat resistance, suitable for turning, milling, and boring. Brands like AG2, LT35, and HS73 are commonly used.
(3) Cubic Boron Nitride (CBN): CBN has a hardness of HV 8000–9000 and can withstand temperatures up to 1400°C–1500°C. It is ideal for semi-finishing and finishing operations. Overall, CBN is the best choice, followed by ceramics and advanced carbide grades.
**3. How to choose the geometric parameters of cutting hardened steel cutters?**
Tool geometry significantly affects the performance of cutting tools. Key considerations include:
(1) Rake angle: Typically set between -10° and 0° to avoid chipping. For harder materials, a larger negative rake angle is preferred.
(2) Clearance angle: Should be larger than standard tools, around 8°–10°, to reduce friction.
(3) Main and auxiliary cutting angles: Usually set between 30°–60° and 6°–15°, respectively, to enhance blade strength and heat dissipation.
(4) Cutting edge inclination: A negative value of -5° to 0° is common, with more negative values used during interrupted cutting.
(5) Nose radius: Typically 0.5–2 mm, depending on the rigidity of the system.
Proper sharpening and geometry selection are essential for maximizing tool life and cutting efficiency.
**4. How to choose the amount of cutting when cutting hardened steel?**
Cutting parameters should be selected based on tool material, workpiece properties, and system rigidity. The general order is: cutting speed, depth of cut, and feed rate.
(1) Cutting speed: Varies by tool material—carbide (30–75 m/min), ceramic (60–120 m/min), and CBN (100–200 m/min).
(2) Depth of cut: Typically 0.1–3 mm, depending on the machining allowance.
(3) Feed rate: Usually 0.05–0.4 mm/r, with lower values for harder materials or intermittent cutting.
**5. How to cut hardened steel with a ceramic tool?**
Ceramic tools outperform carbide in terms of hardness and heat resistance. They can cut hardened steel at speeds 50% higher than carbide. Proper geometry, including small angles and large nose radii, helps prevent chipping. Ceramic tools are not suitable for low-speed cutting and require careful control of feed and cooling.
**6. How to use cubic boron nitride tool to cut hardened steel?**
CBN tools are ideal for cutting hardened steels due to their extreme hardness and heat resistance. They are used for semi-finishing and finishing, offering faster cutting speeds and better surface finish compared to grinding. Geometric parameters like a negative rake angle and specific clearance angles are recommended.
**7. When cutting a hardened steel with a CBN tool, under what conditions is it most effective to replace grinding?**
CBN tools are most effective when machining complex surfaces, internal holes, or multiple surfaces in a single operation. They also help reduce deformation in quenched parts and improve surface quality.
**8. How to Turn the Thread of a Hardened Steel Roller?**
Thread turning is an alternative to grinding for hardened steel rollers. Using appropriate tools and geometries, such as YM052 hard alloy, ensures precision and avoids damage to the workpiece.
**9. How to drill out the tap broken in the threaded hole?**
A carbide drill can be used to remove broken taps. The drill should be slightly larger than the tap core diameter and operated at a controlled speed to avoid damaging the hole.
**10. How to use high-speed steel drills to drill hard materials?**
High-speed steel drills can be used for hard materials if properly modified. Reducing the rake angle and increasing the tip radius improves blade strength and heat dissipation. Drilling at slower speeds with cutting oil enhances performance.
**11. What are the advantages of grinding high-speed steel with cubic boron nitride grinding wheels?**
CBN grinding wheels offer longer life, lower grinding forces, and reduced burns compared to corundum or diamond wheels. They are especially effective for high-vanadium high-speed steels.
**12. What are the examples of cutting hardened steel?**
Examples include turning bearing molds, milling high-speed steel, and drilling hardened holes. CBN and ceramic tools are widely used, achieving high-quality finishes and extended tool life.
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