**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 cooled rapidly, resulting in a hardness greater than HRC50. It is considered one of the most challenging materials to machine due to its high hardness and strength. Traditionally, grinding was the primary method used for machining such materials. However, with advancements in cutting technology, methods like turning, milling, boring, drilling, and reaming are now commonly used, especially when the workpiece has complex shapes or requires precision.
The main cutting characteristics of quenched steel include:
(1) High hardness and strength, with almost no plasticity. When the hardness reaches HRC50–60, the tensile strength can be as high as 2100–2600 MPa. This makes it one of the hardest materials to cut.
(2) High cutting forces and temperatures. The unit cutting force can reach up to 4500 MPa, requiring tools with smaller angles to increase heat dissipation and reduce vibration.
(3) Difficult to form built-up edge due to the brittleness of the material, leading to better surface finish.
(4) Rapid tool wear and chipping due to concentrated cutting forces and heat near the cutting edge.
(5) Low thermal conductivity, which leads to high cutting temperatures and accelerated tool wear.
**2. How to choose cutting hardened steel tool material?**
Selecting the right tool material is crucial for efficient machining of hardened steel. The tool must possess high hardness, wear resistance, heat resistance, and sufficient strength and thermal conductivity.
Common options include:
(1) Cemented carbide: Adding TaC or NbC improves high-temperature strength and hardness. Grades like YM051, YM052, YN05, and others are widely used.
(2) Ceramic tools: Such as Al₂O₃-based and Si₃N₄-based ceramics, offering high hardness and heat resistance. Brands like AG2, AG3, LT35, and HS73 are popular.
(3) Cubic boron nitride (CBN): With a hardness of HV8000–9000, CBN is the best choice for semi-finishing and finishing hardened steel due to its excellent thermal stability and durability.
**3. How to choose the geometric parameters of cutting hardened steel cutters?**
Proper geometry ensures effective cutting performance. Key considerations include:
(1) Rake angle: Usually negative (-10° to 0°), with larger negative angles for high hardness or intermittent cutting.
(2) Clearance angle: Slightly larger than standard, typically 8°–10°, to reduce friction.
(3) Cutting edge angles: Main cutting angle between 30°–60°, and secondary angle 6°–15°, to improve blade strength and heat dissipation.
(4) Nose radius: Typically 0.5–2 mm, depending on rigidity and surface finish requirements.
(5) Inclination angle: Negative values (-5° to 0°) enhance blade strength, while higher negatives (-10° to -20°) are used for interrupted cutting.
**4. How to choose the cutting amount when cutting hardened steel?**
Cutting parameters should be selected based on tool material, workpiece properties, and process rigidity.
(1) Cutting speed: Varies by tool material—carbide at 30–75 m/min, ceramic at 60–120 m/min, and CBN at 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/rev, with lower feeds for high-hardness or intermittent cutting.
**5. How to cut hardened steel with a ceramic tool?**
Ceramic tools offer superior hardness and heat resistance compared to carbide. They allow higher cutting speeds (up to 50% faster) and better tool life. For example, a ceramic tool can machine a 700 mm long keyway without significant wear. Proper geometry, including small primary angles and large tip radii, helps avoid tool damage.
**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 (HV8000–9000) and high heat resistance (1400–1500°C). They are used for semi-finishing and finishing, providing high metal removal rates and excellent surface quality. Common grades include LDP-J and DLS-F1, suitable for various applications.
**7. When cutting hardened steel with a CBN tool, under what conditions is it most effective to replace grinding?**
CBN tools are particularly effective for complex surfaces, small holes, and parts prone to deformation after quenching. They reduce processing steps, improve efficiency, and extend part life by avoiding thermal damage.
**8. How to turn the thread of a hardened steel roller?**
Hardened steel threads can be turned using hard alloy tools with proper geometry. A typical setup includes a lathe, indexable inserts, and careful control of cutting parameters to ensure accuracy and prevent tool breakage.
**9. How to drill out the tap broken in the threaded hole?**
A carbide drill can be used to remove a broken tap. The drill should be slightly larger than the tap core, and care must be taken to avoid damaging the surrounding material.
**10. How to use high-speed steel drills to drill hard materials?**
High-speed steel drills with optimized geometry can successfully drill hard materials (HRC 38–42) if the cutting speed and feed are carefully controlled. Shorter drills and auxiliary support improve rigidity.
**11. What are the advantages of grinding high-speed steel with cubic boron nitride grinding wheels?**
CBN grinding wheels offer longer life, higher grinding ratios, and reduced burn risk. They are more efficient and less prone to clogging compared to conventional abrasives.
**12. What are the examples of cutting hardened steel?**
Examples include turning a hardened bearing roller, milling a high-speed steel component, and drilling a hardened hole. These cases demonstrate how modern cutting techniques can replace traditional grinding, improving efficiency and reducing costs.
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