High-speed steel, also known as air-hardening steel or "sharp steel," refers to a type of steel that can harden even when cooled in air during quenching and retains exceptional sharpness.

 It is a complex alloy steel containing carbide-forming elements such as tungsten, molybdenum, chromium, and vanadium. The total alloy content accounts for approximately 10–25%.  

Under the high temperatures generated during high-speed cutting (around 500°C), high-speed steel cutting blades can maintain high hardness, with an HRC value exceeding 60. 

This is the most critical characteristic of high-speed steel—**red hardness**. In contrast, carbon tool steel, after quenching and low-temperature tempering, exhibits high hardness at room temperature. 

However, when the temperature exceeds 200°C, its hardness drops sharply. At 500°C, the hardness decreases to a level comparable to its annealed state, completely losing its ability to cut metal. 

This limitation restricts the use of carbon tool steel for cutting tools.  

Thanks to its excellent red hardness, high-speed steel overcomes this critical drawback of carbon tool steel, making it suitable for manufacturing high-speed steel cutting blades.