High-Speed Tool Steel

High-Speed Tool Steel (HSS), also known as wind steel, white steel, or sharp steel, has a unique ability to harden in air, maintaining sharpness and high hardness even after quenching. This steel is an intricate alloy comprising tungsten, molybdenum, chromium, vanadium, and cobalt, with a total alloy content ranging from 10% to 25%.

One of HSS’s most remarkable features is its ability to retain high hardness, with a Rockwell Hardness (HRC) of over 60, even under high-temperature conditions up to 500°C. This attribute, known as red hardness, is what sets HSS apart from carbon tool steels, which lose hardness dramatically when temperatures exceed 200°C, rendering them ineffective at around 500°C. Unlike carbon tool steels, HSS maintains its cutting ability at high temperatures, making it ideal for cutting tools.

HSS primarily undergoes metallographic and hardness testing rather than tensile strength testing. Properly heat-treated tungsten and molybdenum-based HSS achieve a Rockwell hardness of 63 or more, while cobalt-based HSS can exceed 65. The steel must exhibit no visible defects such as shrinkage cavities or flakes, with center porosity typically below grade 1.

Metallographic examinations focus on three key aspects:

1. Decarburization: HSS should show minimal decarburization, and the microstructure must be free from fishbone-like eutectic ledeburite.
2. Carbide Non-uniformity: This is critical for quality, and the acceptable level varies depending on the steel’s intended use, typically requiring a uniformity below grade 3.
3. Red Hardness: This refers to the tool’s ability to resist softening at elevated temperatures. A practical test involves heating the steel to 580–650°C, holding for one hour, and repeating this cycle four times before measuring hardness.

For optimal performance, HSS is quenched at temperatures close to its melting point, such as 1210–1240°C for tungsten-based HSS and 1180–1210°C for high molybdenum HSS. Post-quenching, it undergoes tempering three times at 540–560°C. Enhancing quenching temperature can further improve red hardness.

Surface treatments like low-temperature cyaniding, nitriding, and sulfur-nitrogen co-carburizing can significantly extend the lifespan of HSS cutting tools. These processes enhance the surface hardness and wear resistance, ensuring the tools maintain their cutting efficiency over extended use.