Heat Treatment of High-Speed Steels

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Historical Development and Basic Compositions

The T1 type high-speed steel, developed at the century's beginning and designated as 18-4-1, established the foundation for modern high-speed steels. This grade maintains limited use today, with enhanced versions incorporating 5-10% Co and increased carbon and vanadium contents for improved wear resistance. The characteristic hardening temperature range of 1260-1280°C remains safe for processing these steels.

Molybdenum has successfully replaced tungsten in many applications, leading to the development of M2 (6-5-4-2) and M7 grades. The M7 variant, containing higher molybdenum but lower tungsten than M2, offers superior toughness and wear resistance for specific applications. The M42 grade, a cobalt-enhanced version of M7, provides increased hot wear resistance. These M-series steels typically require hardening temperatures of 1200-1220°C, never exceeding 1230°C.

(a) As quenched, 35% retained austenite;	(b) Tempered once at 600°C, 1 h. Tempered martensite and newly formed martensite;
(c) Tempered twice at 600°C. 1 h each time. Tempered martensite 700 x

Figure 1. High-speed steel M42. Microstructure after oil hardening from 1200°C:

Heat Treatment Parameters and Considerations

The hardening process requires careful control due to the high temperatures involved. Chromium carbides dissolve around 1100°C, while approximately 10% of carbides, primarily vanadium and molybdenum-tungsten double carbides, remain undissolved at normal hardening temperatures.

Figure 2. Tempering curves showing inter-depedance of time and temperature for steel M2	Figure 3. Tempering curves showing inter-depedance of time and temperature for steel M42

Tempering Characteristics and Properties

After quenching, these steels contain 20-40% retained austenite. Tempering at approximately 575°C transforms this austenite to martensite, requiring a second tempering treatment for optimal properties.

Figure 4. Influence of tempering temperature on hardness and torsion impact properties of grade T1 high-speed steel, oil hardened from 1290°C (2350°F)

Figure 5. Influence of tempering temperature on hardness and unnotched Izod impact strength of grade T1 high-speed steel, austenitized at 1200°C (2350 °F), and grade M2, austenitized at 1220°C (2225 °F), respectively

Hot Hardness and Applications

High-speed steels maintain significant hardness up to 500°C, crucial for cutting tools operating at elevated temperatures.

Figure 6. Hot-hardness of high-speed steels

These steels excel in machining applications, with highly alloyed grades like M42 performing best in high-speed cutting operations. They also serve in punching, blanking, and hot-work applications, with potential for enhanced wear resistance through carburizing or nitriding.