Tungsten High-Speed Steels

Introduction
High-speed steels are tool steels that find applications in machine tools that have high rates of material removal. Tungsten high-speed steels (group T) and molybdenum high-speed steels (group M) are two types of high-speed steels. These two groups of high-speed steels have similar hardening abilities and other characteristics.

High-speed tool steels are capable of being hardened to 62 - 67 HRC and their hardness can be maintained at service temperatures up to 540°C (1004°F). This makes them suitable for use in high-speed machinery.

The tungsten series include the T1 to T15 class alloys. Tungsten is a good carbide former that prevents grain growth, enhances toughness and increases red hardness and high temperature strength. Tungsten is used in hot forming tool steels and high-speed steels.

Overview
White and Taylor developed the type T1 series of tungsten high-speed steels. In the early 1900s, they discovered that certain steels exhibited red hardness and such steels comprised more than 14% W, about 0.3% V and about 4% Cr. T1 in its earliest form contained about 18% W, 0.68% C, 0.3% V and 4% Cr. An increase in the quantity of vanadium was seen by 1920. The carbon content of most steels also increased to approximately 0.75% over the years.

The most significant alloying elements found in tungsten high-speed steels include carbon, tungsten, cobalt, chromium and vanadium. Tungsten high-speed steels contain 4% chromium. T4 and T15 are the cobalt-base tungsten varieties that contain different amounts of cobalt. The T1 type of tungsten high-speed steels is free of cobalt or molybdenum.

Classification
The American Iron and Steel Institute (AISI) has classified high-speed tool steels into about 40 individual categories. This classification system uses a T for referring to steels in which tungsten the primary alloying element. The letter T is followed by a number which distinguishes each of the tungsten tool steels ranging between T1 and T15.

Hardness
Tungsten high-speed steels have good wear resistance and high red hardness. The maximum hardness of group T steels differs according to the carbon content and also the alloy content. A minimum hardness of 64.5 HRC can be imparted to all types of high-speed steels. Types such as T15 can be hardened to 67 HRC as they have high carbide and carbon content (1.55%). Hence T15 is considered as the most wear-resistant steel of the tungsten high-speed steel series. Tungsten high-speed steels comprising more than 1.0% C and 1.5% V produce a high number of wear-resistant hard carbides in the microstructure due to the presence of high carbon and alloy content. Tungsten high-speed steels are deep hardening when they are quenched from their hardening temperature of 1205 to 1300°C (2200 to 2375°F). Solid tools such as cold extrusion punches and broaches with large diameters are made from tungsten high-speed steels. Full hardness is provided for tools with large diameters using an accelerated oil quench.

Applications
The major applications of tungsten high-speed steels are the following:

•Interrupted-cut applications and delicate tools
•Cutting tools including hobs, milling cutters, bits, reamers, drills, broaches and taps
•Production of dies and punches
•Making high-temperature and high load structural components, e.g., pump parts and aircraft bearings.

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