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
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.
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.
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.
The composition of the T1 to T15 tungsten high-speed steels are tabulated
aGroup W contains 0.025 max S, 0.20 max Cu and 0.025
max P. Other steels except group W contain 0.03 max S, 0.25 max Cu and 0.03 max
P. Where specified, sulfur may be increased to 06 to 0.15% in order to improve
machinability of group A, D, H, M, and T steels.
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.
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.