Jul 9 2012
Topics Covered
IntroductionOverview
ClassificationHardnessApplications
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.
The composition of the T1 to T15 tungsten high-speed steels are tabulated
below:
| Designation |
Compositiona, % |
| AISI |
UNS |
C |
Mn |
Si |
Cr |
Ni |
Mo |
W |
V |
Co |
| T1 |
T12001 |
0.65-0.80 |
0.10-0.40 |
0.20-0.40 |
3.75-4.50 |
0.30 max |
- |
17.25-18.75 |
0.90-1.30 |
- |
| T2 |
T12002 |
0.80-0.90 |
0.20-0.40 |
0.20-0.40 |
3.75-4.50 |
0.30 max |
1.0 max |
17.50- 19.00 |
1.80-2.40 |
- |
| T4 |
T12004 |
0.70-0.80 |
0.10-0.40 |
0.20-0.40 |
3.75-4.50 |
0.30 max |
0.40-1.00 |
17.50- 19.00 |
0.80-1.20 |
4.25-5.75 |
| T5 |
T12005 |
0.75-0.85 |
0.20-0.40 |
0.20-0.40 |
3.75-5.00 |
0.30 max |
0.50- 1.25 |
17.50-19.00 |
1.80-2.40 |
7.00-9.50 |
| T6 |
T12006 |
0.75-0.85 |
0.20-0.40 |
0.20-0.40 |
4.00-4.75 |
0.30 max |
0.40-1.00 |
18.50- 21.00 |
1.50-2.10 |
11.00-13.00 |
| T8 |
T12008 |
0.75-0.85 |
0.20-0.40 |
0.20-0.40 |
3.75-4.50 |
0.30 max |
0.40-1.00 |
13.25- 14.75 |
1.80-2.40 |
4.25-5.75 |
| T15 |
T12015 |
1.50-1.60 |
0.15-0.40 |
0.15-0.40 |
3.75-5.00 |
0.30 max |
1.00 max |
11.75- 13.00 |
4.50-5.25 |
4.75-5.25 |
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.
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.