|
High Speed Steel:
They are used for
cutting of metals where hardness must be
retained at elevated temperature. Sir Robert
Mushat in 1868 discovered that steel containing
2.2% carbon, 2% manganese, 5% tungsten were self
hardening when cooled in air from high
temperature.
In 1898, Frederick W. Taylor & Maunsel White
found that steel retain hardness at elevated
temperatures if high tungsten - high chrome
steel is annealed at 1550 - 1650°
F, and heated rapidly to 2250 - 2350°
F, followed by quenching usually in oil but
sometimes in air, then drawn by heating at 1050
- 1,150° F and then
cooled in air.
Commonly used High Speed Steel is 18-4-1.
Another type of High Speed tool steel has cobalt
added to improve red hardness called Super High
speed Steel. Its toughness is less than that of
18-4-1 type and is rather difficult to forge.
| |
18-4-1 |
Cobalt
Type |
Molybdenum Type |
| Carbon |
0.70 |
0.77 |
0.74 |
| Manganese |
0.25 |
|
0.25 |
| Silicon |
0.20 |
|
0.40 |
| Chromium |
3.75 |
4.50 |
3.80 |
| Tungsten |
18.50 |
18.50 |
1.50 |
| Vanadium |
1.10 |
1.75 |
1.10 |
| Molybdenum |
|
1.00 |
8.50 |
| Cobalt |
|
9.00 |
|
Properties of High speed steel
- Excellent red hardness
- Good wear resistance
- Good shock resistance
- Fair Machinability
- Good non-deforming property
- Poor resistance to decarburization.
Types of High speed steels
There are two main types of high speed steels
1) Tungsten base
2) Molybdenum base
Molybdenum base is more commonly used because
it is cheaper than tungsten base. Besides
tungsten and molybdenum as the primary heat
resisting additive, some other elements are also
present in high speed steel. Carbon for high
hardness, chromium for ease of heat-treating,
vanadium for grain refining and cobalt for
hardness and resistance to heat softening.
Molybdenum High Speed Steel:
It is developed to reduce the amount of
tungsten and
chromium required in steel. It is not as
suitable as 18-4-1 or cobalt type but for many
jobs perform nearly as well.
Cobalt High-Speed Steel:
This is known as super high speed steel.
Cobalt is added from 5 to 8 percent to increase
hot hardness and wear resistance from 18:4:1
type.
Vanadium High-Speed Steel:
This steel contains 0.7 per cent carbon and
more than 1 per cent vanadium.
Effect of Alloying elements on High speed
steel
Carbon produces carbides and a hardenable
matrix. Melting point is decrease with increase
in carbon content. A low carbon content increase
the impact strength but reduces the matrix
hardness. Chromium reduces tendency to scaling.
It is mainly present in the ferritic matrix and
is largely responsible for the air hardening of
High speed steel. Vanadium increases the
abrasion resistance, cutting quality of the
tools and the tendency to air hardening.
Tungsten provides hot hardness by forming
carbides and form-stability. Molybdenum increase
hardenability, while cobalt improves hot
hardness and makes the cutting tool more wear
resistant.
Application of High speed steel:
1) They are used for making all types of
cutting tools such as drills, taps, reamers,
milling cutters, broaches, power-saw blades,
lathe, shaper and planer tool bits etc.
2) They are used for making forming dies,
inserted heading dies, knives, chisels, high
temperature bearings and pump parts
Ultra high speed steels have longer tool
life, can do severe cutting, can cut even gritty
material containing hard particles and posses
higher cutting efficiency. They maintain a very
fine cutting edge. They contain large amount of
vanadium (upto 4%) and cobalt ( 5-12%)
|
