HIGH SPEED STEELS -
High speed steels (HSS) get their name from the fact that they may be operated as cutting tools at much higher speeds than is possible with, plain carbon tool steels. High speed steels operate at a cutting speeds 2 to 3 times higher than for carbon steels. When a hard metal is machined at high speeds with heavy cuts, sufficient heat may be developed to cause the temperature of cutting edge to reach a red heat. This temperature would soften carbon tool steel containing even up to 1.5% carbon to the extent of destroying their cutting ability. Certain highly alloyed steel, designated as high speed steels, have been developed which retain their properties at temperatures up to 600 -620°c.
In high speed steels ordinary iron carbide Fe3C is replaced by double carbides, such as iron tungsten carbide Fe4W2C or iron molybdenum carbide, Fe3Mo3C, and the phases based on these substances are far more temper-resistant than those based on iron carbides.
The main features of a high speed steels are its great hardness in the heat - treated condition, and its ability to resist softening at relatively high temperatures.
Carbon produces carbides and a hardenable matrix. Melting point is decreased with increase in carbon content. A low carbon content increases the impact strength but reduces the matrix hardness. Chromium reduces tendency to scaling. It is mainly present in the ferrite matrix and is largely responsible for the air hardness of HSS. 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 increases hardenability, while cobalt improves hot hardness and makes the cutting tool more wear resistant.
a) 18-4-1 High Speed Steel -
This steel containing 18 tungsten, 4% chromium and 1 % Vanadium with about 0.75% Carbon, is considered to be one of the best of all purpose tool steels.'This is extensively used for lathe, planer and shaper tools, drills and milling cutters.
Cobalt high - speed steel -
This is known as super high speed steel. Normally cobalt is added from 5 to 8% to increase better hot hardness and wear resistance than 18:4: 1 type. One analysis of cobalt high speed steel contains 20% tungsten, 4% chromium, 2% Vanadium and 12% cobalt.
Vanadium High Speed Steel -
This steel contain 0.7% carbon , and more than 1% Vanadium. High Vanadium with carbon, high speed steel is superior to 18 : 4: 1 type for difficult to machine materials. This has excellent abrasive resistance.
Molybdenum high Speed Steels -
This -steel containing 6% molybdenum, 6% tungsten, 4% chromium and 2% Vanadium have excellent toughness and cutting ability.
INDEX ADV ENG NEXT