High Speed Steel (HSS) is a kind of steel containing some other elements: carbon, tungsten, Molybdenum TZM Alloy, chromium, vanadium, and/or cobalt. After heat treatments, it will have high hardness even at high temperatures. When the cutting temperature exceeds 600°C, the hardness of HSS will not decrease apparently. The cutting speed of cutting tools made of HSS can exceed 60 meters per minute, thus High Speed Steel gets its name. HSS can be classified as ordinary HSS and high performance HSS by its chemical composition, and can also be classified as melting HSS and powder metallurgy HSS by its manufacturing process.
1. Ordinary HSS
Ordinary HSS can meet general needs. Two commonly sighted types of it are tungsten series HSS and tungsten-molybdenum series HSS.
a. Tungsten series HSS
Its typical grade is W18Cr4V. After heat treatment, its hardness can reach 63 – 66 HRC, and bending strength reaches 3500 MPa. Its grindability is good.
b. Tungsten-molybdenum series HSS
Its typical grade is W6Mo5Cr4V2, which is now replacing the tungsten series HSS. Its carbide is small and evenly distributed. It has high abradability, and is at low cost. Its hardness after heat treatment is same as W18Cr4V, but its bending strength can reach 4700 MPa. Its toughness and thermoplasticity are higher than W18Cr4V by 50%. It is usually used to manufacture a variety of tools, such as drill bits, taps, and reamers. It can meet the general processing of engineering materials. Its decarbonization sensitivity is a little bit high.
Another Grade is W9Mo3Cr4V, which is a new variety developed in China. Its toughness and thermoplasticity are a little bit higher than W6Mo5Cr4V2. Its hardness is 63 – 64 HRC. It is easy for rolling and forging. Its decarbonization sensitivity is low. Its cost is also lower.
2. High performance HSS
High performance HSS has a better hardness. This is a new developed variety by changing the chemical composition of HSS and improving its performance. Its hardness can stay at more than 60 HRC when the cutting temperature reaches 650°C. Its durability is 1.5 – 3 times of that of Ordinary HSS. It is applicable to manufacture the cutting tools for processing high temperature alloys, stainless steel, titanium alloys, high-strength steel, and other hard materials. There are 4 main varieties, namely high-carbon series HSS (its typical grade is 9w18Cr4V), high-vanadium series HSS (its typical grades are W12Cr4V4Mo and W6Mo5Cr4V3), cobalt series HSS (its typical grade is W2Mo9Cr4VCo8), and aluminum HSS (its typical grades are W6Mo5Cr4V2Al and W6Mo5Cr4V5SiNbAl).
3. Powder metallurgy HSS
Powder metallurgy HSS effectively solved melting HSS’s issue of the big carbide segregation in ingot casting. Grade APM T15 powder metallurgy HSS’s toughness is 2.5 times of that of melting HSS which has the same chemical composition. Its hardness in high temperature is also improved by 0.5 – 1 HRC. Its grindability is good.
Because of its high degree of isotropic mechanical property, powder metallurgy HSS’s hardening deformation is small, and it has larger surface which has uniform distribution of carbide particles. These carbide particles are not easily to fall off from the cutting edge of tools. By using powder metallurgy HSS, the abradability of small size tools can be improved by 1.5 – 2 times, and 20 – 30% for large size tools.
The production process of powder metallurgy HSS is more complicated than that of melting HSS, and its costs are higher.
4. The market position and trends of HSS
HSS has played an important role in the development of modern industry. It can be said that no HSS, there would be no modern metal processing industry. Though the applications of other materials like hard alloy have risen, HSS still has a dominant position in manufacturing the cutting tools which have complex shapes or are difficult to be ground, such as broaches, shaving cutters, pinion cutters, etc. In particular, when high toughness is required, HSS is the very necessary choice.
The main application fields of HSS are machinery and tools manufacturing industries. HSS should adapt to the development of these industries. In the machinery processing, many CNC machine tools, such as processing centers, turning centers, etc., have been used. The metal cutting speed becomes faster and faster. This requires that cutting tools must have high speed performance, that is, good hardness at high temperatures, good abradability and good toughness. Many manufacturers have developed many HSS materials which can meet these requirements.
High Speed Steel, the steel variety which has a century of history, should aim at the specific markets, and improve its own technologies to counter the other materials’ challenges.