High speed steels (HSS), are multi-component alloy carbon steels formulated according to the Iron-Carbon-X system, where "X" can represent one or more other elements, most often Tungsten, Chromium, Molybdenum, Vanadium, or Cobalt. These highly alloyed steels, with alloy additions of more than 5%, are called martensitic steels, because their structure changes after heat treatment by quenching and tempering, giving them very high mechanical characteristics and hardness levels.
The multi-component alloy nature of high-speed steels makes them suitable for processing hard metals: drills, saw blades, taps, milling cutters, grinding wheels.
The most important improvements in the cast microstructure of high-speed steels concern the type, morphology and volume fraction of eutectic carbides, while the improvements obtained thanks to advances in solidification treatment, in particular the cooling speed, are, on the other hand, mainly linked to a significant reduction in the microstructural scale.
Compared to cold work steels, it is possible to achieve cutting speeds three to four times higher and therefore high application temperatures. This is due to the heat treatment in which the steel is annealed at over 1,200°C and then cooled.
Advantages of HSS in general
- Application temperature above 600°C.
- High cutting speed.
- High strength (high breaking strength).
- Good grindability during production.
- Good regrindability of blunt tools.
- Relatively low price.
The presence of cobalt makes it possible to increase thermal resistance and hardness at high temperatures, for example to obtain very fast cutting tools at high temperatures. The HSSE-Co x shape designation is used to indicate the percentage of cobalt content. The higher the cobalt content, the harder the tool steel. The cobalt content increases the resistance to hot hardness, but tends to reduce toughness.
The evolution of alloys to obtain modern high-speed steels led at the beginning of the last century to the creation of the T group, rich in Tungsten, then the M group, giving priority to Molybdenum, with equivalent performance and lower manufacturing costs (95% of high-speed steels manufactured in the United States).
Molybdenum is a silvery-white metal that can be alloyed with other elements; it improves the formation of fine grains, while increasing the brittleness of the quenching process. Molybdenum has high weldability and a strong tendency to secondary hardening during tempering. The melting point of molybdenum steels is lower than that of tungsten steels.
Molybdenum can form a double carbide with iron and carbon, allowing tungsten to be replaced by molybdenum at a ratio of one part by weight of molybdenum to two parts by weight of tungsten.
The main alloying elements in molybdenum high speed steels are molybdenum, tungsten, vanadium, cobalt, chromium and carbon. The approximate percentage of molybdenum in M group tool steels is 3.5 to 10%.
- Types M 1 to M 10 (except M6) contain tungsten but no cobalt.
- The M30 and M40 types are cobalt-, molybdenum-tungsten based, premium types.
- The M40 and higher types are called "super high speed steels".
Properties of high speed molybdenum steels
The mechanical properties of molybdenum tool steels (Group M) are similar to those of high speed tungsten steels, with some differences in performance:
- Better toughness of Group M compared to Group T.
- Wear resistance can be increased by increasing the carbon and vanadium content of the steels in group M: the presence of vanadium results in less brittle behaviour at high temperatures, giving molybdenum tool steels a high resistance to abrasion.
- Better alloying behaviour in the presence of vanadium, giving less brittle behaviour at high temperature: molybdenum tool steels have a high resistance to abrasion.
- Lower hot hardness of Group M, sensitive to overheating in unfavourable austenitising conditions, as these steels must be austenitised at temperatures lower than those required for hardening Group T steels. To compensate for the reduction in hot hardness of Group M high-speed tool steels, tungsten and some vanadium are added to the molybdenum grades. Tungsten-molybdenum grades such as M2, M3, M4 are therefore very popular as high-speed tool steels.
- The total hardness of the high speed tool steels of group M can be obtained by quenching at temperatures from 1175 to 1230°C (2150 to 2250°F).
The maximum hardness that can be achieved in high-speed steels of group M depends on the composition:
- Lower carbon content (M1, M2, M10, M30, M33, M34 and M36) - Maximum hardness 65 HRC.
- Higher carbon content (including types M3, M4 and M7) - Maximum hardness 66 HRC.
- High carbon and cobalt-containing steels (M41, M42, M43, M44 and M46) - Maximum hardness 69 to 70 HRC.
Applications of high speed molybdenum steels
Molybdenum high-speed tool steels can be used in the following areas:
- Cutting tools such as drills, reamers, saw blades, milling cutters, taps, lathe tools, flat tools, punches, spindles and combs,
- Certain grades of molybdenum-based high-speed tool steel are satisfactory for cold work applications, such as threading dies, punches, cold head die inserts and cutting dies,
- Automotive cam rings and valve inserts,
- Tool steels of the M40 series are used for the manufacture of cutting tools for machining the latest, extremely high-strength, high-strength steels.