Element Molybdenum(Mo) was first used as a cost-effective alternative element for tungsten in tool steels and high speed steels(HSS). Mo has about half the atomic weight of tungsten, and 1% molybdenum is about the same as 2% tungsten. Mo improves the hardness and wear resistance of tool steels, and promotes the formation of an optimal martensite matrix by reducing the “critical cooling rate,” even in large and complex molds that cannot be cooled rapidly without deformation or cracking. Mo also works with elements such as chromium to produce large quantities of extremely hard and wear-resistant carbides. The higher grade tool steels also have a higher requirement for mo content. These high alloy steels can be used for machining, cutting and forming metal parts where required high hardness, high strength and good toughness over a wide range of temperatures.
Mo is often added to high-speed tool steels to resist damage caused by high temperature creep. The creep at high temperature is generally diffuse-controlled and the addition of molybdenum is very effective in reducing the creep rate. For the age-hardening alloy precipitated by γ ‘Ni3(Al, Ti) phase, the addition of Mo strengthens the matrix and reduces the lattice mismatch between the matrix and γ’ phase, thus improving the stability of the precipitated phase. High-speed steels can be referred to as Tool steels that contain more than 7% Mo, W, V and more than 0.60% carbon, which describes their ability to cut at “high speed”. High speed steel T1 containing 18% tungsten was the preferred cutting steel until the 1950s, but the development of controlled atmosphere heat treatment furnaces made it feasible and economical to replace some or all of tungsten with molybdenum. The relationship of Mo content and tool steel showed like this:
|Steel Types||Mo content|
|Plastic mold steel||＜0.5|
|Cold working mold steel||0.5-1.0|
|Hot-working mold steel||＜3.0|
The addition of 5-10% Mo can effectively maximize the hardness and toughness of HSS and preserves these properties at the high temperatures generated by cutting the metal. If the primary carbide size of iron and chromium increases rapidly, the steel will soften and brittle. When Mo is in conjunction with vanadium, minimizes this effect by converting carbides into tiny secondary carbides that are more stable at high temperatures.
The superior cutting characteristics of HSS have been further extended by the use of a thin, hard titanium carbide coating that reduces friction and improves wear resistance, thereby increasing cutting speed and tool life. The High-speed steel containing Mo has excellent wear resistance at high temperatures and is an ideal material for new applications such as automobile valve seat ring and CAM ring. It can also be used to manufacture various cutting tools such as a drill, milling cutter, gear cutter, saw blade, etc.