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Tool & High Speed Steel

One of the earliest applications of molybdenum was as an efficient and cost effective replacement for tungsten in tool steels and high-speed steels. The atomic weight of molybdenum is roughly half that of tungsten and therefore 1% Mo is roughly equivalent to 2% tungsten. Because these highly alloyed steels are used in the working, cutting and forming of metal components, they must possess high hardness and strength, combined with good toughness, over a broad temperature range.

Coining dies

Coining dies made of cold work tool steel,
grade K 190 (C2.3%, Cr12.5%, Mo1.1%, V4.0%)
Courtesy of Boehler Edelstahl

Tool steels

Molybdenum in tool steels increases their hardness and wear resistance. By reducing the 'critical cooling rate' molybdenum promotes the formation of an optimal martensitic matrix, even in massive and intricate moulds which cannot be cooled rapidly without distortion or cracking. Molybdenum also acts in conjunction with elements like chromium to produce substantial volumes of extremely hard and abrasion resistant carbides. As the physical demands placed on tool steels increase, so too does the molybdenum content.

% Molybdenum content in tool steels
Steel typeMo
Plastic Moulding steels up to 0.5
Cold work steels 0.5 - 1.0
Hot work steels up to 3.0


High speed steels

When tool steels contain a combination of more than 7% molybdenum, tungsten and vanadium, and more than 0.60% carbon, they are referred to as high speed steels. This term is descriptive of their ability to cut metals at 'high speeds'. Until the 1950's, T-1 with 18% tungsten, was the preferred machining steel but the development of controlled atmosphere heat treating furnaces made it practical and cost effective to substitute part or all of the tungsten with molybdenum.

Typical Compositions of
Selected High-Speed Steels (%)
T-1 0.75 - - 18.0 1.1
M-2 0.95 4.2 5.0 6.0 2.0
M-7 1.00 3.8 8.7 1.6 2.0
M-42 1.10 3.8 9.5 1.5 1.2

Additions of 5-10% Mo effectively maximize the hardness and toughness of high-speed steels and maintain these properties at the high temperatures generated when cutting metals. Molybdenum provides another advantage: at high temperature, steels soften and become embrittled if the primary carbides of iron and chromium grow rapidly in size. Molybdenum, especially in combination with vanadium, minimizes this by causing the carbides to reform as tiny secondary carbides which are more stable at high temperatures. The largest use of high-speed steels is in the manufacture of various cutting tools: drills, milling cutters, gear cutters, saw blades, etc.

The useful cutting characteristics of high-speed steel have been further extended by applying thin, but extremely hard, titanium carbide coatings which reduce friction and increase wear resistance, thereby increasing cutting speed and tool life.

The exceptional high temperature wear properties of molybdenum-containing high-speed steels are ideal for new applications such as automobile valve inserts and cam-rings.

Mill cutter

Mill cutter (courtesy Boehler Edelstahl)