Tool Steels – Molybdenum High-Speed Steels

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Introduction

Tool steels consisting of a combination of more than 7% molybdenum, tungsten and vanadium, and more than 0.60% carbon, are termed high-speed tool steels. High-speed tool steels can be used to cut metals at high speeds and remove large amounys of material in short time frames.

According to the American Iron and Steel Institute (AISI), currently there are more than 40 individual classifications of high-speed tool steels. These steels can be hardened to 62 - 67 HRC and can maintain this hardness even at temperatures as high as 540°C (1004°F), thus making it extremely useful in high-speed machinery.

High-speed tool steels are classified into:

• Molybdenum high-speed steels and
• Tungsten high-speed steels.

Molybdenum high-speed steels are also known as Group M. These steels form more than 95% of all high-speed steels manufactured in the United States. Although both types of high-speed steels are equivalent in performance, Group M steels have lower initial cost. Tools made using high-speed tool steel can be coated with titanium carbide, titanium nitride, and other coatings by physical vapor deposition process for enhanced performance and prolonged tool life.

Overview

Molybdenum is a silvery white metal that can be alloyed together with other elements. It is a pronounced carbide former, increases temper brittleness, and enhances fine grain formation. Molybdenum has high weldability and a high tendency for secondary hardening during tempering. The melting point of molybdenum steels is lower than that of the tungsten steels.

Molybdenum can form a double carbide with iron and carbon which allows molybdenum to be substituted for tungsten at the rate of one part of molybdenum by weight for two parts of tungsten.

Classification

Molybdenum high-speed steels are classified as follows:

• Types M 1 to M 10 (except M6) contain some tungsten but no cobalt
• Types M30 and M40 are cobalt-base, molybdenum-tungsten, premium types
• Types M40 and above are super high-speed steels.

Properties

The main alloying elements in molybdenum high-speed steels are molybdenum, tungsten, vanadium, cobalt, chromium and carbon. The approximate percentage of molybdenum in group M tool steels is 3.5 to 10%.

The mechanical properties of Group M tool steels are similar to those of tungsten high-speed steels, except for toughness. Another advantage with molybdenum steels is that when combined with vanadium, the steel does not become brittle at high temperatures, as vanadium causes the carbides to reform into small secondary carbides which are more stable at high temperatures. Molybdenum tool steels have high abrasion resistance.

Hardness

As Group M tool steels easily decarburize (especially high-molybdenum, low-tungsten compositions), they are sensitive to overheating in unfavourable austenitizing conditions, thus making them far more sensitive than tungsten high speed steels. To overcome this problem, Group M steels must be austenitized at temperatures lower than what is required for hardening Group T steels. This will also avoid incipient melding. Full hardness of Group M high-speed tool steels can be achieved when quenched from temperatures of 1175 to 1230°C (2150 to 2250°F).

Group M high-speed tool steels are tougher than the Group T high-speed tool steels.

To compensate for the reduced hot hardness in Group M high-speed tool steels, tungsten and a bit of vanadium are added to the plain molybdenum grades. This makes the tungsten-molybdenum grades such as M2, M3, M4 very popular as high-speed tool steels.

The maximum hardness that can be achieved in Group M high-speed steels differs according to their composition as listed below:

• Lower carbon contents (M1, M2, M10, M30, M33, M34, and M36) - Maximum hardness is 65 HRC.
• Higher carbon contents (including types M3, M4, and M7) - Maximum hardness 66 HRC.
• Higher-carbon, cobalt-containing steels (M41, M42, M43, M44, and M46) - Maximum hardness is 69 - 70 HRC.

Wear Resistance

By increasing the carbon and vanadium contents of Group M steels, the wear resistance can be increased. By increasing the cobalt content the red hot hardness - the capability of some steels to resist softening at high temperatures causing the steels to emit radiation in the red area of the visible spectrum - is enhanced. However, this simultaneously causes reduction of toughness. Type M2 and other grades in the molybdenum high-speed tool steels group have remarkably high resistance to softening at very high temperatures due to the high alloy content.

Composition

The following table shows the designation and composition limits of molybdenum high-speed steels.

^aAll steels except Group W contain 0.25 max Cu, 0.03 max P, and 0.03 max S; Group W contains 0.20 max Cu, 0.025 max P, and 0.025 max S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group A, D, H, M, and T steels.

Applications

Molybdenum high-speed tool steels can be applied in the following areas:

• Cutting tools such as drills, reamers, saw blades, end mills, milling cutters, taps, hobs, lathe tools, planar tools, punches, routers, gear cutters saws, broaches, and chasers
• Certain molybdenum high-speed tool steel grades are satisfactory for cold-work applications, such as thread-rolling dies, punches, cold-header die inserts, and blanking dies
• Cam-rings and automobile valve inserts
• M40 tool steel series are used to make cutting tools for machining the latest, extremely tough, high-strength steels.