M2 Molybdenum High Speed Tool Steel (UNS T11302)
Equivalent grades: SKH51/SKH9/W6Mo5Cr4V2
Molybdenum high speed steels are designated as Group M steels according to the AISI classification system. Over 95% of high-speed steels manufactured in the US are group M steels. Tungsten is present in all types from M1 to M10, except M6, and cobalt is not present in any these steels. Molybdenum high speed steels have similar performance when compared to tungsten high-speed steels. However, the initial cost of molybdenum tool steels is lower. Titanium nitride, titanium carbide and several other coatings can be used in the tools made of this kind of steels through physical vapor deposition process to improve the performance and life span of the tool.
This article will provide an overview of M2 molybdenum high speed tool steel, which has high carbon content and better wear resistance than M1 molybdenum high speed tool steel.
The following table shows the chemical composition of M2 molybdenum high speed tool steels.
The physical properties of M2 molybdenum high speed tool steel are given in the following table.
|Density||8.16 g/cm3||0.294 lb/in3|
The mechanical properties of M2 molybdenum high speed tool steel are tabulated below.
|Hardness, Rockwell C (tempered at 1150°F, quenched at 2200°F)||62||62|
|Hardness, Rockwell C (as hardened, quenched at 2200°F)||65||65|
|Compressive yield strength (when tempered at 300°F)||3250 MPa||471000 psi|
|Izod impact unnotched (when tempered at 300°F)||67 J||49.4 ft-lb|
|Abrasion (loss in mm3, as-hardened; ASTM G65)||25.8||25.8|
|Abrasion (loss in mm3, tempered at 1275°F; ASTM G65)||77.7||77.7|
|Elastic modulus||190-210 GPa||27557-30458 ksi|
The following table shows the thermal properties of M2 molybdenum high speed tool steels.
|CTE, linear (@20.0 – 100°C/ 68.0 – 212°F)||10 μm/m°C||5.56 μin/in°F|
|CTE, linear (@20.0 – 500°C/68.0 – 932°F)||12.2 μm/m°C||6.78 μin/in°F|
|CTE, linear (@20.0 – 850°C/68.0 – 1560°F)||12.6 μm/m°C||7 μin/in°F|
Other designations that are equivalent to M2 molybdenum high speed tool steels include:
•B.S. BM 2
•ASTM A597 (CM-2)
Fabrication and Heat Treatment
Shaping of M2 tool steels can be carried out using grinding methods. However, they have poor grinding capability and hence they are regarded as “medium” machinability tool steel under annealed conditions. The machinability of these steels is only 50% of that of the easily machinable W group or water hardening tool steels.
M2 tool steels are pre-heated prior to hardening at 2610°C (4730°F) followed by rapid heating from 2610°C (4730°F) to 3960°C (7160°F). These steels are then cooled for 3 to 5 min and quenched in air, salt bath or oil.
M2 tool steels are annealed at 1204°C (2200°F) and cooled at 72°C (162°F) per hour or even less.
M2 steels are tempered at 552°C (1025°F) to obtain Rockwell C hardness from 60 to 65.
Finally, M2 steels are hardened by heat treatment and quenching.
M2 tool steels are suitable for making cutting tools.