Oct 25 2012
Topics Covered
Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
Machinability
Heat Treatment
Hot Work
Cold Work
Annealing
Tempering
Applications
Introduction
High-speed tool steels are broadly classified into molybdenum high-speed steels and tungsten high-speed steels. The molybdenum high-speed steels are also referred to as Group M steels. In the United States, Group M forms 95% or more of all manufactured high-speed steels. Both the high-speed tool steel types have equal performance but the molybdenum high-speed steels have relatively lower initial cost. To improve performance and tool life, tools made of high-speed tool steel can be coated with titanium carbide, titanium nitride, and other coatings using the physical vapor deposition process.
There are many types of molybdenum high-speed steels. Except M6, M1 to M10 contain some tungsten, but none of them have cobalt. The M4 steel has a 1.3% level of carbon content, which is relatively higher than the rest. It has better hardness and superior wear resistance, but low toughness.
Chemical Composition
The following table shows the chemical composition of M4 steels.
| Element |
Content (%) |
| Fe |
Balance |
| C |
1.25-1.40 |
| Mn |
0.15-0.40 |
| Si |
0.20-0.45 |
| Cr |
3.75-4.75 |
| Ni |
0.3 |
| Mo |
4.25-5.50 |
| W |
5.25-6.50 |
| V |
3.75-4.50 |
| Cu |
0.25 |
| P |
0.03 |
| S |
0.03 |
Physical Properties
The physical properties of M4 steels are outlined in the following table.
| Physical properties |
Metric |
Imperial |
| Density |
7.97 g/cm3 |
0.288 lb/in3 |
| Melting point |
1427 °C |
2600°F |
Mechanical Properties
The mechanical properties of M4 steels are tabulated below.
| Mechanical properties |
Metric |
Imperial |
| Hardness, Rockwell C (tempered at 1200°F) |
59.0 - 61.0 |
59.0 - 61.0 |
| Hardness, Rockwell C (as hardened) |
64.0 - 65.0 |
64.0 - 65.0 |
| Izod impact unnotched (when tempered at 900°F) |
47.0 J |
34.7 ft-lb |
| Abrasion (loss in mm3 , tested at 1000°F; ASTM G65) |
20.2 |
20.2 |
| Abrasion (loss in mm3, tested at 1175°F; ASTM G65) |
28.4 |
28.4 |
| Poisson's ratio |
0.27-0.30 |
- |
| Elastic modulus |
190-210 GPa |
27557-30458 ksi |
Thermal Properties
The following table shows the thermal properties of M4 steels.
| Thermal properties |
Conditions |
| T (°C) |
Treatment |
| Thermal expansion |
9.5 x 10-6/ºC |
20-260 |
- |
Other Designations
Other designations that are equivalent to AISI M4 molybdenum high-speed tool steels include:
- ASTM A600
- FED QQ-T-590
- SAE J437
- SAE J438
- UNS T11304
Fabrication and Heat Treatment
Machinability
The machinability of M4 steels is comparatively low rating about 40% than that of water hardening tool steels which are much easier to machine.
Heat Treatment
M4 steels can be preheated at 788°C (1450°F) and then rapidly heated to 1218°C (2224°F) for 3-5 minutes followed by oil, salt bath or air quenching process.
Hot Work
It is better if the M4 steel is not hot forged.
Cold Work
Conventional methods can be applied for the cold working process with the alloy in the annealed condition.
Annealing
M4 steels should be annealed at 885°C (1625°F) followed by a slow furnace cool at -13.77°C (7.2°F) per hour or less.
Tempering
The tempering process can be carried out at 566°C (1051°F) to realize a Rockwell C hardness of 62 to 66.
Applications
M4 molybdenum high-speed tool steels are suitable for all types of cutting tools used in machining operations.