300M (4340M) High Strength Alloy Steel
300M Alloy Steel
300M (4340M) is a high performance alloy steel which is a vacuum melted grade, supplied in the normalised and tempered/softened condition to enhance machinability prior to final heat treatment.
Chemical Component:
| C | Si | Mn | P | S | Cr | Mo | Ni | V |
| 0.38-0.46 | 1.45-1.8 | 0.6-0.9 | 0.01 | 0.01 | 0.7-0.95 | 0.3-0.65 | 1.65-2 | 0.05 |
Mechanical Properties:
| Mechanical & Physical Properties | 21 °C |
| Ultimate Tensile Strength /MPa | 1981 |
| 0.2% Yield Strength /MPa | 1672 |
| Reduction of area % | 35 |
| Elongation % | 10 |
| Room Temperature Hardness/ Rockwell C | 58 |
(Normalised at 927 °C for 1 hour+air cool/Austenitised at 871°C for 1 hour + oil quench/Tempered at 302°C for 1 hour+air cool)
| Hardness at Various Tempering Temperature | 204°C | 316°C | 427°C | 538°C | 649°C |
| Hardness / Rockwell C | 53 | 52.5 | 50 | 47 | 35 |
(Austenitised at 871°C for 1 hour + oil quench/Tempered for 2+2 hour+air cool)
Introduce:
It has a combination of ultra high strength, good fatigue resistance and excellent transverse properties, and as such finds many applications in the aerospace industry, such as undercarriage components, gears and shafts.
Standards:
AMS 6417, ASTM A579
Application:
300M can be used in a variety of applications, here are just a few examples:
Undercarriage components
Gears
Shafts
Aircraft landing gear
High strength bolts
Properties:
Density:
7.85 g/cm^3
Specific Heat Capacity:
448 J/kg*K
Coefficient of Thermal Expansion from -17.8-93°C:
11.34 µm/mm/°C
Minimum Longitudinal Tensile Properties
| Property | Value |
| Tensile Strength | 280 ksi |
| Yield Strength at 0.2% Offset | 230 ksi |
| Elongation in 4D | 7% |
| Reduction of Area | 25% |
Minimum Transverse Tensile Properties, Inch/Pound Units
| Nominal Cross-Sectional Area Square Inches | Tensile Strength ksi | Yield Strength at 0.2% Offset ksi | Average Reduction of Area % | Individual Reduction of Area % |
| Up to 100, incl | 280 | 230 | 30 | 25 |
| Over 100 to 144, incl | 280 | 230 | 25 | 20 |
| Over 144 to 225, incl | 280 | 230 | 20 | 15 |
| Over 225 | 280 | 230 | 15 | 10 |
Principal Design Features
300M is a low alloy, vacuum melted, steel of very high strength. Essentially it is a modified AISI 4340 steel with silicon, vanadium and slightly greater carbon and molybdenum content than 4340. 300M has a very good combination of strength (280 to 305 ksi), toughness, fatigue strength and good ductility. It is a through hardening alloy.
Machinability
Machining is best accomplished with the alloy in the normalized or normalized and tempered condition. Final machining to finish tolerances is done by grinding with care due the hardness of the heat treated alloy (Rockwell C 55). It is important to do a stress relief anneal at 550 F after finish grinding.
Forming
Formability by conventional methods is good in the annealed condition. The alloy behaves much like AISI 4340 steel.
Welding
300M can be welded by fusion methods or by flash resistance welding. Approved procedures must be used for fusion welding, including pre and post-heating practice, because the alloy will air harden due to heat input from welding. Following welding it is essential to re-normalize or re-normalize and temper prior to the final hardening heat treatment.
Heat Treatment
300M must be normalized at 1700 F before hardening. After the normalizing treatment the alloy is hardened by heating to 1600 F and oil quenching. Tempering is then done last.
Forging
No data. The alloy is forgeable – contact the supplier for details.
Hot Working
No data. Heat input can cause this alloy to air harden. Contact the alloy supplier for information as to the advisability of hot working.
Cold Working
In the annealed or normalized condition the alloy has good ductility and can be readily cold worked by conventional methods.
Annealing
Anneal at 1550 F and slow furnace cool at a rate of less than 50 F per hour down to 600 F. From there it may be air cooled.
Aging
Not applicable to this alloy.
Tempering
Temper at 600 F to give a nominal 300 ksi strength.
Hardening
300M hardens from cold working and by heat treatment. See “Heat Treatment” and “Tempering”.
Other Comments
300M may be joined by Nicer brazing, copper brazing and silver brazing.
