300M High Tensile

300M is a low alloy vacuum melted steel of very high strength. It is a modified AISI 4340 with silicon, vanadium and slightly higher carbon and molybdenum content that 4340. It has a very good combination of strength and toughness, fatigue, strength and good ductility. It is a through hardened alloy.
Applications: 300M is used for high strength parts in the range of 55HRc, such as high performance auto parts, aircraft landing gear, airframe parts and any other high strength applications.
Colour Code Stocked Sizes
Silver (Bar End) Rounds
Hollow Bar
Bar Finish
Peeled, Smooth Turned.
Related Specifications
Germany DIN 1.6928
Great Britain
USA UNS K44220ASTM A579, A646AMS 6417, 6419MIL S-8844DMIL S-8844 Class 2
Chemical Composition (Base Material)
Min. % Max %
Carbon 0.38 0.46
Silicon 1.45 1.80
Manganese 0.60 0.90
Chromium 0.70 0.95
Molybdenum 0.30 0.65
Phosphorous 0 0.01
Sulphur 0 0.01
Nickel 1.65 2.00
Vanadium 0.05
Mechanical Property Requirements for Steels in the Heat-Treated Condition for Turned,
Mechanical Property Designation
Tensile Strength Mpa Min 1931
0.2% Proof Stress Mpa Min 1586
Elongation on
5.65√S0 %
Min 7
Izod Impact J Min
Charpy Impact J Min
Hardness Brinell HB Min
Forming by conventional methods is good in the annealed condition.
Heat Treatment
Anneal at 840oC and slow furnace cool at a rate of less than 10oC per hour down to 310C. From there it may be air cooled.
300M must be normalized at 960oC before hardening. After the normalizing treatment the alloy is hardened by heating to 870oC and oil quenched.
Heat to 880 oC - 920 oC, hold until temperature is uniform throughout the section, soak for 10 - 15 minutes and cool in still air.
Stress Relieving
temper at 310oC to give a nominal 55Rc
Notes on Heat Treatment
Perferably done in the normalised or normalised and tempered condition. Final machining to finished tolerances is done by grinding with care due to hardness of (RC55). Stress relieve at 290oC after finish grinding.
300M can be welded. It must be pre and post heated because the alloy will air harden due to heat input from welding. Job must be re-normalised and tempered prior to final heat treatment. 300M can be joined by fusion methods or flash resistance welding.
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