Kg & pieces - No decimal places. The designation “300M” is a commercial name that has become widespread. It is also referred to as AISI 4340M (modified 4340), sometimes written AISI E4340 Mod. Its UNS (Unified Numbering System) designation is K44220. It has no direct counterpart in the French/European standard but would correspond to a modified 40NiCrMo7 steel. As seen earlier, 300M differs from conventional 4340 steel through its higher silicon content (~1.6 % vs ~0.25 % for a 4340) and the addition of a small fraction of vanadium (~0.05–0.1 %). Its carbon and molybdenum contents are also slightly higher than its predecessor. This composition is optimized for superior hardenability and core strength. Silicon at this level delays the tempering of martensite. By inhibiting the precipitation of cementite carbides (Fe3C) during tempering around 572 °F, it preserves very high strength, as illustrated by this diagram: [see schematic below, fig. 1] Finally, the very small addition of vanadium refines the grain size during heat treatment (formation of V carbides/nitrides), improving both strength and toughness. Like most ultra-high-strength steels, 300M shows no fatigue plateau indicating an absolute fatigue limit: the stress-versus-cycle curve continues to decrease beyond 10⁶ cycles. In a more ductile steel such as 17-4 PH (condition H1025), local micro-plasticity blunts the tip of an emerging crack; the stress is redistributed and crack propagation arrests, giving rise to a fatigue limit. Nothing of the sort occurs in 300M, which is low-ductility and also has a low propagation threshold, so a micro-crack advances as soon as it experiences repeated loading. Nevertheless, 300M was specifically designed to improve fatigue life over 4340 steel. It therefore provides excellent fatigue endurance, higher than that of 4340 under comparable conditions. Its cleanliness (thanks to vacuum melting) coupled with its fine tempered martensite give it good toughness, which slows crack growth and shifts the entire fatigue curve upward. Propagation thus remains slower than that of 4340 owing to its toughness and cleanliness. Finally, like all high-strength steels, it exhibits limited work-hardening, and its elongation at break (7–8 %), although modest, is noteworthy given its high strength level. 300M is a through-hardening alloy, with core hardness close to surface hardness. It requires meticulous heat treatment to develop its optimal properties; the slightest deviation can drastically reduce its performance. The green curve in the diagram above (fig.2) represents air cooling carried out at 1697 °F for 1 h after normalising. It homogenises the microstructure (uniforms the internal composition); the grain is refined and becomes more regular. The metal emerges with a fine microstructure, usually bainitic, ready for hardening. This heat treatment is used after hot forming or manufacturing 300M. The orange and yellow curves (fig.2) represent two cooling methods after austenitising (1598 °F for 30 min to 1 h): respectively forced-air cooling (≈ 5.4 °F/s) and oil quenching (36 °F/s). This treatment heats 300M just enough to dissolve carbides, allowing alloying elements and carbon to enter solid solution in the new austenite structure. Because the high Si content increases the opposite tendency to decarburisation, it is carried out in a controlled or vacuum atmosphere to avoid it. Quenching is preferably performed in oil, immediately on leaving the furnace, to obtain martensite throughout the section. 300M shows excellent hardenability: even thick sections (~90 mm) can be through-hardened, achieving hardness above 50 HRC. After quenching, double tempering is mandatory to relieve internal stresses, complete transformation of any retained austenite, and stabilise the result. A lower temper will yield slightly higher strength at the expense of toughness, while a higher one will reduce strength (and hardness) while increasing toughness (see fig.3 below). Precautions are necessary, however: 300M exhibits temper embrittlement between 662 and 1022 °F if the soak lasts too long or cooling is too slow. In aeronautics it is generally performed at 572 °F for 2+2 hours, giving a strength of about 1900 MPa with toughness satisfactory for critical parts such as landing gear. Machining must be carried out in the soft state (annealed or normalised). Welding is delicate because the alloy hardens readily in air, increasing the risk of cracking. It can be welded, by resistance or fusion, only with strict precautions (preheat, post-heat) and a subsequent heat treatment (re-normalising + temper) after welding. In short, 300M steel is a benchmark material when excellent mechanical strength, good damage tolerance (cracks, impacts) and long service life are required. Its flagship historical applications are aircraft landing gear for both military and civilian aircraft, where it long remained the standard. The variations in its chemical composition for aerospace. ROUND BAR SQUARE BAR, RECTANGULAR BAR, ROUND BAR, WIRE, SHEET, ROUND TUBE ROUND BAR RECTANGULAR BAR, ROUND BAR, SHEET, SQUARE TUBE, ROUND TUBE ROUND BAR ROUND BAR SQUARE BAR, RECTANGULAR BAR, ROUND BAR ROUND BAR ROUND BAR, ROUND TUBE SQUARE BAR, HEXAGONAL BAR, RECTANGULAR BAR, ROUND BAR, WIRE SQUARE BAR, RECTANGULAR BAR, ROUND BAR ROUND BAR PROFILE ROUND BAR ROUND BAR ROUND BAR SHEET SHEET ROUND BAR, SHEET ROUND BAR RECTANGULAR BAR, ROUND BAR ROUND BAR ROUND BAR ROUND BAR SHEET ROUND BAR SQUARE BAR, RECTANGULAR BAR ROUND BAR SHEET ROUND BAR ROUND BAR ROUND BAR ROUND BAR ROUND BAR ROUND BAR, SHEET, ROUND TUBE ROUND BAR ROUND BAR The most remarkable properties of this steel alloy ≥4% 1930-2068 MPa ≥1586 MPa ≤99 The practical applications of this steel in aircraft construction. Designed to withstand the severe shocks of landings and takeoffs, landing gears require strong fatigue resistance, such as that provided by 300M steel. Long a competitor of 35NCD16, 300M is now preferred for these applications. Connecting engines to propellers or rotors, aircraft drive shafts demand exceptional torsion and fatigue resistance. 300M steel meets these criteria, ensuring efficient power transmission to helicopters and turboprop engines. The high-strength 300M steel is widely used in critical applications. Its exceptional resistance to wear, high loads, and tensile stress makes it suitable for gears and fasteners in aircraft. Resistant to vibrations, temperature cycles, and high pressures, 300M steel ensures the reliability of propulsion systems and structural components. How it is used in various industries.300M, the result of optimizing the chemical composition of 4340
Mechanical properties of 300M, specialized in fatigue resistance
Heat treatments of 300M, a through-hardened alloy
Normalising and quenching
Double tempering
Machining and welding
Chemical composition of 300M
% C
CarbonCr
ChromiumCu
CopperMn
ManganeseMo
MolybdenumNi
NickelP
PhosphorusS
SulfurSi
SiliconV
Vanadium Min. 0.40 0.70 <0.00 0.65 0.35 1.65 <0.00 <0.00 1.45 0.05 Max. 0.45 0.95 0.35 0.90 0.50 2.00 0.010 0.010 1.80 0.10 Related steel alloys
12NC12, FE-PL61
15CDV6, 15CrMoV6, 1.7734, 1.7736, AIR 9160
16NCD13, 1.6657, 14NiCrMo 13-4
25CD4S, 1.7218, 25CrMo4, FLE-PL1502
30CD12, 1.8515, 30CrMo12, 31CrMo12, FE-PL1501
30CND8, 1.6580, 30CrNiMo8
30NCD16, 1.6747, 30NiCrMo16-6, FE-PL2107, 30Ni4CrMoA
32CDV13, 1.8522, 33CrMoV12, FE-PL1504
35CD4, 1.7220, 34CrMo4, 35CrMo4, FE-PL1503
35NC6, 1.5815, 35NiCr6, FE-PL2102
35NCD16, 1.6773, 36NiCrMo16, FE-PL2108
40CAD6-10
40CDV12, 40CrMoV12, FE-PL1507
40NCD7, 40NiCrMo7
42CD4
45SCD6
C75S
DC04, Fe P04, St 14, ES
E15CDV6
E16NCD13
E32CDV13
E35NCD16
E40CDV12
E4330, 4330 Mod, A646 Grade 5
FER PUR
GENRE STUB
S145F
S145H
S534
S97D
S98D
S99
X210CR12
X30Cr13, Z30C13
XC18S
XC38
Z230KDWVC11
Key properties
Ductility
Tensile Strength
Yield Strength
Brinell Hardness
How 300M is used in aerospace
Landing gears
Drive shafts
Gears and high-strength bolts
Optimizing the use of 300M : treatments, regulations, and options.
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Applications of
300M steel