Steel 300M - A646 - K44220

300M, the result of optimizing the chemical composition of 4340

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]

• Up to ~572 °F, the hardness of both grades remains very high; 300M keeps a slight advantage (~1 HRC).
• Between 572 °F and 806 °F, 4340 starts to soften markedly (about 8 HRC loss), whereas 300M drops much more slowly (-3 HRC).
• Beyond 932 °F, the gap widens: 300M still retains ~47 HRC whereas 4340 falls to ~27 HRC.

Finally, the very small addition of vanadium refines the grain size during heat treatment (formation of V carbides/nitrides), improving both strength and toughness.

Mechanical properties of 300M, specialized in fatigue resistance

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.

Heat treatments of 300M, a through-hardened alloy

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.

Normalising and quenching

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.

Double tempering

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 and welding

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.