Material number G43400: 4340
G43400 is the Unified Numbering System designation for 4340, nickel-chromium-molybdenum low-alloy steel, deep hardening specified in ASTM A29.
What G43400 means
UNS numbers (ASTM E527 / SAE J1086) use a letter for the alloy family and five digits: G = AISI/SAE carbon and alloy steels (G41400 = 4140), S = stainless and heat-resisting steels (S30400 = 304), K = miscellaneous steels (K02600 = A36), T = tool steels, N = nickel alloys. The UNS number identifies a composition, not a product form or property level, so G43400 appears in several ASTM specifications (plate, bar, pipe) with different mechanical requirements.
4340 at a glance
AISI 4340 is the high-strength, high-toughness alloy steel: 0.40 % carbon with 1.8 % nickel, 0.8 % chromium and 0.25 % molybdenum, UNS G43400. Nickel does what chromium and molybdenum alone cannot — it keeps the tempered martensite tough at very high strength and pushes hardenability so far that sections of 100 mm and more through-harden in oil. The result is a steel that can be tempered to any level from 950 to 1900 MPa and still resist impact and fatigue, which is why 4340 is specified for aircraft landing gear (as E4340 or 300M), crankshafts, heavy-duty gears and shafts, power-transmission components, oilfield drilling equipment and armour.
In practice most 4340 is used quenched and tempered to 30–40 HRC (1000–1300 MPa), often bought pre-hardened; the low-temper 1800+ MPa condition is reserved for aerospace parts made from vacuum-melted E4340 with rigorous hydrogen control, since the steel becomes notch- and hydrogen-sensitive above about 1400 MPa. Weldability is poor.
JIS SNCM439, GOST 40KhN2MA, GB 40CrNiMoA and BS 817M40 (EN24) are identical; EN 34CrNiMo6 (1.6582) is the standard European substitute with slightly different Ni/Cr balance.
Full data sheet for 4340 → · Equivalent grades →