AISI 4340 Alloy Steel (UNS G43400)
The deep-hardening high-strength steel — 4340 reaches 155 ksi and beyond in large sections — for crankshafts, drilling tools, aircraft parts and A320 L43 bolting.
What 4340 is.
AISI 4340 adds 1.8 percent nickel to a chromium-molybdenum base. Nickel raises hardenability so sharply that 4340 through-hardens in sections far larger than 4140, and it adds toughness at high strength levels, which is why 4340 is the standard steel for crankshafts, landing gear, drill collars and other parts that must combine 150 ksi tensile with fracture resistance. E4340 is the electric-furnace, low-residual version for aircraft quality.
ASTM A322 governs the chemistry and ASTM A434 the quenched-and-tempered classes. 4340 is the base steel of ASTM A320 L43 and A194 Grade 43. We stock certified 4340 bar for machined components and high-strength fasteners.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | 0.38 | 0.43 |
| Manganese | Mn | 0.60 | 0.80 |
| Phosphorus | P | — | 0.035 |
| Sulfur | S | — | 0.040 |
| Silicon | Si | 0.15 | 0.35 |
| Nickel | Ni | 1.65 | 2.00 |
| Chromium | Cr | 0.70 | 0.90 |
| Molybdenum | Mo | 0.20 | 0.30 |
Values per ASTM A322-13(2018) Table 1 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. E4340 (G43406): manganese 0.65–0.85 %, phosphorus and sulfur 0.025 % max.
Mechanical properties.
| Diameter (in.) | Yield strength, min (ksi) | Tensile strength, min (ksi) | Elongation, min (%) | Reduction of area, min (%) |
|---|---|---|---|---|
| 1½ and under | 130 | 155 | 14 | 35 |
| Over 1½ to 2½ | 120 | 150 | 14 | 35 |
| Over 2½ to 4 | 110 | 140 | 14 | 35 |
| Over 4 to 7 | 105 | 135 | 14 | 35 |
| Over 7 to 10 | 100 | 130 | 14 | 35 |
Values per ASTM A434/A434M-18 Table 1 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
| Diameter (in.) | Yield strength, min (ksi) | Tensile strength, min (ksi) | Elongation, min (%) | Reduction of area, min (%) |
|---|---|---|---|---|
| Over 4 to 7 | 85 | 110 | 16 | 45 |
| Over 7 to 10 | 80 | 105 | 15 | 40 |
Values per ASTM A434/A434M-18 Table 1 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
Specifications that apply.
How 4340 ships from Houston.
- Studs, bolts and threaded rod machined from bar
- Shafts, pins, couplings and machined components
- Certified A322 bar, annealed, normalised or quenched and tempered to A434
4340 fasteners we make.
Every item is manufactured to your drawing or standard in 4340, imperial or metric, with an EN 10204 3.1 certificate.
When to move from 4140 to 4340
4140 hardens fully to about 2½ inches; beyond that its core stays softer than the surface. 4340's nickel pushes full hardening to 4 inches and useful hardening well past that, which is why A434 lists the 4300 series for Class BD in every size and A320 uses 4340 for L43 studs up to 4 inches. 4340 also keeps better toughness at 150 ksi and above. The price is higher cost and slower machining, so for parts under 2 inches at 130 ksi or less, 4140 is normally the better buy.
Testing, certification and marking
- Tensile and hardness tested per lot on the finished product
- Grade or alloy marking by grade, class and heat on the certificate and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
AISI 4140 Alloy Steel (UNS G41400)
The workhorse Cr-Mo steel behind B7, L7 and Grade 7 — the bar stock for studs, shafts, couplings and any part that needs 100 to 150 ksi and good toughness.
ASTM A320 / ASME SA-320ASTM A320 Grade L43
4340 low-temperature bolting — L7 strength in diameters up to 4 in., thanks to the deep hardenability of a nickel-chrome-moly steel.
ASTM A194 / ASME SA-194ASTM A194 Grade 43
The 4340 heavy hex nut — the deep-hardening companion to A320 L43 studs and an alternative to Grade 7 on large low-temperature joints.