AISI 4340 Ultra-High Strength Alloy Steel: Aerospace & Motorsport Properties (UNS G43400)

4340 adds 1.65-2.00% nickel to the 4140 base chemistry (Cr-Mo), which provides three key advantages: (1) superior toughness at equal hardness levels — critical for fatigue resistance; (2) deeper hardenability (Jominy curves show 4340 hardens through ~75 mm vs ~50 mm for 4140); (3) better low-temperature impact properties. The tradeoff is cost — 4340 is approximately 30-40% more expensive than 4140 and harder to machine in the annealed condition. Use 4140 for general shafting and tooling; specify 4340 when component failure is catastrophic (aerospace, motorsport, military).

For maximum strength (260 ksi / 1790 MPa tensile): austenitize at 815-845°C (1500-1550°F), oil quench, temper at 205-230°C (400-450°F). This yields a tempered martensite microstructure at 50-53 HRC. However, at these strength levels, 4340 is notch-sensitive and susceptible to hydrogen embrittlement and stress-corrosion cracking. For most applications, tempering at 425-540°C (800-1000°F) provides a better balance of 140-180 ksi tensile with significantly improved ductility and fracture toughness — this is the recommended range for structural aerospace components per MMPDS.