Grade selection
Should I specify 4140 or 4340 for a shaft?
4140 (42CrMo4) is the default engineering steel for shafts, gears and couplings up to roughly 100 mm diameter. 4340 (34CrNiMo6) adds about 1.8% nickel, which carries hardenability and toughness deep into large sections, so it is the choice above about 100 mm or where impact toughness governs.
Both are chromium-molybdenum through-hardening engineering steels supplied quenched and tempered. The practical difference is section size.
4140 / 42CrMo4 / 1.7225 — good hardenability to about 100 mm diameter, tempered typically to 28 to 32 HRC for shafts. Readily machined, nitrided or induction hardened, and widely available. It covers most shafts, pinions, couplings, bolts and hydraulic components.
4340 / 34CrNiMo6 / 1.6582 — the nickel addition slows the critical cooling rate, so the core of a thick forging still transforms to a tempered martensite structure rather than a weaker mixed structure. That makes 4340 the grade for heavy shafts, large gears, rotor shafts, landing gear and anything with a demanding Charpy requirement at low temperature.
Points worth specifying either way: the delivery condition (as rolled, annealed, or Q+T to a hardness band), whether you need ultrasonic testing on the bar, and the tempering temperature if a specific hardness range is needed for machining or subsequent nitriding. For sour service, hardness must stay inside the NACE cap, which usually means a higher tempering temperature and a lower strength band.
Axicor supplies both grades in round bar and plate with EN 10204 3.1 certificates, cut to length.
Grades referenced
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