The most-specified alloy steel in industrial shafting — 4140 chromoly offers a predictable through-hardening response (55+ HRC achievable) with good weldability and ductility in the annealed condition. Available pre-hardened to 28–32 HRC ("4140 PH") for parts that need moderate strength without post-machining heat treat. The default choice for shafts, gears, couplings, and bolts above M12.
Quench-and-temper to 45–55 HRC through cross-sections up to ~50 mm — unachievable in plain carbon 1018/1045.
Specify "4140 PH" or "4140 HT" to receive bar pre-treated to 28–32 HRC. Saves heat-treat step; machines like medium-hard steel.
Alloy chemistry (Cr-Mo) gives superior fatigue life vs plain carbon in cyclic-load applications.
| Element | Content |
|---|---|
| Iron | Balance |
| Carbon | 0.38–0.43% |
| Chromium | 0.80–1.10% |
| Molybdenum | 0.15–0.25% |
| Manganese | 0.75–1.00% |
| Silicon | 0.15–0.35% |
Composition per ASTM A29. Specific mill test reports (MTR) available on request for production orders.
Annealed machines fast but needs heat-treat. Pre-hardened (PH, 28–32 HRC) is ready-to-use. Q&T (quench-tempered) for max strength.
Below ~-20 °C impact toughness drops. Use 4340 or a nickel-bearing alloy for cold-service applications.
For surface hardening, gas nitriding at 500 °C gives 60+ HRC case without distortion. Carburizing requires full re-heat.
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