The global workhorse of medium-carbon alloy steel — 42CrMo per GB/T 3077, equivalent to AISI 4140, 42CrMo4 and SCM440. C 0.38–0.45%, Cr 0.90–1.20%, Mo 0.15–0.25%. Quenched and tempered to tensile above 1080 MPa and yield above 930 MPa for gears, shafts, bolts and connecting rods.
42CrMo sits at the intersection of strength, toughness and hardenability — the grade engineers reach for when plain carbon steel won't through-harden in the section size, and when a more expensive nickel-chromium alloy (like 4340) is overkill.
40Cr (GB) is chromium-only — lower cost, but lacking the Mo that suppresses temper embrittlement. 40Cr suits smaller sections and lower-stress parts; 42CrMo is the upgrade when the section is larger, the tempering temp is higher, or the part sees impact loading.
42CrMo for larger sections and impact loads; 40Cr for small, low-stress parts.4340 (AISI) adds Ni (1.65–2.00%) for deeper hardening and higher toughness in very heavy sections — aerospace landing gear, large crankshafts. 42CrMo delivers 90% of the performance at lower cost for sections up to ~100 mm. For most automotive and industrial gears, shafts and bolts, 42CrMo is the correct, cost-effective spec.
42CrMo for sections ≤100 mm; 4340 for very heavy, high-toughness parts.35CrMo (GB) has lower C (0.32–0.40%) for better weldability and higher toughness at the expense of peak hardness. 42CrMo (0.38–0.45% C) reaches higher tensile and yield after Q&T. 35CrMo for welded structures and high-toughness components; 42CrMo for gears, shafts and bolts where maximum strength is the priority.
42CrMo for maximum strength; 35CrMo when weldability matters.| Element | 42CrMo (GB/T 3077) % | AISI 4140 (%) | Role |
|---|---|---|---|
| Carbon (C) | 0.38 – 0.45 | 0.38 – 0.43 | Sets base hardness and strength after Q&T |
| Silicon (Si) | 0.17 – 0.37 | 0.15 – 0.35 | Deoxidizer; mild solid-solution strengthening |
| Manganese (Mn) | 0.50 – 0.80 | 0.70 – 0.90 | Hardenability |
| Chromium (Cr) | 0.90 – 1.20 | 0.80 – 1.10 | Deepens hardening; forms wear-resistant carbides |
| Molybdenum (Mo) | 0.15 – 0.25 | 0.15 – 0.25 | Suppresses temper embrittlement; boosts high-temp strength |
| Phosphorus (P) | ≤ 0.035 | ≤ 0.035 | Residual; controlled for toughness |
| Sulfur (S) | ≤ 0.035 | ≤ 0.040 | Residual; controlled for fatigue |
Reference: GB/T 3077; EN 10083-3 (42CrMo4); ASTM A29 (AISI 4140). Values typical per standard, actual confirmed on MTC.
| Condition | Tensile (typ.) | Yield (typ.) | Hardness (typ.) | Typical use |
|---|---|---|---|---|
| Spheroidized / soft annealed | ~650 – 800 MPa | ~450 – 600 MPa | ≤ 225 HB | Blanking, forming, machining — part quenched afterwards |
| Q&T — high strength (500–550°C) | ≥ 1,080 MPa | ≥ 930 MPa | 35 – 40 HRC | Gears, shafts, high-load structural parts |
| Q&T — high toughness (600–650°C) | ~900 – 1,050 MPa | ~750 – 900 MPa | 28 – 32 HRC | Bolts, connecting rods, impact-loaded components |
Values per GB/T 3077 (sample ≤25 mm); actual confirmed on MTC. Elongation ≥12%, reduction of area ≥45% in high-strength Q&T condition. For surface-hardened applications (gear teeth, journals), induction hardening produces 55–62 HRC at the surface with a tough core.
| Standard | Grade | C (%) | Cr (%) | Mo (%) | Notes |
|---|---|---|---|---|---|
| GB/T 3077 (China) | 42CrMo | 0.38 – 0.45 | 0.90 – 1.20 | 0.15 – 0.25 | Base reference |
| ASTM A29 / AISI (USA) | 4140 | 0.38 – 0.43 | 0.80 – 1.10 | 0.15 – 0.25 | Near-identical; routine substitution |
| EN 10083-3 (Europe) | 42CrMo4 (1.7225) | 0.38 – 0.45 | 0.90 – 1.20 | 0.15 – 0.30 | Nearly identical to 42CrMo |
| JIS G4053 (Japan) | SCM440 | 0.38 – 0.43 | 0.90 – 1.20 | 0.15 – 0.25 | Japanese Cr-Mo steel |
Equivalents approximate. For safety-critical parts, confirm full chemistry against your governing spec and the MTC.
1.0–4.0 mm strip, blanked into gear blanks in spheroidized state, then Q&T to 35–40 HRC, with induction or carburizing surface hardening of teeth to 55–62 HRC. Tough core resists tooth bending fatigue; hard surface resists contact pitting. Standard material for transmissions, differentials and industrial gearboxes. See our automotive fine-blanking applications.
3.0–6.0 mm bar/strip, machined or formed into shaft profiles, Q&T to 32–38 HRC. Cr ensures through-hardening in shaft diameters up to ~100 mm; Mo suppresses temper embrittlement during high-temp temper for shaft toughness. Surface induction hardening of bearing journals and seal surfaces is standard.
42CrMo wire/narrow strip, cold-headed into bolt/stud blanks, Q&T to property class 10.9 or 12.9 (tensile 1040–1200 or 1220–1400 MPa). Mo is critical here — it prevents temper embrittlement that can cause delayed fracture in high-strength bolts. Used in engine cylinder heads, connecting rods, wheel hubs and structural flanges.
42CrMo bar/thick strip, forged or blanked into connecting rod shapes, Q&T to 32–36 HRC, fracture-split or machined big ends. High yield strength (≥930 MPa) plus good fatigue resistance makes it suitable for connecting rods under millions of combustion cycles. Shot peening of the shank and beam is standard to raise fatigue limit.
Exact parameters confirmed for your section size and target hardness.
Heat to 840–880°C, hold 15–30 min per 25 mm section to fully dissolve carbides and form austenite. Avoid overheating above 900°C — causes grain growth and reduces toughness. Controlled-atmosphere or vacuum furnaces prevent decarburization.
Oil quench standard — Cr and Mo provide enough hardenability for through-hardening in sections up to ~100 mm. Thin strip (≤3 mm) may air-harden fully. Martempering (hot oil or salt bath at 150–200°C) used for large or thin-walled parts to minimize distortion.
Temper at 500–650°C for 1–2 hours. Mo (0.15–0.25%) suppresses temper embrittlement in the 400–550°C range, allowing high-temp temper for maximum toughness. High-strength parts: 500–550°C → 35–40 HRC. High-toughness parts: 600–650°C → 28–32 HRC.
For gears, shafts and wear surfaces, induction hardening or carburizing plus low-temp temper produces a hard, wear-resistant surface (55–62 HRC) with a tough Q&T core. This dual-property structure — hard surface for wear, tough core for impact — is why 42CrMo is the default gear and shaft material.
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