4140 steel (G41400): properties, composition, equivalents
4140 steel (AISI/SAE 4140, UNS G41400): 0.40 % C, 1 % Cr, 0.2 % Mo chromoly. Annealed 415 MPa yield / 655 MPa tensile; Q&T to 28–32 HRC (pre-hard) or 50+ HRC. Composition, heat treatment, hardness by temper, 4140 vs 4130/1045/4340, 42CrMo4/SCM440 equivalents.
What is 4140?
AISI 4140 is the definitive chromoly steel: 0.40 % carbon, 1 % chromium and 0.2 % molybdenum, UNS G41400. Chromium and molybdenum push hardenability deep enough that a 50 mm bar through-hardens in oil, and the steel tempers over a wide range to any strength from 900 to 1800 MPa — which is why it is the standard for shafts, axles, gears, bolts, tooling, drill collars and every 'machine steel' job that needs more than plain carbon steel can give. It is also the most-stocked alloy steel in North America, usually as pre-hardened bar (4140 HT / PH) at 28–32 HRC, ready to machine and use without further heat treatment.
As an AISI grade 4140 is defined by chemistry, not properties; the tables give typical values for annealed, normalized, pre-hard and quenched-and-tempered conditions at various tempers, and the ASTM A434 or customer specification supplies minimums when required. Weldability is poor (CE ≈ 0.8) — preheat and post-weld tempering are mandatory — and hydrogen embrittlement is a real risk above about 38 HRC, so plating and pickling of hardened parts must be followed by baking.
EN 42CrMo4 (1.7225), JIS SCM440, GB 42CrMo and BS 708M40 (EN19) are the same steel; 4142, 4145 and 4150 are the higher-carbon steps, 4130 the weldable low-carbon cousin, 4340 the nickel-bearing upgrade.
4140 chemical composition
| Element | Mass % | Note |
|---|---|---|
| C | 0.38 – 0.43 | |
| Mn | 0.75 – 1 | |
| P | ≤ 0.035 | |
| S | ≤ 0.04 | |
| Si | 0.15 – 0.35 | |
| Cr | 0.8 – 1.1 | |
| Mo | 0.15 – 0.25 |
SAE J404 / ASTM A29 (G41400). 4142 (G41420): C 0.40–0.45; 4140H (H-band, hardenability guaranteed): slightly wider Mn/Cr. Compare EN 42CrMo4: C 0.38–0.45, Cr 0.90–1.20, Mo 0.15–0.30, Mn 0.60–0.90.
4140 mechanical properties
| Thickness / condition | Yield strength ReH / Rp0.2 MPa (ksi) | Tensile strength Rm MPa (ksi) | Elongation A % | Hardness |
|---|---|---|---|---|
| Annealed (815 °C, furnace cool) — typical | ≈ 415 (≈ 60.2) | ≈ 655 (≈ 95.0) | 25 | ≈ 197 HB |
| Normalized (870 °C, air) — typical | ≈ 655 (≈ 95.0) | ≈ 1020 (≈ 147.9) | 18 | ≈ 302 HB |
| Pre-hardened 'PH / HT' bar (Q&T 28–32 HRC) — typical | ≈ 690–760 (≈ 100.1–110.2) | ≈ 930–1000 (≈ 134.9–145.0) | 16 | 269–321 HB (28–34 HRC) |
| Q&T, tempered 540 °C (1000 °F), 25 mm — typical | ≈ 895 (≈ 129.8) | ≈ 1035 (≈ 150.1) | 15 | ≈ 311 HB |
| Q&T, tempered 425 °C (800 °F), 25 mm — typical | ≈ 1100 (≈ 159.5) | ≈ 1240 (≈ 179.8) | 13 | ≈ 375 HB (40 HRC) |
| Q&T, tempered 315 °C (600 °F), 25 mm — typical | ≈ 1340 (≈ 194.4) | ≈ 1480 (≈ 214.7) | 11 | ≈ 445 HB (47 HRC) |
| Q&T, tempered 205 °C (400 °F), 25 mm — typical | ≈ 1640 (≈ 237.9) | ≈ 1770 (≈ 256.7) | 8 | ≈ 510 HB (52 HRC) |
| As-quenched (oil, 845 °C) | — | — | — | 54–59 HRC |
AISI/SAE grades are specified by chemistry (and hardenability for H-bands); mechanical values are typical for a 25 mm (1 in) round, not guarantees. Order to ASTM A434 (Class BC/BD) or a customer spec when minimums are needed. Charpy at 540 °C temper ≈ 40–50 J.
Impact toughness
4140 requires a minimum Charpy V-notch energy of 45 J at 20 °C (typical, Q&T to ≈ 300 HB). Toughness falls with lower tempering temperature; avoid 230–370 °C (tempered-martensite embrittlement) and slow cooling from 450–600 °C (temper embrittlement in high-P heats).
4140 equivalent grades
| Grade | System | Match | Why / differences |
|---|---|---|---|
| 708M40 (EN19) | BS (superseded British) | Identical | BS 970 708M40: C 0.36–0.44, Cr 0.90–1.20, Mo 0.15–0.25. |
| 42CrMo4 | EN (European) | Identical | 1.7225: C 0.38–0.45, Cr 0.90–1.20, Mo 0.15–0.30 — windows overlap almost completely; dual-certified 4140/42CrMo4 bar is standard. |
| 42CrMo | GB/T (China) | Identical | GB/T 3077 42CrMo: C 0.38–0.45, Cr 0.90–1.20, Mo 0.15–0.25. |
| SCM440 | JIS (Japan) | Identical | JIS G4053 SCM440: C 0.38–0.43, Cr 0.90–1.20, Mo 0.15–0.30. |
| 4142 | AISI / SAE | Near | C 0.40–0.45 — the upper half of the 42CrMo4 range; slightly higher hardness. |
| 4130 | AISI / SAE | Near | 0.30 % C version: lower strength (≈ 1000 MPa max practical), far better weldability; used for tube and welded structures. |
| 40KhM / 38KhM | GOST (Russia/CIS) | Near | GOST 4543 38KhM (C 0.35–0.42) / 40KhMFA (V-bearing); 40KhM is the common Russian equivalent in practice. |
| 4340 | AISI / SAE | Functional | Adds 1.8 % Ni: deeper hardenability (sections > 100 mm), higher toughness at 1500+ MPa. |
| 1045 | AISI / SAE | Functional | Unalloyed medium-carbon: cheaper, through-hardens only ≤ 20 mm. |
Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for 4140 →
Old and superseded designations
| Old name | Standard | Note |
|---|---|---|
| G41400 | UNS | 4140H = H41400 (hardenability band). |
| 42CrMo4 / 1.7225 | EN 10083-3 | European twin; 42CrMoS4 (1.7227) free-machining. |
| 708M40 / 708A42 / EN19 | BS 970 (withdrawn) | EN19 is the old British name still used in the UK and India. |
| SCM440 | JIS G4053 | Japanese twin (SCM4 in older editions). |
| 4145H / 4145H Mod | API Spec 7-1 | Higher-carbon variant for drill collars and tool joints. |
Product forms and tolerance standards
Product forms: hot-rolled and cold-finished bar (ASTM A29 / A322 / A108), pre-hardened bar 'PH' or 'HT' at 28–32 HRC (the most common stock form), forgings (A29, ASTM A788), seamless tube (A519), plate (A829 / A830), wire, downhole and drilling tools per API 7-1 (4145H modified).
Dimensional tolerances: hot-rolled bar: ASTM A29 / A6; cold-finished bar: ASTM A108 (h9–h11 typical); tube: ASTM A519; aerospace: AMS 6349 / 6382 / 6529.
Key property values
4140 steel yield strength
Typical: annealed ≈ 415 MPa (60 ksi); normalized ≈ 655 MPa; pre-hard 28–32 HRC ≈ 690–760 MPa (100–110 ksi); Q&T tempered at 540 °C ≈ 895 MPa (130 ksi); at 425 °C ≈ 1100 MPa; at 315 °C ≈ 1340 MPa; at 205 °C ≈ 1640 MPa (238 ksi).
4140 tensile strength
Typical: annealed ≈ 655 MPa (95 ksi); normalized ≈ 1020 MPa; pre-hard ≈ 930–1000 MPa; Q&T 540 °C ≈ 1035 MPa (150 ksi); 425 °C ≈ 1240 MPa; 315 °C ≈ 1480 MPa; 205 °C ≈ 1770 MPa (257 ksi).
4140 hardness
Annealed ≈ 197 HB; normalized ≈ 302 HB; pre-hardened 28–32 HRC (269–321 HB); as-quenched 54–59 HRC; tempered 205 °C ≈ 52 HRC, 315 °C ≈ 47 HRC, 425 °C ≈ 40 HRC, 540 °C ≈ 33 HRC, 650 °C ≈ 25 HRC. Induction-hardened surface 50–55 HRC; nitrided surface 55–60 HRC (≈ 600–700 HV).
Heat treatment
Anneal 815–870 °C, furnace cool 15 K/h to 480 °C. Normalize 870–900 °C, air. Harden 845–870 °C (1550–1600 °F), oil quench (water for simple shapes ≤ 25 mm). Temper 205–650 °C to the required hardness, 2 h minimum; avoid 230–370 °C. Stress relieve 550–650 °C. Ac1 ≈ 750 °C, Ac3 ≈ 790 °C, Ms ≈ 310 °C. Ideal critical diameter (4140H) ≈ 75–100 mm; Jominy J10 ≈ 47–56 HRC.
Machinability
≈ 65 % of B1112 annealed (197 HB); ≈ 55–60 % pre-hard at 28–32 HRC (excellent chip control, preferred for CNC). 4140 with 0.02–0.04 % S ('4140 resulfurized' / 42CrMoS4) improves to ≈ 70 %.
Physical properties
Density 7.85 g/cm³ (0.284 lb/in³), E = 205 GPa (29.7 × 10⁶ psi), expansion 12.3 × 10⁻⁶/K (20–100 °C), conductivity 42.6 W/m·K, specific heat 473 J/kg·K.
Weldability
Poor weldability (carbon equivalent ≈ 0.75–0.85). Weld only in the annealed or normalized condition where possible: preheat 250–350 °C (higher for thick or restrained joints), maintain interpass ≥ preheat, use low-hydrogen consumables (E8018-B2 / ER80S-B2 for matching Cr-Mo, or E7018 / ER70S-6 when the joint is not strength-critical), slow-cool under insulation, then temper immediately at 550–650 °C. Welding pre-hardened or Q&T bar softens the HAZ and risks cold cracking; for hardened assemblies design mechanical joints.
Machining, forming and heat treatment
Turning, milling, drilling and threading are all good; pre-hard 28–32 HRC gives the best surface finish and chip control with coated carbide. Rough in the annealed state and finish after hardening only if distortion allows (grinding stock 0.2–0.4 mm). Cold forming is limited to the annealed state; hot forging 1200–950 °C then normalize or anneal. Surface treatments: induction/flame hardening (50–55 HRC), gas or ion nitriding (excellent — 0.3–0.6 mm case at 600+ HV), QPQ, hard chrome plating (bake 190 °C ≥ 4 h after plating to relieve hydrogen), phosphate and black oxide.
Typical applications
- Shafts, axles, spindles, arbors and couplings
- Gears, sprockets, pinions (through-hardened or nitrided)
- High-strength bolts and studs: SAE Grade 8, ASTM A193 B7 (4140/4142 Q&T), A320 L7, ISO 10.9/12.9
- Hydraulic cylinder rods, pistons and glands (chrome-plated)
- Drill collars, tool joints, drill-string subs (4145H Mod), downhole tools
- Injection-mould bases, holder blocks and machine frames (pre-hard)
- Crankshafts, connecting rods, steering components, axle housings
- Rifle barrels and firearm receivers, hand tools, wrenches and sockets (forged)
4140 compared with related grades
Frequently asked questions
Is 4140 the same as 42CrMo4?
Yes for practical purposes. 4140 (C 0.38–0.43, Cr 0.80–1.10, Mo 0.15–0.25) and EN 42CrMo4 (C 0.38–0.45, Cr 0.90–1.20, Mo 0.15–0.30) overlap almost entirely and are dual-certified by mills; JIS SCM440 and GB 42CrMo are also identical.
What is 4140 HT or 4140 PH?
Heat-treated / pre-hardened bar: quenched and tempered at the mill to 28–32 HRC (≈ 900–1000 MPa tensile). It machines well and is used without further heat treatment for shafts, mould bases and fixtures — the most common way 4140 is sold.
How hard can 4140 get?
54–59 HRC as-quenched; in service usually 28–32 HRC (pre-hard) up to about 50 HRC (tempered 205–260 °C) for wear parts. Induction hardening gives 50–55 HRC on the surface; nitriding gives 55–60 HRC (600–700 HV) with a soft core.
What is the difference between 4140 and 4130?
Carbon: 0.40 % vs 0.30 %. 4130 is weldable with modest preheat and is used for tubing, aircraft and roll-cage structures at up to ~1000 MPa; 4140 reaches higher strength and hardness but must be welded with 250–350 °C preheat and post-weld tempering.
Which is stronger, 1045 or 4140?
Both have ~0.45 % carbon, but 4140's Cr and Mo let it through-harden in sections up to 50–75 mm and temper to a tougher structure. In thin sections both reach similar hardness; in anything thicker than about 20 mm, or at the same tempered hardness, 4140 is stronger and tougher. 1045 is cheaper and slightly more machinable.
Can 4140 be welded?
Only with preheat (250–350 °C), low-hydrogen consumables and post-weld tempering, and preferably in the annealed condition. Welding pre-hard or Q&T 4140 without these steps causes hydrogen cracking in the hard heat-affected zone.
Is 4140 good for knives?
It is used for large choppers and tools where toughness matters more than edge retention, hardened to 52–56 HRC. Its 0.4 % carbon limits edge hardness; dedicated knife steels (1095, 5160, 52100, O1) hold an edge better.
Related grades
Verified against ASTM A29/A29M-20 (SAE J404 chemistry); ASTM A29/A29M-20 Table 2 (G41400); SAE J404 / J1268 (4140H hardenability); ASTM A434/A434M (Q&T bar classes); ASM Handbook Vol. 1 (typical properties). Last checked: September 2026.