1045 vs 4140: differences explained
1045 vs 4140: same 0.45 % carbon, but 4140's Cr-Mo gives deep hardenability (50–75 mm vs 20 mm), higher Q&T strength (to 1600 MPa) and better toughness. 1045 is cheaper and machines easier. Table and buying guide.
Overview
1045 and 4140 both contain about 0.4–0.45 % carbon, so in thin sections they quench to almost the same hardness — which is why the question 'which is stronger?' has a subtle answer. The difference is hardenability: 4140's 1 % chromium and 0.2 % molybdenum let it through-harden in bars of 50–75 mm, while 1045 hardens only to a depth of about 10 mm and through-hardens only below ~20–25 mm. 4140 also tempers to a tougher structure at the same strength and resists softening at temperature, and it costs 1.5–2× more.
In practice 1045 is the surface-hardened shaft and general medium-strength steel; 4140 is the through-hardened, higher-strength part. The table compares the conditions in which each is actually bought (hot-rolled/cold-drawn 1045, pre-hard 4140) and their heat-treated maxima; the European twins C45 and 42CrMo4 follow the same logic.
1045 vs 4140 side by side
| Property | 1045 | 4140 |
|---|---|---|
| Type | plain medium-carbon steel | Cr-Mo low-alloy steel |
| UNS / EN twin | G10450 / C45 (1.0503) | G41400 / 42CrMo4 (1.7225) |
| Carbon | 0.43–0.50 % | 0.38–0.43 % |
| Alloying | Mn 0.60–0.90 % | Mn 0.75–1.00, Cr 0.80–1.10, Mo 0.15–0.25 % |
| Yield, as supplied | ≈ 310 MPa hot-rolled; ≈ 530 MPa cold-drawn | ≈ 415 MPa annealed; ≈ 690–760 MPa pre-hard |
| Tensile, as supplied | ≈ 565 MPa hot-rolled; ≈ 625 MPa cold-drawn | ≈ 655 MPa annealed; ≈ 930–1000 MPa pre-hard |
| Q&T tensile, 25 mm, 540 °C temper | ≈ 760 MPa (surface); core lower | ≈ 1035 MPa (through) |
| Max. Q&T tensile (thin section) | ≈ 960 MPa (315 °C temper) | ≈ 1770 MPa (205 °C temper) |
| As-quenched hardness | 55–60 HRC (water, surface) | 54–59 HRC (oil) |
| Through-hardening limit | ≈ 20–25 mm (water) | ≈ 50–75 mm (oil) |
| Quench medium | water/brine (cracking risk) | oil (low distortion) |
| Toughness at 30 HRC (Charpy, typical) | ≈ 20–30 J | ≈ 40–50 J |
| Fatigue strength (rotating bend, Q&T) | ≈ 300–350 MPa | ≈ 450–550 MPa |
| Surface hardening | induction/flame 55–58 HRC (excellent) | induction 50–55 HRC; nitride 55–60 HRC |
| Machinability (vs B1112) | ≈ 55–60 % | ≈ 65 % annealed; 55–60 % pre-hard |
| Weldability | limited: 200–300 °C preheat + temper | poor: 250–350 °C preheat + temper |
| Carbon equivalent | ≈ 0.55–0.65 | ≈ 0.80 |
| Temper resistance / hot strength | low | good to ~450 °C |
| Relative price per kg (bar) | 1.00 | ≈ 1.5–2.0 |
| Typical use | TGP shafting, induction-hardened shafts, Grade 5 bolts, gears, hand tools | high-stress shafts, gears, Grade 8 / 10.9 bolts, hydraulic rods, drill collars, tooling |
Which should you choose?
Choose 4140 when the part is thicker than about 20 mm and must be strong or hard through the section, when it sees fatigue or shock, when it runs warm, or when you need a predictable oil quench with low distortion: gearbox shafts, heavy bolts, hydraulic rods, tooling, anything over 1000 MPa.
Choose 1045 when strength is needed only at the surface or in a thin section, and cost matters: induction-hardened shafting, small gears and sprockets, spindles, hand tools, Grade 5 bolts, general machine parts under 25 mm. Its water quench and shallow hardening are fine for a hard skin over a soft core.
The classic error is specifying 1045 for a 40 mm shaft that must be 30 HRC through — it will be hard outside and pearlite inside. The opposite error, using 4140 for a linear shaft that only needs an induction-hardened case, wastes money but at least works.
- Induction- or flame-hardened shafts and rollers
- TGP and chrome-plated linear shafting
- Small gears, sprockets, cams (surface-hardened)
- Bolts to SAE Grade 5 / ISO 8.8
- Hand tools, hammers, wrenches
- Machine parts < 25 mm needing moderate strength
- Through-hardened shafts and gears > 25 mm
- SAE Grade 8, ISO 10.9/12.9 bolts, studs
- Hydraulic cylinder rods and pistons
- Drill collars, tool joints, downhole tools
- Injection-mould holder blocks (pre-hard)
- Fatigue- and shock-loaded parts, hot-service parts
Frequently asked questions
Which is stronger, 1045 or 4140?
4140. In thin sections both quench to ~55–58 HRC, but 4140 keeps that hardness through 50–75 mm bars and tempers to 1000–1600 MPa with better toughness; 1045 through-hardens only to about 20–25 mm and tops out around 960 MPa in thin sections. As supplied, pre-hard 4140 (≈ 950 MPa) is far stronger than cold-drawn 1045 (≈ 625 MPa).
Is 1045 the same as 4140?
No. They share the ~0.45 % carbon level, but 4140 adds 1 % chromium and 0.2 % molybdenum, which change hardenability, toughness and price. 1045 = EN C45; 4140 = EN 42CrMo4.
Can 1045 replace 4140?
Only for parts thinner than ~20 mm that need surface hardness rather than through-strength, and that are not fatigue- or shock-critical. For a through-hardened shaft or a Grade 8 bolt, no.
Which is easier to weld, 1045 or 4140?
Neither is easy, but 1045 (CE ≈ 0.6) is somewhat less critical than 4140 (CE ≈ 0.8). Both need preheat (200–300 °C for 1045, 250–350 °C for 4140), low-hydrogen filler and post-weld tempering; weld before hardening.
Which machines better?
Roughly equal; 1045 hot-rolled rates 55–60 %, annealed 4140 about 65 % and pre-hard 4140 55–60 %. Pre-hard 4140 gives the best surface finish and chip control with carbide.
What about C45 vs 42CrMo4?
Same comparison in EN terms: C45 (1.0503) ≈ 1045 and 42CrMo4 (1.7225) ≈ 4140. EN 10083 gives the same picture — 42CrMo4 +QT 900–1300 MPa versus C45 +QT 630–850 MPa at ≤ 16 mm.
Related comparisons
Verified against ASTM A29/A29M-20; EN 10083-2/-3:2006. Last checked: September 2026.