Overview
1045 is the standard medium-carbon, through-hardenable carbon steel. The 0.45% carbon content is the sweet spot — enough to develop meaningful martensite on quench (reaching ~58 HRC max), but not so high that the steel becomes brittle in the through-hardened condition.
The selection logic:
- Hardenable. Q&T to any strength level from ~90 ksi UTS (high- temper) to ~285 ksi UTS (low-temper, brittle). Tempering temperature controls the strength-toughness trade-off.
- Cheap. ~$0.90/lb in bar form. Among the cheapest hardenable steels.
- Reasonable machinability. Better in annealed condition; Q&T above ~35 HRC requires harder tooling.
- Through-hardening limited to ~25 mm in water quench, ~12 mm in oil quench. For deeper hardening, use 4140 alloy steel.
The position in the steel hierarchy:
| Need | Choose |
|---|---|
| Hot-rolled structural | A36 |
| Machined low-strength bar | 1018 |
| Through-hardenable carbon | 1045 |
| Spring steel | 5160 or 1095 |
| Tool blade / knife | 1095 |
| Deeper-hardening alloy | 4140 |
| Premium aerospace toughness | 4340 |
| Free-machining (no heat treat) | 12L14 |
| Case-hardening | 8620 |
Heat treatment — the design space
For 1045, the temper specifies the part properties. The same alloy at different tempers behaves like different materials:
| Temper | UTS | Yield | Elong. | Hardness | Typical use |
|---|---|---|---|---|---|
| Annealed | 75 ksi | 50 ksi | 17% | 150 HB | Machining stock |
| Cold-drawn | 85 ksi | 75 ksi | 15% | 200 HB | Standard supply |
| Normalized | 90 ksi | 55 ksi | 18% | 180 HB | Pre-Q&T treatment |
| Q&T 650°C | 110 ksi | 80 ksi | 17% | 220 HB | High toughness shafts |
| Q&T 540°C | 130 ksi | 95 ksi | 14% | 270 HB | Balanced |
| Q&T 425°C | 175 ksi | 145 ksi | 12% | 400 HB | High strength, brittle |
| Q&T 200°C | 285 ksi | 245 ksi | 5% | 58 HRC | Maximum hardness (rarely used) |
For most applications, the typical 1045 supply form is cold-drawn or normalized, machined to near-net shape, then Q&T to 25–35 HRC for the combination of moderate strength + acceptable machinability for finish grinding.
Surface hardening (induction or flame) on 1045 is the standard for shafts requiring hard wear surfaces on bearing journals. Core remains soft and ductile; the surface achieves 54–60 HRC with case depth 1–5 mm typical. Crankshafts, camshafts, axles — the standard treatment.
Machining notes
Annealed/cold-drawn 1045 machines reasonably:
- HSS adequate for low-volume; carbide for production
- Speed: 80–180 SFM (HSS) or 150–350 SFM (carbide)
- Feed: 0.005–0.020 in/rev
- Cutting fluid recommended
Q&T 1045 above ~35 HRC becomes specialized:
- Coated carbide (TiAlN preferred)
- Speed: 50–120 SFM
- Feed: 0.003–0.010 in/rev
- Tool life significantly reduced
Above 50 HRC, machining requires CBN or ceramic tooling. Most 1045 parts are machined soft, heat treated, then finish-ground to final dimension.
Welding considerations
1045 is weldable but requires more care than 1018 due to the higher carbon content. The HAZ has more hardenability — fast cooling produces hard, brittle martensite that can crack.
Mitigation:
- Preheat to 200–300°C before welding sections above ~12 mm
- Use low-hydrogen electrodes (E7018, E8018) to prevent hydrogen cold cracking
- Slow cooling in still air or under insulation
- Post-weld stress relief at 590–650°C for critical welds
For Q&T 1045 assemblies, post-weld heat treatment is essentially mandatory — otherwise the weld zone is over-quenched and brittle. Most 1045 weldments are designed to be:
- Welded in annealed or normalized condition with preheat
- Heat treated as an assembly afterward
- Finish machined to final tolerances
Welding hardened 1045 in service (field repair) requires careful preheat, low-hydrogen practice, and post-weld stress relief. Bringing a Q&T 1045 part back to original properties after weld repair is difficult — design for replacement, not repair.
Through-hardening limitations
1045 has only moderate hardenability. The Jominy curve for 1045 shows surface hardness 58 HRC at the quenched end, dropping to ~30 HRC at 12 mm depth and ~22 HRC at 25 mm depth in standard end-quench test. Practical implications:
- Sections <\12 mm thick — oil quench gives reasonable through- hardening
- Sections 12–25 mm thick — water quench needed for through- hardening (with quench-crack risk)
- Sections >25 mm thick — through-hardening not practical; use alloy steel (4140 has much better hardenability)
The standard alternative for through-hardened thick sections:
| Section | Recommended alloy |
|---|---|
| <\12 mm | 1045 oil quench |
| 12–25 mm | 1045 water quench OR 4140 oil quench |
| 25–75 mm | 4140 oil quench |
| 75–150 mm | 4340 oil quench |
| >150 mm | H-series tool steel or premium alloy |
Corrosion considerations
Same baseline as A36 / 1018 — none. 1045 rusts in any moist environment. Protection options identical to 1018:
- Oil for storage and inter-process handling
- Black oxide + oil for cosmetic finish
- Zinc or cadmium plating for moderate corrosion resistance
- Electroless nickel for industrial wear+corrosion
- Hard chrome for cylinder rods and bearing surfaces
- Paint for outdoor service
Galvanic risks with stainless, copper, and graphite same as A36.
Applications by industry
- General manufacturing — power transmission shafts, axles, spindles, machine components. The default through-hardenable carbon steel.
- Automotive — lightly stressed engine components, axles, forged hardware. Heavily stressed parts use 4140 or 4340.
- Agriculture — agricultural equipment shafts, axles, components. Often induction-hardened for wear surfaces.
- Industrial machinery — gears (light to medium duty), spindles, arbors, machine guideways.
- Bicycles — crank arms, hubs, chainring spider arms (induction hardened for wear).
- Hand tools — hammer heads (drop forged 1045, induction hardened striking face), wrench bodies, hand-tool blanks.
- Sprockets and chains — sprocket teeth (induction or flame hardened), chain components.
- Forklift forks — heavy-duty handling equipment.
Failure modes worth designing around
Quench cracking during heat treatment of thick sections — 1045’s moderate hardenability with high-carbon content creates quench-crack risk in water quench above ~12 mm thickness. Mitigations:
- Use oil quench for sections >12 mm (accept partial hardening)
- Pre-machine to near-net shape before heat treatment
- Provide gradual thickness transitions (avoid sharp section changes)
- Switch to 4140 for thick-section through-hardening
Specifying 1045 for thick-section through-hardening is a common error — designers see “through-hardenable” and assume any thickness will harden. For sections above ~25 mm, use 4140 from the start; 1045 produces soft cores in thick sections.
Hydrogen embrittlement in high-strength Q&T tempers (>1000 MPa UTS / >35 HRC). Pickling, plating, and cathodic protection introduce diffused hydrogen. Standard practice: bake-out at 200°C for 4 hours after plating. Mandatory for critical fasteners and high-strength service components.
Fatigue at machined fillets and sharp transitions — 1045 in high-strength temper conditions is fatigue-sensitive. Generously radius internal corners, finish-machine fatigue-critical surfaces, shot peen for cyclic service.
Through-hardening of welded assemblies without post-weld heat treatment — the weld zone is over-quenched and brittle. PWHT essentially mandatory for welded Q&T 1045 service parts.
Decarburization during heat treatment without protective atmosphere — reduces surface carbon and case hardness. Use protective atmosphere (endothermic) or salt bath for critical parts; alternatively, leave grinding stock and remove decarb after heat treatment.
Surface finish in fatigue service — machined fillets and tool marks are crack initiation sites. Polish fatigue-critical surfaces to 16 Ra or better; shot peen for compressive surface stress.
Brittle fracture in low-temper Q&T condition at low temperature — ductile-brittle transition temperature varies with temper. For sub-zero service, use higher temper temperatures (high-temper Q&T) or alternative alloys (4340 has better cryogenic toughness).