Steel
Steel is the backbone of machined parts. But "steel" means nothing without a grade — 1045 and D2 behave nothing alike. This page helps you pick the right grade, know what heat treatment to specify, and avoid the expensive mistake of over-specifying hardness you can't actually achieve.
Which Steel Do You Need?
Start here. Your answer determines material cost, machining time, and whether heat treatment is required.
| Your Situation | Use This | Why |
|---|
| General shafts, pins, brackets | 1045 | Cheapest carbon steel. Adequate for most non-critical parts. H&T to 25–35 HRC. |
| Gears, axles, high-load parts | 4140 | The workhorse alloy steel. Good through-hardening, decent machinability. H&T to 28–38 HRC. |
| High-strength structural parts | 4340 | Higher hardenability than 4140. Used when 4140 can't reach the needed core hardness. 40–50 HRC. |
| Cutting tools, punches, dies | D2 | High-carbon high-chromium tool steel. 58–62 HRC. Wear-resistant but brittle. |
| Die casting molds, forging dies | H13 | Hot-work tool steel. Maintains hardness at 500°C+. 44–52 HRC. |
| Corrosion resistance + hardness | 420 SS | Stainless with decent hardenability. 40–50 HRC. Surgical instruments, pump shafts. |
| Welded assemblies | 1045 or 4140 | Preheat required for 4140 (200°C+). D2 and H13 are not weldable. |
| Budget priority | 1045 | Cheapest steel. Fastest machining. Widest availability. |
Steel Data at a Glance
| Property | 1045 | 4140 | 4340 | D2 | H13 | 420 SS |
|---|
| Category | Carbon | Alloy | Alloy (NiCrMo) | Tool (high Cr) | Hot-work | Martensitic SS |
| Tensile (MPa) | 585 (annealed) | 655 (annealed) | 745 (annealed) | — | — | 655 (annealed) |
| Hardenability | Low | Medium | High | Very high | High | Medium |
| Max HRC (H&T) | 35 | 38 (q: oil) | 50 (q: oil) | 62 (q: oil/air) | 52 (q: air) | 50 (q: air/oil) |
| Quench medium | Water | Oil | Oil | Oil / Air | Air | Air / Oil |
| Machinability | Good | Good | Fair | Poor (before HT) | Fair | Fair |
| Weldable | Yes | Preheat req. | Preheat req. | No | No | Preheat req. |
| Density (g/cm³) | 7.85 | 7.85 | 7.85 | 7.70 | 7.80 | 7.80 |
| Relative cost | 1.0x | 1.3–1.8x | 1.5–2.0x | 3–5x | 3–5x | 2–3x |
Heat Treatment Results
Steel without heat treatment is just... steel. Most machined parts need some form of H&T. The table below shows what to expect.
| Steel | Quench | Temper (°C) | Result (HRC) | Result (HB) | Application |
|---|
| 1045 | Water | 400–550 | 25–35 | 255–320 | Shafts, pins, brackets |
| 4140 | Oil | 400–600 | 28–38 | 270–350 | Gears, axles, couplings |
| 4340 | Oil | 200–430 | 40–50 | 380–480 | High-strength bolts, shafts |
| D2 | Oil / Air | 200–300 | 58–62 | — | Cutting tools, forming dies |
| H13 | Air | 500–600 | 44–52 | — | Die casting molds, hot forging |
| 420 SS | Air / Oil | 200–400 | 40–50 | — | Surgical tools, valve stems |
Hardenability trap
1045 water-quenches, but only achieves through-hardening on small cross-sections (<20mm). For thicker parts, the core stays soft. If you need 35 HRC through a 50mm shaft, you need 4140 (oil quench), not 1045. This is the most common steel selection error.
4140 vs 4340 — When to Upgrade
4140 handles 80% of alloy steel applications. But 4340 has nickel, which gives deeper hardenability and better toughness at high hardness levels.
| Requirement | Choose | Reason |
|---|
| General gear/shaft (most cases) | 4140 | Cheaper, good enough, widely stocked |
| Large cross-section (>50mm) needing high core hardness | 4340 | Nickel gives deeper hardenability |
| Impact resistance at 45+ HRC | 4340 | Notch toughness is significantly better |
| Cost-sensitive | 4140 | 20–30% cheaper than 4340 |
Tool Steel: D2 vs H13
D2 and H13 are both expensive, both wear-resistant, but used for completely different applications.
| Property | D2 | H13 |
|---|
| Type | Cold-work tool steel | Hot-work tool steel |
| Hardness | 58–62 HRC | 44–52 HRC |
| Wear resistance | Excellent (high Cr) | Good |
| Toughness | Low (brittle) | Good |
| Red hardness | Poor (softens >300°C) | Excellent (holds to 500°C+) |
| Machining (annealed) | Poor (abrasive carbides) | Fair |
| Use for | Blanking dies, punches, shear blades | Die cast molds, forging dies, extrusion |
Do NOT use D2 for impact-loaded parts
At 60 HRC, D2 is hard but brittle. Under impact, it chips and cracks. If your tool will see shock loads, use A2 or S7 instead. D2 is for sliding wear, not impact.
Procurement Tips
| Tip | Detail |
|---|
| Specify condition | "4140" is incomplete. Say "4140 annealed" (for machining then H&T) or "4140 Q&T to 30–35 HRC" (pre-treated). Pre-treated saves time but harder to machine. |
| Request MTR | Material test report for critical parts. Verify chemistry matches the spec. |
| Chinese domestic steel | Domestic 4140 is usually good quality (45CrMo4 equivalent). D2 and H13 quality varies — verify with the supplier. For tool steels, import (Bohler, Uddeholm) is often worth the premium. |
| Pre-machined vs pre-treated | Order annealed if you need to machine first, then H&T. Order pre-treated if dimensions are simple and you just need grinding/finishing. Pre-treated material machines 2–3x slower. |
| Size availability | 1045 and 4140 available in any size. D2 and H13 often limited to standard plate/bar sizes. Allow extra lead time for non-standard sizes. |
Common Mistakes
| Mistake | What happens | Correct approach |
|---|
| Specifying 1045 for thick parts needing through-hardening | Core stays soft, part fails in service | Use 4140 or 4340 for cross-sections >20mm |
| Specifying 60 HRC on 4140 | Can't achieve it consistently — max is ~38 HRC | For 60 HRC you need D2, not 4140 |
| Quenching D2 in water | Cracking. D2 must be oil or air quenched. | Follow the correct quench medium for each grade |
| Using D2 for impact tools | Chipping and cracking under shock | Use S7 or A2 for impact applications |
| Welding 4140 without preheat | HAZ cracks due to hardenability | Preheat to 200–250°C minimum before welding |
| Not leaving grinding stock after H&T | Quench distortion makes part out of tolerance | Leave 0.2–0.5mm grinding allowance on critical surfaces |
| Ordering pre-hardened for complex machining | Tool wear triples, cycle time doubles | Order annealed, machine, then H&T |