CNC Machining Carbon & Alloy Steel
1018, 1045, 4140, 4340, A2 and D2 machined, heat treated and finished — for parts where strength, hardenability or wear life matters more than corrosion resistance.
- Tolerances to ±0.01 mm on critical features
- Heat treatment arranged with the sequence
- Black oxide, zinc and nickel plating
Quick answer
Carbon and alloy steels are the workhorse materials for machined parts that need strength, wear resistance or hardenability at a reasonable price. Unlike stainless steel, they are not corrosion resistant on their own — they rely on a finish such as black oxide, zinc plating or nickel plating. Choose them when strength or wear life matters more than appearance, and specify the finish at the same time as the material.
We quote carbon and alloy steel work within 48 hours in the normal case and handle heat treatment and finishing as part of the same job.
Common grades
From free-machining 1215 to D2 tool steel at 62 HRC.
| Grade | Type | Condition | Best for |
|---|---|---|---|
| 1018 | Low carbon | Cold drawn | General purpose pins, brackets, shafts; excellent weldability, low cost |
| 1045 | Medium carbon | Hot rolled or cold drawn | Shafts, bolts, couplings; stronger than 1018, can be flame or induction hardened |
| 1144 (stressproof) | Free machining | Cold drawn | High-volume turned parts; machines fast with good strength |
| 1215 | Free machining | Cold drawn | Very fast machining, low cost; for non-critical parts |
| 4140 | Chromoly alloy | Annealed or pre-hardened | High-strength shafts, couplings, tooling; through-hardens well |
| 4340 | Nickel-chromoly | Annealed | Higher toughness than 4140 at similar strength; heavily loaded parts |
| A2 | Air-hardening tool steel | Annealed, hardened to 57 – 62 HRC | Dies, punches, gauge blocks; minimal distortion in heat treatment |
| D2 | High-carbon tool steel | Annealed, hardened to 58 – 62 HRC | High wear resistance; cutting tools, forming dies, wear strips |
| O1 | Oil-hardening tool steel | Annealed, hardened to 58 – 62 HRC | General tooling where A2 is not required |
How the grades compare
Strength and wear resistance are different requirements — pick the one your part actually needs.
| Grade | Tensile strength (annealed) | Hardness | Machinability | Weldability | Relative cost |
|---|---|---|---|---|---|
| 1018 | ~440 MPa | ~130 HB | Good | Excellent | $ |
| 1045 | ~570 MPa | ~170 HB | Good | Fair | $ |
| 1144 | ~640 MPa | ~190 HB | Very good | Poor | $$ |
| 4140 | ~655 MPa | ~200 HB | Fair | Fair — preheat required | $$ |
| 4340 | ~745 MPa | ~220 HB | Fair | Poor — preheat and post-heat | $$$ |
| A2 / D2 | Up to ~2000 MPa hardened | 57 – 62 HRC | Poor when hardened | Poor | $$$ |
Machining Capability
Capacity across the carbon and alloy range, including heat-treated work.
Work Envelope
Up to 1000 mm
On milling and turning centres.
Finishing
Oxide · Zinc · Nickel
Black oxide, zinc, nickel, phosphate and powder coating.
Lead Time
2 – 4 weeks
Add time for heat treatment and plating.
Heat treatment options
Heat treatment is what turns an inexpensive alloy into a high-performance part — we plan the machining sequence around it.
| Treatment | What it does | Typical result | Used for |
|---|---|---|---|
| Annealing | Softens the material so it can be cut | ~150 – 200 HB | Preparing 4140, 4340 and tool steel for machining |
| Stress relieving | Removes internal stress without changing hardness | Hardness unchanged | Large parts and welded fabrications before finish machining |
| Through hardening | Quench and temper for uniform strength | 28 – 60 HRC depending on grade | 4140 and 4340 shafts, couplings, heavily loaded parts |
| Case hardening | Hard surface with a tough core | 58 – 62 HRC case | Cams and wear surfaces on low-carbon steel |
| Nitriding | Hard case at low temperature, minimal distortion | 55 – 70 HRC case | Finished parts that must not move afterwards |
Design tips for carbon and alloy steel
The usual sequence is rough machine, heat treat, then finish — designing for it saves money.
- Specify the finish at the same time as the material. Bare carbon steel rusts; zinc, black oxide or nickel costs little and prevents a failure in transit or in service.
- Machine 4140 and 4340 in the annealed condition where possible. Pre-hardened stock cuts noticeably slower and wears tools faster.
- Plan for heat-treat movement. Hardening changes dimensions slightly — if a feature must be exact afterwards, finish it after heat treatment, not before.
- Allow a larger corner radius on alloy steels. They work-harden and are tougher than 1018; sharp-cornered tools wear quickly.
- Consider 1144 or 1215 for high-volume turned parts. Faster cycle times and better chip breaking usually beat the slightly higher material price.
- Do not weld 1144 or 1215 — their sulfur content makes them crack. If the assembly is welded, specify 1018 or 1020.
- Tool steel costs more to machine than to buy. If the geometry allows, machine D2 annealed and let us handle hardening plus a final grind.
Frequently Asked Questions
Answers to the questions we get asked most about this process.
Carbon steel or stainless steel?
If the part sees moisture, chemicals or food contact, use stainless — see our comparison of 304 and 316. If it lives inside a machine, in oil, or in a dry environment and needs strength or wear resistance, carbon or alloy steel plus a finish is usually stronger and cheaper.
Will it rust?
Bare, yes, fairly quickly in humid conditions. Black oxide slows it but is not a barrier on its own; zinc or nickel plating protects properly. Tell us the service environment and we will recommend a finish.
Can you supply heat-treated parts?
Yes. Through hardening, case hardening and stress relieving are all arranged as part of the job, with the machining sequence planned around the treatment.
What is the strongest grade you machine?
4340 at high strength, or A2 and D2 tool steel at 58 – 62 HRC for wear resistance. Strength and wear resistance are different requirements — tell us which one the part actually needs and we will recommend accordingly.
Can you machine a part after it has been hardened?
Do you provide material certificates and inspection reports?
Material certificates come with the stock from the mill and can be supplied with your order. Dimensional inspection reports are available on request.
What about prototyping in steel?
Same process, no minimum quantity. See prototyping and low-volume production for lead times on one-off and short-run work.
Typical applications
Shafts, pins and couplings for automation equipment, structural brackets and machine frames, jig and fixture bodies, wear strips and guide components, tooling and die parts, and low-volume production parts where stainless would be over-specified.
Ready to Start Your Project?
Send drawings in STEP, IGES or DWG. You get a priced quote within 48 hours in the normal case, with tolerance and lead time stated up front.