Choosing Steels
Commercially Available Steels for CNC Machining
Material selection doesn't end at strength or modulus tables. In practice, the steel you specify determines how quickly a part can be sourced, how it machines, what heat treatments are realistic, which finishes are compatible, and how predictable the final result will be.
This page focuses on commercially available steels — the grades that machine shops can reliably source, machine, and finish without extended lead times or custom mill orders. The intent is not to catalog every steel that exists, but to highlight the materials designers most often encounter in real-world CNC machining.
What “Commercially Available” Actually Means
Many steel grades exist on paper but are not regularly stocked by distributors. If a material requires a custom mill run, lead times can stretch into months and minimum order quantities may exceed what a custom machine build can justify.
As a general rule:
If a steel grade is not commonly stocked in bar or plate form, it should be assumed to be special order unless confirmed otherwise.
Commonly Stocked Steels
These grades are widely available in North America in a range of sizes and are routinely used in CNC machining.
| Steel | Typical Availability | Practical Notes |
|---|---|---|
| 1018 / 1020 | Widely stocked (bar, plate) | Low carbon, predictable, easy to machine; limited strength without additional processing |
| 4140 | Widely stocked | Strong, versatile alloy steel; responds well to heat treatment; common for shafts and load-bearing parts |
| 4340 | Moderately stocked | Higher strength and toughness than 4140; often used where fatigue resistance matters |
| 12L14 / 11L17 | Widely stocked | Free-machining steels; excellent surface finish and tool life; limited structural performance |
| A36 / Hot-rolled steels | Extremely common | Structural grades; variable chemistry and tolerances; rarely ideal for precision components |
| 4160, 6150, similar alloys | Rarely stocked | Typically require special order and long lead times; usually avoided for short-run or one-off parts |
Design takeaway:
If a grade is unfamiliar to your machine shop or not available from local distributors, it will likely drive schedule and cost more than the material properties justify.
Machining and Finishing Considerations
Steel choice directly affects machinability, achievable tolerances, and finish compatibility.
| Steel Category | Typical Finishing Options |
|---|---|
| Mild steels (1018, 1020, A36) | Black oxide, zinc plating, paint, powder coat, electroless nickel |
| Alloy steels (4140, 4340) | Heat treat + black oxide, nitriding, electroless nickel, selective plating |
| Free-machining steels (12L14, 11L17) | Plating and paint; typically not heat treated |
| Stainless steels (303, 304, 316) | Passivation, electropolishing, polishing; limited hardening options |
Finishing choices should be considered alongside heat treatment and tolerance requirements. Some coatings add material, others convert the surface, and some processes introduce risks such as hydrogen embrittlement or distortion if not planned for.
Heat Treatment in Practice
Not all steels respond meaningfully to heat treatment, and not all parts benefit from being fully hardened.
| Steel | Heat Treatable | Practical Use |
|---|---|---|
| 1018 / 1020 | Limited | Stress relief and annealing only; not suitable for through-hardening. These can be carburized for surface hardness. |
| 4140 | Yes | Commonly quenched and tempered; good balance of strength and machinability |
| 4340 | Yes | High strength and toughness; often used in demanding mechanical applications |
| 12L14 / 11L17 | Not recommended | Not suitable for through hardening. These can be carburized for surface hardness. |
| 303 / 304 / 316 stainless | No | Strength comes from alloying, not heat treatment |
| 17-4 PH stainless | Yes | Precipitation hardened; often chosen when corrosion resistance and strength are both required |
Design takeaway:
When tight tolerances matter, parts are often machined in a softer state and heat treated afterward. This requires planning for distortion, post-treatment finishing, or selective hardening.
Practical Guidance for Designers
- Availability drives feasibility. Choosing a stocked grade often matters more than choosing the “perfect” alloy.
- Machinability affects cost and predictability. Free-machining steels behave very differently from high-strength alloys.
- Heat treatment is a process, not a checkbox. It affects distortion, finish compatibility, and final tolerances.
- Finish and material are linked. Some finishes work best on specific steels or hardness ranges.
- Early conversations help. A short discussion with your machine shop during material selection can prevent weeks of delay later.
Design Takeaway
Steel selection is a systems decision. Strength, machinability, availability, heat treatment, and finishing all interact, and optimizing one at the expense of the others often creates downstream problems.
Designs that move smoothly from CAD to finished parts typically rely on well-understood, readily available materials, specified with a clear understanding of how they will be machined, treated, and finished in practice.