I machine carbon steel every week — brackets, shafts, housings, fixturing, tooling plates, hydraulic manifolds. It's not glamorous. Nobody writes press releases about 1045 steel. But when an engineer sends me a drawing and asks "can you make this in something cheaper than stainless?", carbon steel is usually the answer.

Carbon steel costs less than stainless, machines faster than titanium, and welds better than aluminum. The catch is rust. You have to deal with surface treatment. But if your part lives in oil, paint, or a controlled environment, carbon steel gets the job done for half the material cost of 304 stainless.

Here's what I've learned machining the grades that show up on real drawings — not the textbook grades, the ones engineers actually specify.

The five carbon steel grades that show up on real drawings

1018 — the default mild steel

If a drawing just says "steel" and doesn't specify a grade, 1018 is usually what they mean. Low carbon (0.18%), weldable, machines easily. It's the 6061 aluminum of the steel world — the general-purpose baseline.

Machinability: 78% compared to 1212 (the benchmark). It cuts clean, breaks chips reasonably well, and doesn't work-harden. You can run carbide tooling at 350-450 SFM with 0.006-0.012 IPT (inch per tooth).

The downside is surface finish. 1018 has a slight gummy quality when turning — it tears more than it cuts if the tool isn't sharp. For a smooth finish, keep the feed up and the speed moderate. A light finish pass at higher speed cleans up any tearing.

I use 1018 for fixturing, brackets, mounting plates, spacers, and anything structural that doesn't see high stress or wear. If the part is getting painted or powder coated, 1018 is the obvious choice.

1045 — medium carbon, the shaft material

Step up from 1018 with 0.45% carbon. Stronger, harder, and more wear-resistant. This is the classic shaft material — you'll see it on motor shafts, pump shafts, rollers, pins, and anything that spins under load.

Machinability: 57% — significantly tougher than 1018. The higher carbon content makes it harder and more abrasive. Tool life is shorter. Surface speed drops to 250-350 SFM. You need to feed aggressively enough to stay ahead of the work hardening, but not so hard that you break the tool.

1045 heat-treats well. Induction hardening the OD of a shaft while keeping the core tough is standard practice. If your drawing calls for a hardened wear surface, 1045 is usually where to start looking.

The one thing to watch: 1045 doesn't weld as nicely as 1018. The higher carbon content means preheat and post-weld stress relief become important above certain thicknesses. If welding is a major part of your manufacturing process, stay with 1018 or drop to a dedicated weldable grade.

4140 — the alloy steel workhorse

4140 is chromium-molybdenum alloy steel. Heat-treatable to high strength (RC 28-32 is common, up to RC 50+ if you push it). Used for gears, axles, bolts, tool holders, and anything that needs to be both strong and tough.

Machinability: around 55% in annealed condition — harder than 1018 but predictable. In hardened condition, machinability drops and you're looking at carbide-only tooling at conservative speeds. For parts that need machining after heat treat, staying at RC 32 or below is workable. Above RC 35, every operation becomes a negotiation with the material.

The big advantage of 4140 over plain carbon steels is hardenability. 1045 only hardens at the surface. 4140 hardens through the cross-section (within limits). For a 50mm diameter shaft, 4140 quenched and tempered gives consistent properties all the way through. 1045 gives a hard case and a soft core — which is sometimes what you want, but not always.

4140 also machines with a better surface finish than 1018 or 1045. The alloying elements reduce the gummy tearing behavior. If you need a smooth steel surface right off the machine, 4140 is a better bet than 1018.

8620 — case-hardening steel

Low-carbon alloy steel (0.20% C) with nickel, chromium, and molybdenum. Designed for case hardening — carburize the surface to RC 58-62 while the core stays tough at RC 30-40. Gears, cam followers, pins, rollers, and anything that needs a hard wear surface on a tough core.

Machinability: 66% in annealed condition. Cuts similarly to 4140 — not as easy as 1018, not as tough as 1045. The nickel makes it slightly gummier than 4140 but still manageable.

The post-machining process defines 8620. After machining, the part goes through carburizing (carbon-rich atmosphere at roughly 900°C for several hours), then quench and temper. The surface gets hard and wear-resistant. The core stays tough and impact-resistant. You can't get this combination from a through-hardening steel.

The catch is that 8620 after case hardening can't really be machined — the surface is too hard. All machining happens in the annealed state, then heat treat is the final step. Grinding after heat treat handles final dimensions on critical features.

A36 — structural steel, not for machining

A36 is the hot-rolled structural steel you see in buildings and bridges. I mention it because engineers sometimes write "steel" on a drawing meaning A36 — and that's usually the wrong call for a machined part.

A36 is inconsistent. The chemistry spec is loose — whatever comes out of the melt shop within a broad range. One batch machines fine. The next batch from a different heat has hard spots that eat carbide inserts. Surface finish on A36 is unpredictable because the material isn't homogeneous.

If your part is a welded frame that gets a few drilled holes, A36 is fine. If it's a precision-machined component with tolerances under ±0.05mm, specify 1018 or better. The twenty cents per pound you save on A36 over 1018 disappears the first time you scrap a part to a hard spot.

Machining data: how carbon steel compares

Here's what I actually run on the floor. These are production numbers for a medium-complexity part — roughly 100mm × 60mm × 30mm with pockets, holes, and threading:

Grade Cutting Speed (SFM) Feed (IPT) Cycle Time vs 1018 Tool Life vs 1018 Relative Cost
1018 350-450 0.008-0.012 1.0× 1.0× 1.0
1045 250-350 0.006-0.010 1.25× 0.7× 1.3
4140 annealed 200-300 0.006-0.010 1.4× 0.6× 1.6
4140 RC 28-32 120-180 0.004-0.008 2.2× 0.3× 2.8
8620 annealed 250-350 0.006-0.010 1.3× 0.65× 1.4
A36 200-300 0.005-0.008 1.5× 0.5× 1.5

Relative Cost factors material + cycle time + tooling + scrap risk, normalized to 1018.

For context, the same part in 304 stainless runs about 2.0× the cycle time of 1018 and costs about 2.5× overall. 6061 aluminum machines roughly twice as fast as 1018 and costs about 0.6×. So the quick reference: 6061 is cheapest overall, 1018 is next, then carbon steels scale up from there — hardened 4140 can cost more than annealed stainless.

The rust problem and how to solve it

Carbon steel rusts. There's no chromium to form a passive oxide layer like stainless. Leave a 1018 part on the bench for a week in humid air and you'll see orange spots. In a marine or outdoor environment, rust appears in hours.

The fix is surface treatment. Here are the common options:

Black oxide. Thin (1-2μm), matte black finish. Minimal corrosion protection. Mostly cosmetic — it looks good on fixturing and internal tooling. Can be oiled for improved protection. Cheap. Not for outdoor use.

Zinc plating. Sacrificial coating — the zinc corrodes instead of the steel. Thickness 5-25μm. Common for fasteners, brackets, hardware. Options: clear (silver), yellow, black. Good indoor and moderate outdoor protection. Cost: moderate. This is the most common finish I see on commercial carbon steel parts.

Electroless nickel plating. Uniform thickness regardless of part geometry because it's chemical deposition, not electroplating. 25-75μm typical. Excellent corrosion resistance, decent hardness (RC 48-52 after heat treat). More expensive than zinc but delivers better protection on complex geometries with blind holes and recesses.

Zinc phosphate + oil. Porous crystalline coating that holds oil. Good wear resistance during break-in. Common on gears, bearing surfaces, and internal engine or transmission parts. Not for external corrosion protection.

Powder coating or paint. Thick barrier coating. Good for external, structural, and architectural parts. Not for precision surfaces — the coating is too thick (50-150μm) for close-tolerance fits.

One thing I tell every engineer: plan the coating before you machine. Coatings add thickness. Electroless nickel at 25μm per side means a 50μm diameter hole gets about 50μm tighter. Threads need to be cut to accommodate the coating. Masking threaded holes from coating is sometimes easier than adjusting thread dimensions — but specify it on the drawing.

When to use carbon steel instead of stainless or aluminum

Carbon steel isn't always the right answer. Here's my decision framework:

Use carbon steel when:

Use stainless steel when:

Use aluminum when:

I've made plenty of parts where the engineer started with 4140, we looked at the requirements, and realized 1018 with zinc plating would work fine at 40% lower cost. I've also had the reverse — trying to save money with carbon steel, only to find out the coating process added more cost than just using 304 stainless from the start. For small, simple parts, the coating setup cost can outweigh the material savings.

What to put on the drawing

Four things your shop needs to see for any carbon steel part:

  1. The grade. "Steel" on a drawing is like "plastic" — it doesn't narrow things down. Write "1018 steel" or "4140 annealed" or "8620 carburized RC 58-62 case, 0.5mm min depth."

  2. The heat treat condition. Annealed? Quenched and tempered? Case hardened? If it's post-machining heat treat, call it out so the shop plans for distortion and stock allowance.

  3. The surface treatment. What coating, what thickness, what specification. "Zinc plate per ASTM B633, Type II, 8μm min" is actionable. "Rust protection" is not.

  4. Critical surfaces that must stay coating-free. If a bearing bore or thread needs to be masked from coating, mark it clearly. It's easier to mask before coating than to chase threads after.

Bottom line

Carbon steel isn't exciting, but it shows up on more drawings than any other metal family. For the right application — coated, oil-bathed, heat-treated — it does the job for a fraction of what stainless costs.

The key decisions happen before the drawing is finalized: which grade, which heat treat, which coating. Change any one of those and the part cost can swing 30-50%.

Send us your drawing with the operating environment and mechanical requirements. We'll recommend a grade and surface treatment, and tell you honestly if carbon steel is the right call — even when the answer is "you should use stainless for this one."