1018 and Low-Carbon Steel CNC Machining

The mild steel family

The 1008–1018 range covers the low-carbon steels, and the whole family shares one defining limitation: there is not enough carbon in any of them to harden through the section.

Grade	Carbon	Character
1008	~0.10% max	Softest and most formable; chosen for deep drawing and severe forming
1010–1015	~0.10–0.15%	General forming and light machining
1018	~0.15–0.20%	The general machining, welding and case-hardening grade
1020	~0.20%	Marginally stronger, otherwise similar

What the family buys you is everything except hardness:

• It is the cheapest steel we machine, which matters on any large part
• It welds more easily than any other steel family — low carbon means no hardening in the weld zone, so no preheat and no cracking risk
• It forms and bends freely, because it is ductile
• It machines reasonably in the cold-drawn condition, though never well

1018 is the grade in the family that turns up most often on a drawing, because it balances formability, weldability and machinability better than its neighbours. The rest of this page is about 1018 and about where the family’s limits apply.

Case hardening: hardness where you need it

Low carbon rules out through hardening, but it makes case hardening work well — and case hardening is a genuinely useful treatment rather than a consolation prize.

The part is heated in a carbon-rich atmosphere at roughly 900–930 °C so that carbon diffuses into the surface, then quenched. The result is a hard skin, typically in the region of 58–62 HRC, over a core that stays soft and tough. Case depth is a design variable, commonly somewhere between a fraction of a millimetre and about 1.5 mm depending on the application and the time at temperature.

That combination — hard where it wears, tough where it takes load — is exactly what a gear tooth, a cam follower, a pivot pin or a chain link needs. A through-hardened part of the same hardness would be brittle throughout and would fail by cracking; a case-hardened part wears at the surface and absorbs shock in the core.

Two consequences for the drawing:

• Specify case depth and surface hardness, not a core hardness. The core follows from the material.
• Plan for distortion. Carburising and quenching move dimensions, and thin or asymmetric sections warp. The route is rough machine, case harden, then grind or take light finishing cuts. Where the case is shallow, grinding stock has to be kept smaller than the case depth, or the hard layer is ground away and the part fails its own specification.

For parts that need to be hard all the way through rather than at the surface, the answer is 1045. Our heat treatment page covers how these cycles are arranged.

Machining 1018

Low-carbon steel with no free-machining addition is the least pleasant material in the steel group to cut, and it is worth being precise about why.

The chip does not break. Ductile steel produces a long continuous ribbon. It tangles around the tool and the workpiece, it is a handling hazard, and on a finishing pass it drags across the surface that was just produced and scratches it.

The edge builds up. 1018 tends to weld to the cutting edge, particularly at lower speeds. The built-up material rounds the edge geometry, degrades the finish, and then breaks away unpredictably, often taking part of the edge with it.

It tears rather than shears. At parameters that suit a harder steel, low-carbon steel smears and tears instead of cutting cleanly, which shows up as a rough finish and a poor surface for any subsequent coating.

What works:

• Sharp, positive-geometry tooling with a chipbreaker matched to the depth of cut — on a grade with no free-machining addition, geometry does most of the work
• Adequate feed per tooth, so the edge cuts under the deformed layer instead of burnishing it
• Flood coolant, for temperature and for chip evacuation
• Rigid workholding, especially on thin plates and long slender parts
• Cold-drawn stock, which starts you at a better surface and tolerates more

The sourcing point is worth repeating because it changes the quotation: cold-drawn bar machines considerably better than hot-rolled, and it carries a smooth, accurate, scale-free surface into the first operation. Hot-rolled bar has mill scale and a decarburised skin that must be removed before the part is sound. Our hot rolled vs cold rolled article sets out the difference and when each is worth specifying.

Where the part is a turned item at volume and does not need to be hard or welded, the free-cutting grades are a different quotation entirely: 1215 and 12L14 cut several times faster and produce a far better finish. Where the part must be welded, 1018 is the right answer and the machining cost is simply part of it.

Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold.

Where the 1008–1018 family is the right choice

• Weldments with machined details — frames, bases, brackets, mounting plates
• Parts that will be case hardened — pins, gears, cam followers, wear plates, chain components
• Low-stress shafts, spacers and bushings that do not need hardness
• Formed and bent components where ductility is the requirement
• Large parts where material cost dominates, and the cheapest steel that does the job is the right one
• Any part where the alternative would be an alloy steel used far below its capability

Where it is the wrong choice

The part must be hard through the section. Case hardening gives a hard skin, not a hard core. Where the whole section has to carry load at hardness, the grade is 1045.

The part is a turned item in volume. The machining penalty on a low-carbon steel with no free-machining addition is the largest single cost difference in the carbon steel group. Where welding is not required, a free-cutting grade will cut the price substantially.

The part has to resist corrosion. No chromium, so it rusts. Where resistance is needed without a coating, use 304 or 316.

A fine finish is specified straight off the machine. 1018 will not produce what 12L14 produces on the same operation, and where finish is the requirement that gap decides the grade.

Very high strength. The family tops out well below what a medium-carbon or alloy steel provides in the hardened condition.

Finishing low-carbon steel

The finish is functional — protection against rust, not decoration.

• Zinc plating — the usual choice for brackets, fasteners and hardware. Sacrificial, so it protects the steel even where the coating is scratched
• Powder coating — a thick, durable colour finish for visible parts and fabricated assemblies
• Black oxide — a uniform dark finish that carries a light oil film, common on tooling and machine hardware
• Nickel plating — a harder, brighter barrier where appearance or wear resistance matters
• Sand blasting — a uniform matte surface before coating, or as a finish in its own right

All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.

What to put on the drawing

Instead of	Write
Steel	1018, and say whether it is cold drawn or hot rolled
1018, harden through	Wrong treatment — case harden, or use 1045
1018, case hardened	State surface hardness and case depth, not core hardness
1018, case hardened and ground	Keep grinding stock below case depth, or the case is ground away
1018, turned part in volume, no welding	1215 or 12L14 machines several times faster
1018, exposed to weather	State the finish — it rusts unprotected
1018, fine finish off the machine	12L14 produces a better finish on the same operation

The three lines that decide most low-carbon steel specifications are whether the part is welded, whether it needs a hard surface, and whether it is turned in volume. Those answers point to 1018, to a case-hardening route, or to a free-cutting grade — and getting them wrong is usually the difference between a competitive quotation and an uncompetitive one.

Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade to its mill heat plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the case depth and grinding allowance where case hardening applies.