1045 Carbon Steel CNC Machining

Enough carbon to harden, not enough to be an alloy steel

Carbon content is what separates the plain carbon steels into three working groups, and 1045 sits in the middle one.

Group	Carbon	Hardenable?	Typical grades
Low carbon	Up to ~0.25%	Only at the surface, by case hardening	1008, 1018
Medium carbon	~0.25–0.60%	Yes — through hardening and induction	1045
High carbon	Above ~0.60%	Yes, and to high hardness	1095

At roughly 0.45% carbon, 1045 has enough carbon to form martensite when it is quenched, and enough to respond to induction heating on a local surface. That is the whole commercial reason for the grade: it is the cheapest steel that can be made hard, in bulk, in the section sizes most engineering parts actually use.

Everything else about the material follows from the fact that there is nothing else in it. No chromium, no molybdenum, no sulphur, no lead. That keeps the price down and makes 1045 easy to source in bar, plate and forgings — and it is also why it machines moderately, rusts freely and has limited hardenability.

How 1045 is hardened

Two routes are used, often on the same part.

Through hardening. The part is austenitised at roughly 815–845 °C, held long enough to dissolve the carbides, then quenched — water, polymer or oil depending on section and the hardness required — and tempered. As-quenched it reaches roughly 55 HRC and above in thin sections; it is then tempered back to a working range, typically somewhere between about 25 and 50 HRC depending on whether the requirement is toughness, wear resistance or a balance.

Induction hardening. A coil heats a band of the surface, which is then quenched immediately. Only the surface transforms, so the wear face goes hard while the core stays tough and shock-resistant. This is the standard treatment for a shaft journal, a spline, a cam face or a wear collar, and it is the single most common reason 1045 turns up in a drawing package.

Hardenability is the limit. 1045 has no alloying element to slow the transformation during quenching, so how deep it hardens depends almost entirely on how fast the section cools. A thin part hardens clean through; a heavy one can come out with a hard skin over a soft core, or fail to harden at all at the centre. Where a large part must be hard throughout, the answer is an alloy steel with better hardenability rather than a heavier 1045 section — our 1045 vs 4140 comparison sets out where that line falls in practice. Our heat treatment page covers how the cycle is arranged.

Machining 1045

1045 is cut in the annealed, normalised or cold-drawn condition — before hardening — and it machines at a moderate rate: considerably faster than any stainless, considerably slower than a free-machining grade.

The characteristic problems are worth naming, because they are what a quotation reflects:

• Stringy chips. With no sulphur addition the chip does not break on its own. Long ribbons tangle around the tool and the workpiece, interrupt the cut and, on a finishing pass, scratch the surface they just produced.
• Built-up edge. 1045 tends to weld to the tool edge, particularly at lower cutting speeds, and the built-up material degrades the finish and grows until it breaks away and takes part of the edge with it.
• Moderate tool life. It is abrasive enough to wear edges without being hard enough to need special tooling.

What the material rewards:

• A sharp, positive-geometry insert with a chipbreaker matched to the depth of cut — on a grade with no free-machining addition, geometry does most of the work
• Adequate speed and feed, so the edge cuts cleanly under the built-up layer rather than rubbing over it
• Flood coolant, for both temperature and chip evacuation
• Rigid workholding, particularly on long slender shafts, where deflection is a dimension problem before it is a finish problem

One sourcing detail matters more than buyers expect: cold-drawn (CD) bar machines better than hot-rolled (HR). Cold drawing leaves a smooth, accurate, scale-free surface with better machinability, so the first cut is productive rather than a cleanup pass. Hot-rolled bar carries mill scale and a decarburised skin, both of which have to be removed. The difference between the two forms and when each is worth specifying is set out in our hot rolled vs cold rolled steel article.

For a part that is turned in real volume and does not need to be hard, the free-machining grades are a different quotation altogether — 1215 and 12L14 cut several times faster. Where hardness is the requirement, 1045 is the economical 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.

The sequence on a hardened part

This is the part of a 1045 job that most often goes wrong, and it is entirely avoidable.

Quenching puts a part through two things at once: a rapid thermal cycle from above 800 °C down to room temperature, and a martensitic transformation that changes the crystal structure and therefore the volume. Both move dimensions, and thin, long or asymmetric sections warp on top of that.

So the route is fixed:

1. Rough machine, leaving grinding or finishing stock distributed where the distortion will actually appear
2. Harden and temper to the specified condition
3. Finish — grind, or take light machining passes, to bring critical dimensions in
4. Inspect after finishing, not before

The failure mode is a part cut to ±0.01 mm in the soft condition, approved by inspection, then hardened — and then out of tolerance everywhere. Where a drawing calls for both a tight tolerance and a hardened condition, saying which dimensions actually matter at the enquiry is what lets the allowance be placed correctly rather than guessed.

Where 1045 is the right choice

• Shafts, axles and spindles, particularly with induction-hardened journals
• Gear blanks and couplings that will be cut and then hardened
• Pins, tie rods and studs in load-bearing service
• Wear collars, bushes and rollers that need a hard surface over a tough core
• General engineering parts that need to carry load and take some wear, at the lowest material cost that does the job
• Large turned parts where the section is heavy enough that 1045’s hardenability is not the binding constraint

Where 1045 is the wrong choice

Heavy sections that must be hard to the core. Limited hardenability is the grade’s main weakness. Where a large part needs uniform hardness through the section, an alloy steel with chromium and molybdenum is the correct answer.

Any part that has to resist corrosion. 1045 has no chromium at all. It rusts in damp air and it will rust in transit without an oil film or a coating. Where corrosion resistance is required without a coating, the answer is 304 or 316.

High-volume turned parts that do not need to be hard. The free-machining grades cut several times faster for the same geometry. On a screw, a fitting or a bushing in mild service, 1045 is paying a machining penalty for hardening capability that is never used.

A welded fabrication. The carbon content that makes 1045 hardenable also makes its weld zone hard and crack-prone. Where welding is structural rather than incidental, a low-carbon grade is the better starting point.

A fine surface finish straight off the machine. 1045 with no free-machining addition will not match the finish that 12L14 produces on the same operation, and where the finish is the requirement that difference decides the grade.

Finishing and protecting 1045

Plain carbon steel rusts, so the finish is functional rather than decorative. Parts normally leave with an oil film at minimum, and one of three finishes where the part will be handled, stored or used in a damp place:

• Black oxide — a uniform dark finish that holds a light oil film, common on tooling and machine hardware
• Zinc plating — a sacrificial coating that protects the steel even where the coating is scratched, the usual choice for fasteners and brackets
• Nickel plating or chrome plating — where a harder or more decorative barrier is wanted
• 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	1045, with the condition — annealed, normalised, or hardened and tempered
1045, harden and temper	State the hardness and the duty, not the tempering temperature
1045, ±0.01 mm as machined	Harden first, then finish — the quench moves dimensions
1045, heavy section, hard throughout	Limited hardenability — an alloy steel is the safer route
1045, will be welded	Limited weldability — a low-carbon grade is easier
1045, exposed to weather	State the finish — it will rust unprotected
1045, high-volume turned part, no hardening	1215 or 12L14 machines far faster

The three lines that decide most 1045 specifications are the hardness and duty, the section size, and whether the machining sequence has room for a post-hardening finishing operation. Get those right and the grade, the cycle and the process route all follow.

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 hardening sequence where the hardness and the tolerance are working against each other.