A Comprehensive Collection of Steel Knowledge for CNC Machining

When a steel specimen is pulled, stress and strain rise together in a straight line until the elastic limit. Past that point the steel keeps deforming even if the load stops increasing. The stress at which this begins is the yield strength, written σs. Designers use yield strength, not tensile strength, when they size a part that must not bend.

Tensile strength (σb) is the peak stress the specimen reaches before it necks and breaks. For most carbon and alloy steels the tensile value sits 1.3 to 1.6 times above yield. A 1045 bar at 585 MPa yield typically tests near 850 MPa tensile. That ratio matters when you read a certificate and compare it to your drawing callout.

Elongation (δ) and reduction of area (ψ) describe ductility. A 1018 low-carbon steel may elongate 25% in 50 mm; a hardened 4140 at 40 HRC drops under 12%. Low ductility means the part resists bending but chips poorly and cracks at sharp internal corners. Hardness, usually Rockwell C for steel above 20 HRC and Brinell below that, tracks roughly with tensile strength and is the number the shop actually measures.

Impact toughness is the fourth item and the one most often left off a drawing. Charpy V-notch energy falls sharply as hardness rises and as service temperature drops. A part that passes every tensile check can still shatter in a cold workshop if nobody specified toughness.

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  Yield strength (σs)Stress at which plastic deformation starts. Size stiffness-critical parts on this value.
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  Tensile strength (σb)Peak stress before fracture. Runs 1.3–1.6× yield for common steels.
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  Elongation (δ)Ductility measure. Above 20% machines and forms easily; below 12% needs care.
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  HardnessHRC above 20, HB below. Quick shop-floor proxy for strength and machinability.

Carbon is the single biggest lever on machinability. Low-carbon steels such as 1018 and A36 hold under 0.25% carbon. They are soft, weld well, and cut with generous depth of cut and high surface speed. They also gum up on light finishing passes and produce a built-up edge if the insert is too sharp.

Medium-carbon steels sit between 0.25% and 0.60%. The 1045 and 4140 grades are the workhorses of shafts, pins, and housings. At 1045 in the normalized state you can run near 180–200 m/min with coated carbide. Push much harder and the tool edge breaks down because the material work-hardens at the shear zone.

High-carbon and tool steels run above 0.60% carbon. They take a keen edge and hold hardness after heat treatment, which is exactly why they are hard to machine in the annealed state and nearly impossible above 45 HRC with carbide. Below that hardness, carbide still works. Above 55 HRC, the job moves to ceramic or CBN inserts, or to grinding.

Alloying elements shift the picture. Chromium and molybdenum raise hardenability, so a 4140 section hardens deeper than a 1045 section of the same size. Nickel improves toughness. Sulfur and lead are added deliberately in free-machining grades such as 12L14 to break chips, at a small cost in weldability and strength.

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  Below 0.25% C1018, A36. Fast cutting, gummy finish, excellent weldability.
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  0.25–0.60% C1045, 4140. Balanced strength and machinability; the common choice.
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  Above 0.60% CTool steel. Machines well annealed, needs grinding or CBN when hard.

Heat treatment sets the final properties, and it sets the machining sequence too. Annealing softens steel for roughing. Normalizing refines grain after forging. Quenching and tempering raise strength and hardness to the drawing value. Case hardening puts a hard skin on a tough low-carbon core.

The sequence usually runs rough machine, heat treat, then finish machine. Distortion during quench is real. A long 4140 shaft can move 0.1–0.3 mm over 500 mm, so we leave stock for the finishing pass. If a drawing calls for ±0.005 mm on a hardened feature, that tolerance is achieved by grinding or by hard milling after treatment, not by turning the soft bar.

Nitriding and induction hardening are shallow-case processes. Nitriding adds a very hard, thin layer at low temperature with little distortion, which suits finished parts that cannot be re-cut. Induction hardening hardens only the surface under the coil, so the core stays ductile. Both change the finishing plan.

One practical rule: state the final hardness and where it applies on the drawing. A blanket note like 'harden to 45 HRC' on a part with thin walls and drilled holes invites cracks at the holes and distortion everywhere else. Mark the surfaces.

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  Pre-hard4140 at 28–32 HRC. Machines and holds tolerance without post-treatment.
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  Through-hardQuench and temper to drawing HRC. Expect distortion; leave finishing stock.
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  NitridedThin hard case, minimal movement. Best for finished precision parts.

Machinability ratings are relative, not absolute. With 1018 at 100%, free-machining 12L14 rates around 170%, 1045 near 60%, and 304 stainless near 45%. Lower ratings mean slower surface speed, shorter tool life, or both. They do not mean the part cannot be made. They mean the cycle time and the tooling bill change.

Tolerance and finish are linked to the material state. On annealed 1018 we hold ±0.005 mm on turned diameters and reach Ra 0.8–1.6 μm with a sharp finishing insert and high surface speed. On 316L the same tolerance is achievable, but the finish tends to run one band coarser because the material work-hardens under the tool nose. On hardened 4140 above 40 HRC, milling cutters wear quickly and the finish depends on the rigidity of the setup.

Thin walls and long slender parts are where steel fights back. Deflection under cutting force scales with the cube of the length-to-diameter ratio. A Ø12 mm shaft hanging 120 mm out of the chuck will chatter no matter what insert you use. Support it with a tailstock, a steady rest, or split the operation.

Surface finish on steel is also a function of feed per revolution. Doubling the feed roughly doubles the theoretical scallop height. If a drawing asks for Ra 0.4 μm on a steel bore, the answer is often a reaming or honing step, not a slower turning pass.

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  Free-machining12L14, 1215. Fast, short chips, lower strength. Good for fittings.
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  General purpose1018, 1045, 4140. Predictable tool life with coated carbide.
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  Difficult304, 316L, 17-4PH, Inconel. Expect slower speeds and more passes.