Machining steel: from free-cutting to tool steel

Steel is not one material. Between free-cutting steel and tool steel there is a factor of several in cycle time, and the grade on the drawing is usually chosen for strength, not for the lathe.

We machine steel; we do not produce it. That distinction matters more than it sounds, because a turning shop's whole relationship with steel is about how it behaves under a cutting edge — and the grades behave very differently indeed.

The spectrum, from the machine's point of view

• Free-cutting steel. Sulphur and manganese form inclusions that break the chip. Fast, predictable, long tool life. If the application allows it, this is the cheapest steel to turn by a wide margin.
• Case-hardening steel. Machined soft, then carburised so the surface is hard and the core stays tough. The catch is distortion in hardening, which is why critical features are often finished afterwards.
• Tempering steel. Higher strength through the section. Slower to cut, higher forces, shorter tool life.
• Spring steel. Elastic and work-hardening; it pushes away from the tool and hardens where it is not cut cleanly.
• Cold extruding steel. Very ductile — excellent for forming, awkward for turning, because the chip does not want to break.
• Tool steel and hardened steel. The slow end. Hardened material is turned with ceramic or CBN on our hard turning machines, after heat treatment, where grinding used to be the only option.

Two things worth knowing before specifying

Free-cutting is not always allowed. The sulphur that makes it machinable reduces toughness and weldability, and some applications and customer standards exclude it. That is legitimate — but if the exclusion is habit rather than requirement, it is worth revisiting, because the cost difference is large.

Hardening moves the part. A tolerance held perfectly before heat treatment can be outside afterwards. Either finish after hardening, or agree tolerances that account for the movement. Deciding this at the drawing stage is far cheaper than discovering it at first article.

What we run

Free-cutting, structural, carbon, case-hardening, tempering, tool, spring, cold extruding and hardened steel, alongside stainless V2A and V4A — from Ø 2 to Ø 52 mm, bought from European mills with a certificate that stays with the batch.

More on the parts we make, or send a drawing to info@kovinski-izdelki.si.