Steel covers an enormous range of materials that behave completely differently under a cutting tool, from free machining grades that cut like butter to hardened tool steels that no cutter will touch at all. Choosing the grade decides the machining cost as much as the geometry does, and the decision is often made without that in mind.
The grades in rough order of difficulty
Free machining carbon steels with added sulphur cut fastest and break chips cleanly. Plain low and medium carbon steels are straightforward. Alloy steels are tougher and slower. Stainless work hardens. Tool steels in the annealed condition machine acceptably and in the hardened condition not at all with conventional cutters. Hardness and toughness both work against you, and they are not the same thing.
The hardness ceiling
Conventional carbide tooling runs out somewhere in the mid forties on the Rockwell C scale, beyond which cutting becomes uneconomic and unreliable. That is why the standard sequence for a hardened part is to machine it soft with grinding stock left on, heat treat it, then grind or use electrical discharge machining to reach final size. Trying to machine fully hardened steel is a common and expensive mistake.
Heat treatment and distortion
Heat treating moves parts. Thin sections, asymmetric geometry and residual stress from machining all contribute, and the movement can exceed the tolerance easily. That is another reason the finishing operation comes after hardening rather than before, and why stress relieving between roughing and finishing is standard practice on precision parts.
Specifying steel sensibly
Name the grade and the condition, because a grade in the annealed and hardened conditions are effectively two different materials to the shop. Say if a free machining grade is acceptable, since it can cut the machining cost substantially where the reduced weldability and toughness do not matter. And say what hardness is required where, since selective hardening is often cheaper than through hardening the whole part.
Questions people ask about steel machining
Which steel is cheapest to machine?
Free machining carbon steels with sulphur additions, which cut fast and break chips cleanly. Alloy and stainless grades cost appreciably more per unit removed.
Can hardened steel be machined?
Not economically with conventional cutters above the mid forties on the Rockwell C scale. Hardened parts are ground or cut by electrical discharge machining.
Why machine soft and then harden?
Because cutting hardened steel is impractical and because heat treatment distorts parts, so the finishing operation has to come after hardening.
Does specifying the condition matter?
Very much. The same grade annealed and hardened are effectively two different materials to a machine shop, and the price difference is large.