Work Hardening

Work hardening — strain hardening — is the property of metal that makes it harder and stronger as it is plastically deformed. Bend a piece of mild steel, cut it, hammer it, and the worked regions grow harder than the metal around them; the more the metal is deformed, the more its crystals are strained and tangled, and the more force it takes to deform it further. Every machining operation is, in part, an act of work hardening, because the cut deforms the layer it removes and the layer beneath it. Where the effect becomes the machinist’s problem is in the metals that work-harden readily — above all the austenitic stainless steels and the nickel alloys — which grow a hard, tough skin at the surface they are cut, and that skin fights the next cut. This entry sets out what work hardening is, why it makes some metals hard to machine, and the cutting discipline that keeps the machinist ahead of it.

The metal that gets tougher as you work it

Work hardening is one of the oldest observations in metalworking, and its cause lives in the metal’s structure. When a metal is deformed beyond its elastic limit — when it is bent, stretched, pressed or cut — its crystal grains slip and multiply dislocations, and the tangled dislocations make further slip harder. The metal is literally strengthened by its own deformation: the more it has been worked, the greater its resistance to being worked further. That is why a sheet of copper bent back and forth snaps at the bend, why a spring is formed by cold-working wire that is then too hard to bend, and why the hardness of a part is not only what the material started with but what the working of it has added. For the machinist the effect appears at the surface, because that is where the tool works: every cut deforms the thin layer of metal it removes, and leaves the layer immediately beneath it strained and hardened.

Why it matters in the cut

The difficulty arrives when the metal that work-hardens readily is machined, because each cut makes the surface it leaves harder than the metal below — and the next cut must cut through that hardened skin. The austenitic stainless steels are the classic case: they work-harden aggressively, and a cut that does not penetrate cleanly below the layer left by the previous cut finds itself trying to shear a surface that is harder than the bulk material, which is why a stainless part that is machined wrongly becomes progressively harder to machine as it goes. The problem is made worse by the tool’s own behaviour. A worn tool or a rubbing cut does not shear the metal — it burnishes and deforms it, hardening the surface instead of removing it, and the hardened surface then wears the edge faster, which makes it rub more, which hardens the surface more: a vicious circle that ends in a ruined edge and a surface too hard to finish. The metals that work-harden are thus unforgiving of the conditions that other metals tolerate, and they punish the timid cut most of all.

The discipline that beats it

The machining of work-hardening metals is governed by one rule: cut beneath the hardened layer, and keep the edge cutting. A work-hardened skin is only a fraction of a millimetre deep, and a cut that goes beneath it shears clean, undamaged metal and leaves a fresh skin for the next pass — so the discipline is to use enough depth of cut to get under the hardened layer rather than skating on top of it, and to keep cutting the same surface with successive passes before the skin has time to develop. The second rule is a sharp, positive tool: a keen edge with a positive rake shears the metal instead of rubbing it, which is why the tool geometry chosen for stainless and the tough alloys favours sharp, honed edges and positive geometries that cut with a true shearing action rather than a pressing one. And the third rule is to keep the edge in the cut and moving: the work-hardening metals are cut with steady, adequate feeds — enough feed that each tooth takes a real chip — because a feed so light that the edge rubs is exactly the condition that hardens the surface and starts the vicious circle.

Where work hardening is useful

Work hardening is not only a nuisance — it is also a tool the machinist uses, sometimes deliberately and sometimes unavoidably. It is the reason a thread rolled or a surface burnished is stronger than one cut, because the cold working has strengthened the surface layer; it is part of why machined surfaces are generally a little harder than the interior of the part; and it is why the shop that needs a hard surface without a heat treatment can sometimes get it by cold working rather than by heating. The same effect also appears where it is not wanted: the burr left at the edge of a cut is work-hardened and tough, which is why a hardened burr on a stainless part must be dealt with by a sharp, cutting deburring pass rather than by a scraping one, and why a work-hardened surface that must be finished calls for a cut that removes the hardened layer entirely rather than polishing it.

The measure of the effect

Work hardening is, in the end, a property the machinist reads in the behaviour of the cut as much as in a number. The sign of a metal that work-hardens is in how it responds to abuse: rub it and it grows hard, cut it cleanly and it behaves. The practical measures are the ones this entry has described — a cut deep enough to get under the hardened skin, a tool sharp and positive enough to shear it, a feed heavy enough to keep the edge cutting — and they are the same measures that machine the stainless steels and the tough alloys successfully. The machinist who understands work hardening understands why those metals ask for a firm hand rather than a timid one, and why the cut that seems most aggressive — sharp, deep and well fed — is often the cut that treats the work-hardening metal most gently, because it cuts clean metal instead of fighting the hardened skin that a tentative cut would leave behind.