Work-hardening means that the hardness increases considerably as the material is deformed, e.g. during the process of producing chips.
It also means that, after a first pass, the machined surface has become harder, changing the working conditions for the cutting edge during subsequent posses, as the edge then has to work in a hard surface layer.
Austenitic and ferritic-austenitic steels have the greatest tendency to work-harden, which results in both high cutting forces and a high hardness of the machined surface, see figure. This means that the cutting edge has to work in a material that is much harder than is indicated by the hardness of the base material. This often results in problems during finishing, as the depth of cut is then often so small that the cutting edge is working entirely in the work-hardened layer.
It is, of course, not only cutting that causes work-hardening, but also all other types of cold working. Shot or sand blasting, for example, or a straightening operation can all produce a hard surface layer that affects machinability. In such cases, the hardness gradient is similar in principle to that resulting from machining, but the increase in hardness can extend to a greater depth into the metal. After a substantial straightening operation the increase in hardness can extend to a depth of 2-4 mm below the surface.