How can depth and hardness of strainhardened surface layer be minimized?
Patrick de Vos, Corporate Technical Education Manager at Seco Tools Group, answers your questions about machining. This time he answers the question:
“In finish machining of strain-hardening materials such as austenitic stainless steels, how can the depth and hardness of the strainhardened surface layer be minimized?”
Strain-hardening materials are materials that show a noticeable tendency to become stronger and harder when they are deformed. Machining is a process through which material is deformed to such a level that the material shears off in the form of chips. When machining strain-hardening materials, the machining process can unintentionally create a thin layer of strain-hardened material on the machined surface. For the actual cutting edge this is not a problem, but the next cutting edge (such as in milling applications) will have to cut through this strain-hardened surface layer, influencing the tool-deterioration process and leading to a shorter tool life.
To minimize the thickness and the strength of the strain-hardened layer, the actual material deformation during machining has to be minimized. This can be done by using sharper cutting edges and smaller nose radiuses, and through the accurate selection of cutting speed (not too high to minimize the deformation speed) and feed.
The feed in particular deserves careful consideration. Feeds that are too high will create more deformation and will lead to higher strength of the strain-hardened layer. But too small a feed will lead to the effect that the ‘next’ cutting edge is constantly cutting in strain-hardened layers created by the previous cutting edge. In general the correct balance between not too high and not too low will be more towards higher feeds, especially in operations where several cutting edges cut simultaneously, such as milling.