Formation of reaction layers in steel machining: Impact on surface treatment processes

Abstract

During production and machining, metal parts come into contact with cooling lubricants which protect the tool from wear. Cooling lubricants are responsible for heat and swarf removal as well as for friction reduction. For this reason they contain additives which form reaction layers not only at the tool surface, but also on the surface of the workpiece. Residues of cooling lubricants are thought to cause problems in surface treatment processes if they are not removed by efficient cleaning/degreasing. This is hardly practicable as long as it is unknown what contamination (and how much of it) has to be removed to obtain a desired physicochemical surface condition. Thus it is necessary to identify the composition of reaction layers and to determine critical surface concentrations which must not be exceeded if the surface treatment process is to function properly. Reaction layers have been prepared on steel by milling using different cutting oils, consisting of a mineral or synthetic base oil and additives in different concentrations. X-ray photoelectron spectroscopy was used to analyse surface composition after milling, and electrochemical measurements were carried out to establish the condition of the metal surfaces. Gas nitriding was used as a model surface treatment process to examine the impact of these reaction layers on the reactivity of a steel surface.