Influence of material properties on the drilling thrust to hardness ratio

A drilling method developed by the authors for the measurement of the hardness profile for surface heat treated steels is based on the proportionality between drilling thrust and material hardness. However, parameters other than hardness related to the material can modify the cutting thrust level. In order to study the influence of the microstructure, the ductility, the toughness and the work hardening of material on the drilling thrust, a few tests were carried out in this work. Tensile and impact tests were carried out on different materials. Moreover, micrographic observations and Vickers micro-hardness were performed on the chips. The study allowed two groups of materials to be distinguished according to the drilling thrust to hardness ratio ( R/ HV). The first includes not very ductile materials presenting an about constant and low value of R/ HV, short, thin and not very work hardened chips. The second includes ductile materials presenting a non constant and high value of R/ HV, long, thick and very work hardened chips. As a consequence, the hardness profile determination by drilling method is reliable only if the R/ HV ratio is about the same for the treated layer and for the substrate, i.e. if the difference in ductility between the treated layer and the substrate is not very high.