Material Mechanical Properties Influence on the Ratio of Drilling Thrust to Hardness
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Material Mechanical Properties Influence on the Ratio of Drilling Thrust to Hardness G. Mauvoisin, O. Bartier, R. El Abdi and A. Nayebi University of Rennes Applied Mechanic Research Laboratory I.U.T. de Rennes, 3, Rue du Clos Courtel – B.P. 90422 -35704 Rennes cedex 7, France
ABSTRACT A new approach using an experimental hole drilling technique has been developed to determine the hardness profile of superficially heat-treated steels. This method was based on the proportionality between the hardness of the material tested and the cutting thrust measured during the drilling test. Although it was observed that, overall, the increase in the cutting thrust was linear with hardness, locally the variation of thrust was not necessarily regular and linear in particular for iron-carbon alloys. In fact, other parameters related to the material, other than hardness, can modify the cutting thrust level. We report here on the influence of parameters, such as the type of microstructure and the ductility or the work hardening of material, on drilling thrust. The cutting thrust obtained for various brittle or ductile materials will be discussed using results obtained from hardness tests, micrographic observations and microhardness of the chips obtained by drilling test.
INTRODUCTION The drilling thrust depends on the geometry of the drill (diameter, point angle, lip length, evolution of the cutting angles along the edges etc.) as well as on the cutting conditions (cutting speed, feed rate, lubrication etc.) and on the material properties. The influence of the material properties on the cutting thrust is characterized, for the majority of the models by the hardness or by a coefficient which depends on the shear flow stress [1]. The cutting force measured during a drilling test on a superficially heat-treated steel, integrates heterogeneous resistance along the drill cutting edge. Thus, measured, thrust does not correspond exactly to the hardness profile. In order to obtain the hardness profile, an analytical model based on the discretization of the hardened layer was developed to separate the heterogeneous resistance along the cutting edges of the drill [2]. Figure 1 (a) gives the hardness profile of a nitrided 32CrMoV13 steel obtained by the drilling test and the Vickers micro indentation technique. V ick er s H a r dness H V 0.2
V ick
100 0
M icr V icke rs T e chn iq ue Moicro 800
D r illin g T e st
6 00 40 0 2 00
D e pth (m m ) 0 0
0.5 0 1 .00 1.5 0 2 .0 0 2.50 3.0 0
(a)
(b)
Figure 1. Hardness curves for (a) nitrided 32CrMoV13 steel and (b) nitrided X6CrNi17-04 steel.
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The good agreement of the hardness profile by the drilling test with hardness measurements proves the validity of this method. This result is interesting because the drilling test does not require any cutting or preliminary sample preparation and gives the profile in a few minutes. However, the precision of the results depends on the type of tested steel and on the type of heat treatment. Figure 1 (b) for example, shows that the hardn
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