Hardness profile control on a carbo-nitriding steel by a drilling test

The surface treatments concerns many parts of machines particularly exposed to corrosion, friction or high level of pressure. Different techniques are available to carry out a superficial heat treatment (carburizing, nitriding, carbo-nitriding, induction hardening, …). Then, to control the heat treatment quality one uses to measure the hardness profile by using the micro Vickers indentation technique. This well known method is long and tedious. In this paper, we propose a new experimental setup allowing to measure the thrust during a drilling test. The measured values correspond to the material penetrating resistance which is reliable to its ultimate resistance. The drilling test applied to a workpiece specimen after superficial heat treatment gives a signal similar to the hardness profile along a normal axis to the surface under test. Because of the high hardeness of surface tested and the small diameter required to minimize the material damage, twist drill tool is more recommended than milling cutter. Then, the thrust measured during test integrates heterogeneous resistances along the cutting edge. Consequently, the relation between the hardness profile and the measured signal is shown to be non-linear. Eventually, the hardness profile must be extracted from the actual signal by some mathematical operations. Indeed, by the discretization of the drilled thickness, it is possible to constitute a system of equations giving the resistance of each elementary thickness. In this paper, two algorythms for hardness profile unwrapping are presente. Results on carbo-nitriding steel are shown to be very close to those obtained by Vickers technique and the accuracy is also very similar for both methods. Given the hardness level of the tested materials a good reproducibility on measurements is only obtained by using a drill of excellent quality. The automated drilling test gives the hardness profile in a few minutes and seems quite well adapted to industrial implementation.