Quantitative measurement of the hardness profile on carbo-nitriding steel by the hole drilling method

The thrust measured during a drilling test is related to the hardness of the material being tested. When the test is performed on a superficially heat-treated specimen, the results do not correspond exactly to the hardness profile obtained by the standard Vickers micro hardness technique.Because of the extreme hardness of the surface tested and the small diameter required to minimise material damage, a drill is more suitable than a milling cutter. The thrust measured during testing thus integrates heterogeneous resistance along the cutting edge of the drill. Consequently, correspondence between the hardness profile and the measured signal is shown to be non-linear. By discretising the thickness drilled, we have developed an algorithm which yields the hardness of each elemental thickness. Results for carbo-nitriding steel are very close to those obtained by the Vickers technique and the accuracy is also very similar for both methods.