Nondestructive Determination of Case Depth by Barkhausen Noise Method

Demands for better products and more environmentally friendly manufacturing advance the technological developments in quality control applications. Especially car manufacturers which mostly use steel components strive to produce higher quality components with cost efficient ways. In order to improve the quality of steel components, raw material needs to be processed with special methods. One of the common methods to increase the quality characteristics of steel components is surface hardening process which also increases the service life and fatigue properties of components. There are different methods available to harden the surfaces of steel components to increase their strength. In principle, surface hardening provides a case of hardened layer on top of the soft core which also prevents the crack initiation and propagation. The quality control of the produced case can be made by measuring the thickness of the hardened layer which is called case depth measurement. Case depth needs to be measured because it verifies the quality of the hardening process as well as the thickness left for re-grinding operations. There are various methods available for case depth measurements. However, industries seek a reliable nondestructive method which could possibly be applied to entire volume of manufacturing, be fast, environmentally friendly and possibly cheap. Currently, hardness profiling is the most common technique for quality control of the surface hardening process. However, this method is destructive, expensive and slow. In order to develop a nondestructive technique to overcome this problem, many methods have been tried so far. Barkhausen noise (BN) has been one of the promising methods to success this. In this thesis, BN is investigated by using newly developed sensors, software and other hardware to measure the case depth of various hardened steel components. In the thesis, it was indicated that recently developed method of using BN for nondestructive case depth measurement is promising although it still requires further testing period. The main conclusion of the research was that the method works case by case. Challenge with the research was the lack of suitable sample sets and measurement systems. BN measurements are sensitive and any change in the system and samples, dramatically change the results and affect the correlations. The improvements and ideas to develop the current methodology are also given in this study