Carburization depth evaluation from magnetic nondestructive testing

Abstract

Precise estimation of carburization depth is mandatory to avoid failure and critical degradation of high-performance mechanical parts. In this work, we studied magnetic methods as an indirect way to obtain this information. Usual techniques consisting of complex combinations of unrelated indicators have been left aside and replaced by a theoretical approach based on the magnetization mechanisms.A series of rod specimens were pre-characterized using destructive hardness tests and classified into four categories from virgin to very deep carburization depth. Then, rods from each batch were tested with different magnetic testing techniques. Identical experimental conditions were kept to limit the degrees of freedom and ensure the viability of the conclusions. The Magnetic Incremental Permeability (MIP) maximal amplitude ΔZ which is associated with the domain wall bulging mechanism showed the highest linear correlation with the carburization depth.Based on these first observations, new experimental conditions were proposed: MIP AC component frequency was reduced to 1 kHz, leading to a magnetic penetration large enough to reach the specimen core. A sudden change in slope was observed on the MIP signature of the treated specimens when the excitation field was close to the core coercivity (of similar magnetic behavior as untreated specimens). The corresponding local slope obtained a very high 0.987 linear correlation coefficient vs. carburization depth and revealed low-frequency MIP as an effective magnetic nondestructive technique for thick carburization evaluation.