Towards reliable micromagnetic detection of white etching layers in deep drilled quenched and tempered steels

Ultrafine-grained white etching layers (WEL) can be formed in the machining of steels, titanium alloys and nickel-based superalloys due to high forces and temperatures in the contact area of the tool and the workpiece. In general, these layers are associated with very high hardness and brittleness as well as (tensile) residual stresses. These mechanical properties of WEL can have a severely negative impact on the lifetime and reliability of components. As a result, it is of crucial importance to reliably detect WEL, understand the underlying mechanisms and physical relationships in their formation and finally control their emergence in machining. Currently, WEL are usually detected using destructive metallographic analyses. In recent years, therefore, the applicability of alternative non-destructive methods for the reliable detection of WEL has been increasingly investigated. In this context, methods such as X-ray diffraction, acoustic emission (AE) and eddy current testing were used. The analysis of magnetic Barkhausen noise (MBN) was identified as a particularly suitable method for the detection of WEL in steels with a very high potential for application in production technology. In this study, MBN analysis is employed for the time-efficient and non-destructive detection of WEL in deep drilled components made of the quenched and tempered steel AISI 4140. It is shown that WEL form in drilling, especially at high cutting speeds and feeds. The use of coated guide pads and cutting edges promotes the formation of WEL. Hardness in the WEL exceeds the hardness of the bulk material up to three times. Specimens with thick WEL can be separated from specimens free of WEL by significantly lower maximum magnetic Barkhausen noise amplitudes