WEB Micro-magnetic assessment of functional properties in the sub-surface zone of deep drilled AISI 4140 and 304L steels
In the past decades, the Boring Trepanning Association (BTA) deep hole drilling process has become increasingly important in fields like oil exploration or the aerospace industry for machining bores with large length-to-diameter ratios. Industrial applications of the process include the manufacturing of drill collars, hydraulic cylinders or other seamless pipes . Despite the high demand for BTA deep drilled components, research on surface integrity in BTA deep hole drilling remains far from conclusive, particularly in terms of functional properties in the sub-surface zone of deep drilled components. One reason for this lack of knowledge is that conventional approaches towards the characterization of surface integrity tend to be time-consuming and expensive. Beyond this backdrop, micro-magnetic techniques have been developed with the aim of providing a fast and reliable method to assess surface integrity.
In this study, the sub-surface zones of deep drilled AISI 4140+QT and AISI 304L steels were analysed. Specimens, drilled with three different feeds and three different cutting speeds were characterized by eddy current testing, as well as Magnetic Barkhausen Noise (MBN) analysis to analyse the effect of cutting parameters on surface integrity. During machining, the occurring process forces, as well as the drilling torque, were recorded to correlate them with the functional properties in the sub-surface zone. To separate the effects of residual stresses, microstructure, and hardness on the micro-magnetic properties, the results of the micro-magnetic investigations were correlated with X-ray diffraction measurements as well as microstructural analyses of the sub-surface zone. It was found that the cutting parameters have a significant impact on the residual stress state. The induced compressive residual stresses between σ = -200 and -700 MPa could be assessed reliably by the micro-magnetic techniques.