MS Thesis Presentation by Ian S. Harrison
Tuesday, November 23, 2004
(Dr. Thomas Kurfess, Chair)
"Detecting White Layer in Hard Turned Components Using Non-Destructive Methods"
Hard turning is a machining process where a single point cutting tool removes material harder than 45 HRC from a rotating workpiece. Due to the advent of polycrystalline cubic boron nitride (PCBN) cutting tools and improved machine tool designs, hard turning is an attractive alternative to grinding for steel parts within the range of 58-68 HRC, such as bearings. There is reluctance in industry to adopt hard turning because of a defect called ‘white layer.’ White layer is a hard, 1-10 micron thick layer on the surface of the specimen that resists etching and therefore appears white on a micrograph. There is some debate if white layer actually decreases the strength or fatigue life of a part, but nevertheless it is not well understood and therefore should be avoided. This research will examine two different non-destructive methods for detecting white layer.
The first technique uses the Barkhausen sensor to detect white layer in hard turned components. The Barkhausen sensor is an instrument that works by applying a magnetic field to a ferromagnetic metal and observing the induced electrical field. The amplitude of the signal produced by the induced electrical field is affected by the hardness of the material and surface residual stresses.
Another approach for detecting white layer is electrochemical impedance spectroscopy. This method is useful for modeling the interface between a metal and an aqueous solution. White layer has traditionally been identified by etching the material with a nitric acid solution. This suggests that the corrosion properties of white layer are different than the bulk material. This idea is verified by measuring the electrochemical potential of surfaces with white layer and comparing to surfaces without any.
The goal of this project is to determine if it is possible to infer white layer thickness reliably using either of these two methods. The general procedure is to use typical process parameters and a PCBN content tool to examine a process that will not initially produce white layer. As the tool wears, the likelihood for white layer to form increases. Measurements from the non-destructive techniques are compared with direct observation of the microstructure in order to determine if either technique can reliably detect the presence of white layer.