Ph.D. Thesis Defense by Stephen Ray Smith
Thursday, April 5, 2001

(Dr. Shreyes Melkote, advisor)

"An Investigation Into the Effects of Hard Turning Surface Integrity on Component Service Life"


Hard machining is defined as the machining of materials with a hardness of 45 HRC or greater. The technology to machine such materials has been around since the 1970ís with the advent of Ceramic and CBN tooling but recent improvements in the rigidity of machine tools and the quality of the tool material have made hard machining a viable option.  The hardness of the material generates extremely high specific forces in the contact area between the tool and the workpiece resulting in thermal and mechanical mechanisms that influences the surface of the part.  In order for hard machining to gain acceptance, it must be shown that as a finishing process, it will provide surfaces that meet the same quality standards as grinding.

This research addressed the fundamental relationship between surface integrity generated by finishing processes and component service life.  Specifically, fatigue and wear testing was utilized to determine the impact of hard turning as compared to the traditional finishing process of grinding.  Test specimens used in the experiments were generated under carefully controlled manufacturing processes and included five distinct surface conditions: hard turned with continuous white layer on the surface, hard turned with no white layer, ground, and hard turned and ground specimens subsequently super finished to improve surface finish.  Extensive fatigue and wear testing programs were conducted to determine the performance of each surface condition relative to the other conditions (wear testing was limited to hard turned specimens only).  To provide an understanding of the performance, the surface integrity of each surface condition was thoroughly characterized through surface topography mapping, metallographic inspection, residual stress measurement, TEM analysis, and nano-indentation hardness measurements.

Perhaps the most significant findings were the conclusions that the presence of white layers generated using tooling with limited wear did not affect the fatigue life or wear characteristics of the specimens.  In fact, the physical evidence resulting from this investigation suggests that under controlled conditions (i.e. limited tool wear), the white layer is nothing more than an artifact resulting from the manufacturing process and does not contribute to the service life of the specimen.  On the other hand, evidence was presented demonstrating that while the phase and crystallographic structure of the white layer is identical to the bulk material, the grains within the white layer have undergone significant grain refinement and the white layer is harder.

Finally, experiments were conducted that demonstrated that the physical characteristics of white layers resulting from excessive tool wear differ from those generated with new tooling.  Although both white layers appeared similar under microscopic inspection, nano-indentation results indicate that white layers developed though tool wear is harder.