Ph.D. Dissertation Defense by Sangil Han
Tuesday, February 15, 2005

(Dr. Shreyes Melkote, Chair)

"Mechanisms and Modeling of White Layer Formation in Orthogonal Machining of Hardened and Unhardened Steels "

Abstract

With the advent of new cutting tool materials such as polycrystalline cubic boron nitride (PCBN), turning of hardened materials (>45 HRC ) is becoming increasingly feasible. However, while surface finish in hard turning can be comparable to that in grinding, undesirable microstructure change, specifically, the formation of white layer, has become a hurdle in its widespread acceptance by industry.

Despite knowledge of the various characteristics of the white layer, the mechanisms of its formation have not been fully understood. It is commonly believed that white layer in hard turning is produced primarily by severe plastic deformation and/or phase transformation resulting from rapid heating and quenching. However, while phase transformation effects have been well studied separately, little effort to quantify the mechanical effect has been reported. Also, both effects have not been studied together in terms of their contributing to white layer formation. This work proposes to fulfill this need. Therefore, the research objectives are as follows: (1) Prove/disprove that the temperature for phase transformation in machining is the same as the phase transformation temperature (A s ) in the Fe-C phase diagram, (2) Investigate the roles of carbon content and/or alloying elements, and heat treatment on white layer formation, (3) Quantify the contributions of thermal and mechanical effects to white layer generation in machining, (4) Develop a semi-empirical model of white layer formation that accounts for both thermal and mechanical effects.

The above objectives will be realized in this thesis via: (1) Measurement of workpiece surface temperature and X-Ray Diffraction (XRD) characterization of the machined surface, (2) Orthogonal machining of unhardened steel and hardened steel, (3) Calculation of workpiece surface temperature and stress, (4) Semi-empirical modeling of white layer formation.

The expected contribution of this work is to explain the roles of thermal and mechanical phenomena in white layer generation in different types of steels. This work will also attempt to conclusively prove or disprove the thermal phase transformation effects reported in the literature. The semi-empirical model generated by correlating maximum temperature and effective stress on the workpiece surface and white layer thickness can then be used to optimize the hard turning process for minimal surface damage.