THE SIGNIFICANCE OF THE WHITE LAYER IN A HARD TURNED STEEL CHIP

Abstract

Abstract The white layer that forms in the nonetching region of a saw-tooth chip plays an important role in the chip forming process. Such white layers in the chip were found to consist of a mixture of retained austenite and untempered martensite by electron diffraction of electron transparent chip specimens. The time available for transformation from α to γ iron to start is less than the time available for the first saw tooth to form on the rake face of the tool. However, despite this short time, austenite forms because the carbon is already dispersed in a hard steel and the high strain associated with chip formation encourages an α/γ transformation. Since the white layer is austenite during chip formation before cooling, it will offer little resistance to deformation. Reasons why the white layer may be beneficial in saw-tooth chip formation and methods of avoiding penetration of the white layer to the. finished surface are discussed.