Investigation on White Layer Formation in PCBN Hard Turning GCr15

It is commonly believed that the white layer formed during hard machining of steels is caused primarily by a thermally induced phase transformation resulting from rapid heating and quenching. The focus of this study is to investigate the white layers produced on the machined surfaces and on the inner side part of the chips in dry hard turning GCr15 with PCBN tools. Samples of machined workpiece and chips were metallographically processed and observed under a microscope to determine whether white layers were present or not. Some properties of white layers were deduced in order to verify some of the prevalent theories. More specifically, chip shapes were studied to determine how they developed during machining with potential appearance of white layers, with a view to correlating the chip shapes with the white layer formation.