Myths of white layers in hard turning: a critical review

This article challenges the common academic and industrial assumption that white layers formed during the hard turning of bearing steel are inherently detrimental. Using statistical analysis and data mining, it synthesizes peer-reviewed literature on hard-turning-induced white layers in bearing steels, while using white layers produced by other steel-processing routes as comparative baselines. The review integrates more than a century of literature across applications such as gun barrels, railway wheels, gears and bearings and emphasizes that optical appearance alone is insufficient to infer formation mechanism or performance response. The article addresses four areas: the historical development of white layers in steels, the statistical assessment of hard-turning white layers in bearing steels, their performance implications and improved metallographic procedures to reduce false positives. Non-ferrous systems are outside the scope. The analysis shows that white layers are not metallurgically equivalent across processes such as grinding, nitriding, EDM and hard turning. For hard-turned bearing steels, probabilistic regression indicates that a 2 µm white layer has an 87% probability of beneficial performance. Overall, the review provides a mechanism-based framework for distinguishing detrimental white layers from those that may enhance surface integrity and component performance.