White Layer Suppression and Surface Integrity Enhancement in Hard Turning of AISI D3 Steel Using CBN Inserts under Dry and Gas Mixture Cooling

Abstract

Superalloys and hardened steels are widely used across industries due to their superior mechanical properties, including high hardness, toughness, wear resistance, and fatigue strength. Because of these characteristics, they typically require grinding for final finishing; however, grinding operations demand expensive machinery, specialized equipment, high-cost abrasive wheels, lengthy setup procedures, and extended machining cycles. As a result, hard turning has emerged as a preferred alternative for finishing hard-to-cut materials, offering advantages such as reduced lead time, lower setup and tooling costs (no fixtures or form wheels), and decreased energy and coolant requirements. In addition, hard turning (HT) can achieve better part quality compared to conventional grinding. However, the process is often affected by the formation of a detrimental surface feature known as the white layer (WL), which significantly reduces fatigue life and undermines surface reliability due to dynamic phase transformations. In this study, super-hardened AISI D3 steel was machined using a semi-worn CBN insert under dry machining and gas-mixture cooling conditions. The results indicate that dry machining neither eliminated the WL nor improved surface finish, whereas gas-mixture cooling effectively suppressed WL formation and produced superior surface quality.