TRIBOLOGICAL AND WEAR PERFORMANCE OF PVD COATINGS FOR MACHINING SAF 2507 (UNS S32750) SUPER DUPLEX STAINLESS STEEL

Abstract

Super duplex stainless steels are applied in highly corrosive environments. To withstand such conditions, they designed with increased content of alloying elements and a duplex microstructure consisting of austenitic and ferritic phases. The result of this combination is the desired improvement in corrosion resistance, but also, the enhancement of mechanical properties. Machining super duplex stainless steels involves strain hardening of the workpiece, intense adhesive wear and elevated temperatures within the cutting zone leading to rapid tool wear and poor machined surface integrity. This research pertains to the application of commercially available PVD coatings to minimize the detrimental effects when turning super duplex stainless steel S32750. The selected coatings for this study were Alcronos (AlCrN), Alnova (AlCrN + AlCrSiN), Formera (CrN + CrAlTiN), Croma Plus (Cr + CrN + OX), Fortiphy (CrN) and Certiphy (TiAlN). The wear behavior and mechanisms in two distinct machining experiments were evaluated, and all tools failed by chipping preceded by intense adhesive wear and BUE formation. The use of AlCrN coatings improved tool life significantly. Data on the cutting force, chip formation, and workpiece surface integrity indicate less workpiece strain hardening effects, improved friction conditions at the tool/chip interface, as well as thinner chips being formed when machining with Alcronos coated carbide inserts. The micro-mechanical properties of the selected coatings were assessed and Alcronos combined high hardness and elastic modulus with a high plasticity index value that allows this coating to better manage the friction in the cutting zone and better dissipate the energy generated during cutting.