Tool wear and machinability of HIPed P/M and conventional cast duplex stainless steels

In this study, active wear and failure mechanisms of TiN-coated cemented carbide tools with internal coolant supply when drilling of HIPed P/M Duplok 27 and conventionally-produced duplex stainless steel ASTM A8190 1A have been investigated. Stainless steels are often considered as poorly machinable materials. In P/M-produced duplex stainless steels, there are more hard oxide particles causing machining difficulties from the wear point of view. High strength and work hardening rate cause also difficulties from the machining point of view. In this study, drilling tests carried out by using a machining centre and optical microscope are presented. Chips were analysed by SEM electron microscopy and EDS-analysis. The machinability of duplex stainless steels is examined based on tool life and cutting speed presented by upsilon -T-diagrams. The effect of cutting speed and the differences between powder metallurgically and conventionally-produced duplex stainless steels are also analysed by chip formation and tool wear mechanisms. Based on the cutting tests, cutting speeds of 20-100 m/min, feed rate of 0.15-0.25 mm and solid carbide drills, diameter of 8.6 mm, can be applied from machinability point of view. P/M duplex stainless steels with hard oxides decrease machinability. Tool wear criterion, VB-value of 0.3 mm, is reached after drilling time of 10 min, when 50 m/min cutting speed and 0.2 mm/r feed rate are utilised. (C) 2001 Elsevier Science B.V. All rights reserved.