Investigation on the Behavior of Austenite and Ferrite Phases at Stagnation Region in the Turning of Duplex Stainless St

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INTRODUCTION

DUPLEX stainless steels are known in machining for a high tendency to form built-up edge (BUE) on cutting tools. Built-up edge is an undesired effect in machining when the cutting material adheres to the cutting tool. It contributes to poor surface finish and machined dimensional tolerance control, and also accelerated tool wear. Previous machinability studies by the authors[1] identified that duplex alloys SAF 2205 and SAF 2507 had a higher occurrence to built-up edge formation compared to austenite 316L, causing accelerated tool wear and very poor surface finish. Paro et al.[2] found similar results with drilling cast duplex, finding adhesion wear triggered by built-up edge, as the dominant tool failure mechanism in their machinability study. In an earlier study, Carlborg,[3] observed built-up edge to be an issue in turning, and further suggested that higher ferrite content in duplex was triggering frequent built-up edge, though no mechanism for this was suggested. Williams reported that two-phase materials promote an additional fracture point occurring along the chip-tool rake face during BUE formation, while single-phase materials maintain only one fracture point. Williams[4] attributed the fracturing caused by cracking as a result of reduced ductility in the second phase, but could not J. NOMANI, Lecturer, T. HILDITCH, Associate Professor, and G. LITTLEFAIR, Professor, are with the School of Engineering, Deakin University, Waurn Ponds, Australia, A. PRAMANIK, Lecturer, is with the Department of Mechanical Engineering, Curtin University, Bentley, WA, Australia. Contact e-mail: Alokesh.pramanik@ curtin.edu.au Manuscript submitted August 24, 2015. Article published online April 4, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A

show how this occured. Most machining-related studies have mainly focused on the machinability aspect toward machining duplex. There has been little research conducted to focus on understanding the mechanisms triggering these long-established machinability issues such as built-up edge. The purpose of this paper is to therefore understand the plastic behavior of the duplex microstructure during chip formation, particularly at the stagnation zone, since it is an area widely known for where built-up edge is most likely to develop. Details on the plastic flow of the microstructure in this region can provide insight to the triggering mechanisms to frequent built-up edge occurrence. As a two-phase material, duplex stainless steel combines the inherent benefits of both a-ferrite and c-austenite phases, in relative equal amounts, see Figure 1. The a-ferrite phase contains a body-centered cubic crystal structure. It is responsible for the excellent pitting and crevice corrosion resistance properties, while the c-austenite phase, a face-centered cubic structure promotes the superior strength and toughness.[5] There has been little research which focuses on observing how these phases behave plastically during chip formation, since both phases naturally respond differently to an applied load.

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