Regime features of austempered ductile iron cutting

Abstract

Machining regimes and sub-regimes of dry straight finish-turningof structural austempered ductile iron (ADI) with low cBN-content PcBN-based cutting inserts were examined. The global and local characteristics of TC contact and chip formation processes tied to this turning, as estimated by Katuku [Journal of Manufacturing Processes 78, 2022, 288–307], were used in this attempt. The significance of each prominent feature of these regimes was discussed from multiple perspectives. Across primary deformation zones (PDZs) and secondary deformations zones (SDZs), drag-controlled (or shock-induced) and defect-controlled plasticity regimes are, in turn, dominant and secondary. Coupled shear localisations in PDZs and SDZs occur via strain softening for cutting speeds beyond ~175 m min−1. BUE softening turns active at ~650 °C (~125 m min−1). Two bordering machining regimes emerged in close link with this cut-off temperature alongside a ⁓16 % cut-off SDZPDZ softening-degree, to name a few. Higher SDZPDZ softening-degree and moderately low mean contact stresses (high TC contact-length factor), TC contact temperature and sliding-friction coefficient, prevailing for Machining-regime-I (50–150 m min−1), hint at lower-pace wear regimes. The opposite conditions, standing for Machining-regime II (200–800 m min−1), suggest temperature-affected and stress-backed higher-pace wear regimes. Mean total energy levels (3476–7122 and 4364–4596 MPa) imply residual stresses built-up and microstructure damage below the machined surface. Machining sub-regimes are linked to solid-state phase transformations in PDZs/SDZs. Shear energy stored in PDZs/SDZs (166–291/1.5–43 MPa and 159–134/33–72 MPa) matches these transformations. Machining sub-regimes are sensitive to shallow variations in TC settings past ~175 m min−1. Powder lubrication by smeared-out graphite nodules is worthless in Machining-regime II. Cutting speeds of 150–400 m min−1 (TC contact temperature of ~685–930 °C and chip compression ratio of 3.4–2.32), sampled at the confluence of Machining-regimes I and II, are recommended for dry straight finish-turning structural ADI with low cBN-content PcBN-based cutting inserts.