Machinability of martensitic and austempered ductile irons with dual matrix structure

Abstract

The main goal of present paper is to highlight the influence of type of hard phase at different volume fraction onto the cutting force of dual matrix structure ductile iron (DMS-DI) in order to achieve the DMS-DI articles having the suitable machining performance for the DMS-DI market development. DMS-DI specimens were prepared by casting, followed by partially austenitising at temperatures of 870 °C for various times and then quenching into water or molten salt with the temperature 350 °C. The machinability of the workpieces was investigated by adopting, cutting index (relative cutting force with respect to AISI 1110 steel) as a general criterion. Fitting the data to the Johnson-Avrami model revealed the ferrite-to-austenite phase transformation kinetics with two consecutive distinguishable stages with Avrami exponents of 3.5 and 2. In both series of DMS-MDI (martensitic DMS) and DMS-ADI (ausferritic DMS) samples, with increasing percentage of hard phase (Vf), the hardness value was increased, while, surprisingly the cutting index was improved by enhancing of Vf to ≈37%, and then worsened. The dependence of the cutting force on the cutting depth was linear for steel AISI 1110, DMS-MDI and DMS-ADI. The slope of the line and its y-intercept depend on the base materials, type of hard phase, the feeding rate and the spindle turning speed. The hardness value of the DMS-MDI was higher than the DMS-ADI, despite this, for Vf≈37%, the cutting index of the former was up to 20% and 60% better than the DMS-ADI and regular austempered ductile iron, respectively.