Met. Mater. Int., Vol. 23, No. 5 (2017), pp. 855~864 doi: 10.1007/s12540-017-6704-y Assessment of the Microstructure Evolution of an Austempered Ductile Iron During Austempering Process Through Strain Hardening Analysis Riccardo Donnini 1 , Alberto Fabrizi 2 , Franco Bonollo 2 , Franco Zanardi 3 , and Giuliano Angella 1,* 1 Institute of Condensed Matter Chemistry and Technologies for Energy (ICMATE), National Research Council of Italy (CNR), Milan 20125, Italy 2 Department of Engineering and Management, University of Padua, Vicenza 36100, Italy 3 R&D Department, Zanardi Fonderie S.p.A., Minerbe 37046, Italy (received date: 6 October 2016 / accepted date: 17 January 2017) The aim of this investigation was to determine a procedure based on tensile testing to assess the critical range of austempering times for having the best ausferrite produced through austempering. The austempered ductile iron (ADI) 1050 was quenched at different times during austempering and the quenched samples were tested in ten- sion. The dislocation-density-related constitutive equation proposed by Estrin for materials having high density of geometrical obstacles to dislocation motion, was used to model the flow curves of the tensile tested samples. On the basis of strain hardening theory, the equation parameters were related to the microstructure of the quenched samples and were used to assess the ADI microstructure evolution during austempering. The microstructure evo- lution was also analysed through conventional optical microscopy, electron back-scattered diffraction technique and transmission electron microscopy. The microstructure observations resulted to be consistent with the assess- ment based on tensile testing, so the dislocation-density-related constitutive equation was found to be a powerful tool to characterise the evolution of the solid state transformations of austempering. Keywords: austempering, alloys, microstructure, ductility, tensile test 1. INTRODUCTION Plastic behaviour of metallic alloys comes from glide resi- stance of mobile dislocations and strain hardening that is the raise of glide resistance because of dislocation density incre- ase with straining. Both glide resistance and strain hardening are sensitive to the microstructure of metallic alloys, and strain hardening analysis has been often used to give indications on the microstructure evolution during the industrial processes of metallic alloys that are characterized by solid state transfor- mations. Austempered ductile irons (ADIs) [1] are nodular ductile irons produced through proper alloying and heat-treat- ments, and their outstanding mechanical properties are due to the ausferrite [2-10]. During austempering, the alloys are first austenitized in the range of temperatures 850-890 °C and then isothermally held in salt bath at temperatures typically between 250 and 400 °C for the austempering transformation [9-15]. γ → α + γ HC (1) Depending on the chemical composition and process tem- peratures, ausferrite consists of high volume fraction (up to ~ 70%) of body-centered cubic (BCC) α ferrite with residual metastable face-centred cubic (FCC) γ HC austenite with high carbon content, resulting in an optimal compromise between ductility and strength [9,16,17]. Actually, α ferrite is in the form of Widmanstätten acicular laths with hardness of about HB 600, whilst γ HC austenite is softer with about HB 270 [18]. Transmission Electron Microscopy (TEM) investigations [19,20] have revealed that no significant precipitation occurs in ADIs, and the high strength of this materials is due to strain harden- ing because of very high increase of dislocation density in fer- rite α during deformation, whilst austenite γ HC is strengthened by solution hardening mechanism supported by grain refine- ment because of twinning. However, for longer austempering times, the metastable γ HC decomposes according to γ HC → α + ε' (2) where ε' is a metastable carbide Fe-C that causes embrittle- ment of ADIs [12,13]. During austempering the microstructure evolution passes through different stages [9-15]. In the early stage, individual Widmanstätten laths of α ferrite that are separated from each other by thin layers of carbon-saturated austenite γ HC , nucle- ate at austenite grain boundaries and grow into the grain interiors. If the alloy is quenched from the austempering temperature at *Corresponding author: giuliano.angella@cnr.it KIM and Springer