Issue 51

D. Fernandino et alii, Frattura ed Integrità Strutturale, 51 (2020) 477-485; DOI: 10.3221/IGF-ESIS.51.36 478 I NTRODUCTION pheroidal Graphite cast Iron (SGI) is a kind of cast material which covers a wide range of mechanical properties via microstructural control. SGI can be produced with ferritic, pearlitic, martensitic or ausferritic matrices depending on its chemical composition and processing. The SGI with ausferritic matrix is usually obtained after an austempering heat treatment and it is called Austempered Ductile Iron (ADI). It shows very high strength and acceptable toughness. These characteristics, together with the relatively low cost and ease of production, make SGI and ADI to be increasingly used in the construction of high stressed parts for machines and vehicles [1]. However, as most structural materials nowadays, SGI faces a strong competition from different materials available for similar applications. In this context, over the last 10 years, increasing attention has been given to the development of a new kind of SGI, referred to as Intercritical Austempered Ductile Iron (IADI) or sometimes as Dual-Phase Austempered Ductile Iron (DPADI) [2-4]. The metallic matrix of this material is formed by different amounts of ausferrite (a fine mix of ferrite and austenite) and free-ferrite. The aim of this development is to bridge the gap existing between the properties of the pearlitic grades of SGI and the much stronger ADI grades, by producing materials with similar strength as the pearlitic grades but displaying greater toughness and better machinability. The usual methodology used to produce IADI consists of submitting SGI to a partial austenitization by heating within the ternary ferrite-austenite-graphite phase field, followed by an austempering treatment [3-5]. Depending on the intercritical temperature selected, different amounts of austenite will be present in the sample before cooling. The subsequent austempering aims at transforming the austenite into a mixture of acicular ferrite and retained austenite, sometimes referred to as “ausferrite”, while leaving the ferrite untransformed. Thus, after heat treatment, the metallic matrix of IADI is composed of ausferrite and free ferrite. Under this experimental procedure, the relative amounts and morphology of ferrite and ausferrite can be controlled by using different austenitizing temperatures and thermal cycles. The literature reports several experimental procedures and considerations to obtain IADI [2-5] and different mechanical properties based on the amount of ferrite and ausferrite [6-8]. In general, tensile strength, yield stress, and fracture toughness increase when the amount of ausferrite increases, while elongation diminishes. Nevertheless, such a decrease is very small and the IADI keep having the minimum value of elongation required by ASTM A 536 standard. The fracture surfaces of IADI for different amounts of ferrite-ausferrite were also evaluated by several authors [8-9], nevertheless, there is not a complete understanding about the sequence and occurrence of damage mechanisms, nor about the influence of the ausferrite (in terms of morphology and volume fraction) on crack propagation and damage evolution. Recent studies carried out by the authors improved the understanding of the role of different microconstituents during fatigue and tensile testing by means of in-situ analysis [10-13]. This procedure provides a direct observation of the damage evolution of IADI. In consequence, in this work, a step by step analysis of IADI samples during tensile testing is carried out with the main goal of improving the understanding of the relation between the microstructure of IADI and the damage micromechanisms under tensile loading. M ATERIAL AND M ETHODS he Spheroidal Graphite Cast Iron (SGI) samples were taken from 25 mm “Y” blocks of a melt used in a previous work [14]. The SGI material was submitted to a ferritizing annealing following standard heat treatment procedures [15]. The annealing is commonly used to standardize the starting microstructure of the samples to be heat treated to obtain IADI. Electrical furnaces were employed to carry out the austenitization and ferritization heat treatments and 500 kg of low-melting-point salt bath (50% NaNO 2 and 50% KNO 3 ) were used for the austempering treatments. Determination of the intercritical temperature interval The determination of the upper and lower temperatures of the ferrite-austenite-graphite phase field is necessary to properly select the heat treatment temperature to obtain the desired IADI microstructure. In consequence, the kinetics of austenite transformation as a function of the heating temperature was investigated. Several 10mm diameter cylinders were heated and held for one hour at temperatures in the range of 720–900 °C, at steps of 20 °C. After the holding stages were completed, each sample was quenched in water. They were then sectioned and prepared for metallographic observation. The microconstituents were quantified (% in volume) by using an optical microscope and the Image Pro Plus software. Reported values are the average of at least three determinations. The graphite areas were not accounted for in the reported percentage of the microconstituents. S T