ASSESSMENT OF RETAINED AUSTENITIC PHASE IN INDEFINITE CHILL CAST IRON MICROSTRUCTURE TOWARDS VARIOUS TECHNIQUES

摘要

Indefinite Chill Cast Irons also called ICDP when used as rolling mill rolls are high alloyed cast iron alloys belonging to the Fe-Cr-C system. ICDP exhibit improved properties regarding hot oxidation and wear resistances as their microstructure is a mixture of a martensitic with bulky cementite (M3C) and lamellar graphite fully dispersed into the matrix.Mechanical properties involved depend either on the chemical composition of the alloy or the crystallisation behaviour and the heat treatments performed subsequently in order to lead to the desired microstructure.Thus hardenability of the matrix is of great concern as heat treatments sequence is intended to transform austenite in a more stable structure like martensite or even bainite, depending on Cr content.In fact austenite is known to be unstable during hot working conditions as hot temperature and external stresses could yield transformation of this phase into martensite with major volume distortions and even multiple cracks network on the component involved.To avoid such a hitch in service it is quite necessary to make sure all the austenite is completely transformed into martensite while performing subsequent heat treatments like quenching or tempering on the material obtained after casting process.Various techniques are allowable starting from classical ones (X-Ray Diffraction (XRD), Image Analysis, Vibrating Sample Magnetometer (VSM), Dilatometry) to more exotic one like Conversion Electron Mössbauer Spectroscopy (CEMS).From an industrial point of view, it is important to set a fast and reliable method to assess retained austenite presence into ICDP microstructure at the end of any step of the thermomechanical process.The present work is intended to compare different techniques used for retained austenite assessment in ICDP alloys as a first step prior to the calibration of a classical method like dilatometry. This calibration is to be set starting from complex XRD and CEMS spectrometers analyses. Dilatometry seems to overrate retained austenite volume fraction in the as-cast conditions while good agreements are made on the overall batch of techniques when considering heat treated material where retained austenite transformation occurred