Papers by Author: Irene Calliari

Abstract: Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Abstract: The combination of different types of deformation can create a continuous method that ensures the formation of ultrafine-grain structure in medium carbon steel wire. The method is based on drawing operation combined with torsion and bending. Tools and equipment applied in the wire and cables manufacturing are used for the implementation of this method. As a result of the combined strain effect the ultrafine homogeneous structure is formed in the medium carbon steel wire. The wire has increased strength while maintaining the plastic properties when compared with the corresponding properties after drawing.

Abstract: Coins are widely studied in archaeometry because they provide a lot of information on social, economic and technological history of people and territories which they are related to. A lot of chemical analyses have been performed by different methods, but only a few of metallographic data are known. The combination of chemical analysis with metallurgical investigation results in a complete description of the coin and its production steps. This paper reports a summary of the results on the Cu base coinage in Italy (Roman and Byzantine Empires) with the aim to develop a protocol of investigation based on microstructural and chemical analysis in order to reinforce the numismatic classification.

Abstract: Duplex Stainless Steels (DSS) are very attractive steels and their application is presently of increasing interest, especially as structural materials in aggressive environments. DSS are austeno-ferritic biphasic steels, having an austenite-to-ferrite phase ratio of about one, giving the best combination of mechanical and corrosion-resistance properties. However, these steels must be handled with extreme care, especially if thermal cycles are involved, owing to the formation of dangerous secondary compounds that highly worsen their excellent properties.The production of big pipes requires manufacturing welding operations on steel plates or sheets and the final products must satisfy specific requirements in terms of material properties. DSS approximately contain equal volume fraction of the phases that, in practice, cover a slightly wider range within 40/60 and 60/40. Therefore, since DSS properties depend on phase ratio, ferrite quantification on an industrial scale represents a topic of great interest, which must be as reliable as possible and, at the same time, quickly executable.The present paper gives a comparison of different methods currently employed for ferrite determination in DSS weldments, in order to understand the limits derived from the specific employed technique.

Abstract: Addition of some active elements such as yttrium, cerium, lanthanum and other rare earths elements (REE) to austenitic stainless steels helps to improve their high temperature oxidation resistance and tribological properties. The high intensity plasma pulses were used to introduce Ce and La into AISI 316L austenitic stainless steel. The plasma pulses contained both ions/atoms of Ce-La and those of the working gas. The pulse energy densities were sufficient to melt the near surface layer of the steel and introduce those elements into the surface layer. Scanning electron microscopy (SEM) as well as energy dispersion spectroscopy (EDS) was used during each one part of surface characterisation. Obtained results allowed us to make decision about directions of modified material successive investigations.

Abstract: This paper concerns the phase transformation induced by heat treatment and cold rolling in four duplex stainless steel. In 2205 and 2507 , during the isothermal heat treatments, chi-phase precipitates as small particles at the ferrite/austenite boundaries, followed by sigma precipitation. At the lowest temperature the formation kinetic of chi-phase is favoured, with the increasing of time and temperature a progressive transformation of chi to sigma occurs and the kinetic of sigma is favoured. During continuous cooling, the chi -phase appears at low cooling rates. In low Ni grades the grain boundaries precipitation of chromium nitrides were detected , but no sigma and chi. In 2101 the austenite transforms to martensite both after cold rolling and quenching

Abstract: The continuous improvement of the applications of duplex stainless steels (DSS) is justified mainly by their excellent corrosion resistance in very aggressive environments and by their mechanical strength, often twice than that of conventional austenitic grades. Therefore more extensive applications have been suggested, like constructions, structural components, automotive applications, etc. However for such not advanced but quantitatively important applications the base cost of the alloy is one of the more constraining features. A way to obtain leaner alloys is to reduce the Ni content and to compensate with manganese and nitrogen additions. In the present paper the structural and mechanical properties of two low Ni duplex grades are analyzed in order to investigate the structural stability of the austenite against its possible transformation to martensite and the secondary phase’s precipitation. The detailed characterization has been performed with SEM-EDS on as received and heat treated (600-850 °C) alloys. A limited precipitation of chromium carbides and nitrides at the grain boundaries has been detected in both grades. The martensite formation has been revealed only in the 2101 grade. With regard to the impact toughness the behaviour of 2101 grade is quite similar to that of the common DSS, while in the 2304 the drastic drop of toughness is avoided. The corrosion behaviour of both grades in aggressive chloride environments is quite similar to that of austenitic AISI 304 grade.