Influence of the Carbo-Chromization Process on the Microstructural, Hardness, and Corrosion Properties of 316L Sintered
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INTRODUCTION
STAINLESS steel provides superior mechanical and physical properties for a reasonable cost. Among the different grades, the austenitic 316L stainless steel is widely used to manufacture products that require a good corrosion resistance such as plates for proton exchange membrane fuel cells[1] or biomedical applications.[2] These products can be obtained by conventional metallurgy techniques or by sintering techniques such as Spark Plasma Sintering (SPS). This is a relatively new process that uses electric current with low voltage and high amperage to sinter powder materials at elevated temperature, in a much shorter time compared to the traditional sintering techniques.[3–7] Samples with small SORIN IORGA and ADRIANA CHIVU Ph.D. Students, MIHAI COJOCARU, Professor, and SORIN CIUCA, Associate Professor, are with the Polytechnic University of Bucharest, Splaiul Independentei 313, 77204 Bucharest VI, Romania. MIHAIL BURDUSEL, Research Assistant, and PETRE BADICA, Senior Researcher, are with the National Institute of Materials Physics, Atomistilor Str. 105bis, 077125 Magurele, Ilfov, Romania. CE´DRIC LEUVREY, Engineer, GUY SCHMERBER and CORINNE ULHAQ-BOUILLET, Research Engineers, and SILVIU COLIS, Associate Professor, are with the Institut de Physique et Chimie des Mate´riaux de Strasbourg (IPCMS), UMR 7504 CNRS and Universite´ de Strasbourg (UDS-ECPM), 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France. Contact e-mail: [email protected] Manuscript submitted February 4, 2013. Article published online March 18, 2014 3088—VOLUME 45A, JUNE 2014
crystalline grains and reduced porosity can be thus obtained. The improvement of the mechanical properties while keeping the corrosion properties as close as possible to those of the original material can enlarge the application field of 316L steel. In the case of metallic samples, such improvement can be obtained by surface hardening. This can be achieved by carrying out thermo-chemical treatments such as carbo-chromization,[8,9] nitro-chromization,[10,11] or boro-chromization.[12] As a result, superficial layers containing iron and chromium carbides can be obtained. These layers offer a superior hardness, and thus, enhanced mechanical properties along with good anticorrosion properties.[13,14] 316L stainless steel is a heavily alloyed steel (Fe, Cr, Ni) which has a very small C content in order to prevent reactions with Cr. Since carbides generally precipitate at the grain boundaries, such an eventual reaction can lead to a local reduction of the Cr concentration in the solid solution, and therefore, a decrease of the corrosion resistance can occur. On the other hand, due to the very low C content (usually below 0.03 pct), it is clear that the mechanical resistance of the 316L steel is relatively small. Within carbo-chromization, the carburizing process aims bringing carbon to the system in order to form a carbide diffusion layer with enhanced hardness. In this process, chromium (which has larger affinity with respect to Fe to form carbides) will be extracted
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