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Influence of sulfur on the nature and morphology of carbides formed on the surfaces of Fe-Ni-Cr alloys at high temperatures

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The influence of sulfur upon surface carbide formation on a range of Fe-Ni-Cr alloys has been investigated at a temperature of 1273 K. The addition of small amounts of H2S, up to 100 ppm (range of pS2 p_{S_2 } = 2.210–12 to 5.510 su–11 bar), to a H2-CH4 carburizing atmosphere (ac=0.8) was found to significantly modify the behavior from that normally observed in the sulfur-free environment. The carburization of these alloys in the H2-CH4 atmosphere led to the formation of globular particles of M7C3 on the surface of the alloy, but the addition of H2S affected the type, morphology, and distribution of the surface carbides. Initially, the lower carbon containing M23C6 was formed, which transformed to M7C3 at a rate determined by the concentration of sulfur in the environment. The morphology of the M7C3 was modified by the presence of sulfur, and the carbide exhibited a preferred crystallographic orientation in the [001] direction. Particles of manganese sulfide were formed on the commercial alloys at pS2 p_{S_2 } > 2.3 10–11 bar at 1273 K, and these served as nucleation sites for carbides sothat, in contrast to the behavior in sulfur-free conditions, complete surfacecarbide layers were formed.
... The addition of small amounts of sulfur is known to be beneficial against the carburization and metal dusting which often lead to the formation of internal carbides or carbon deposits on austenitic Fe-Ni-Cr alloys widely used as heatresistant materials in petrochemical and chemical industries. It is generally accepted that the sulfur atoms may be adsorbed on the surface of alloys and of external carbides [1][2][3][4]. The adsorbed sulfur blocks the sites for carbon absorption and retards graphite nucleation. ...
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