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

Abstract

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$p_{S_2 }$ = 2.2×10⁻¹² to 5.5×10 su−11 bar), to a H₂-CH₄ carburizing atmosphere (a_c=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$p_{S_2 }$ > 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.