Morphological changes in the microstructure in new high mo super ferritic stainless steels

Abstract

High Mo content in stainless steels improves the resistance to naphthenic corrosion in equipment used in the processing of heavy crude oil. However, causes the precipitation of intermetallic phases such as Sigma (σ), Chi (χ) and Mu (μ), nitrides (Fe5Mo13N4), carbides [(Cr, Fe, Mo)23C6] or Nb and Ti carbonitrides [Nb(CN) and Ti(CN)]. These precipitations result in embrittlement, decreased corrosion resistance at elevated temperatures, and reduced toughness. Experimental alloys of superferritic stainless steel (25% Cr-X% Mo-Y% Ni) with (X = 5 and 7%) and (Y = 2 and 4%) with additional Ti and Nb were developed. The effect of Mo contents higher than those of the known commercial alloys was studied. The influence of the chemical composition variation on the kinetics precipitation of intermetallic phases and evaluated effects on the microstructural characteristics and mechanical properties were analyzed. The σ phase was formed before the χ phase in the samples treated at 600°C and 700°C. However, at higher temperatures of 800 ° C and 900 ° C, the χ phase precipitated earlier than the σ phase. The formation of Gamma (γ) phase in samples with more Ni occurred by eutectoid reaction (α → σ + γ) in the form of lamellar agglomerates.