Oxidational and Tribological Studies of Austenitic Stainless steels under CO2-Based Environments

Abstract

The oxidational and tribological behaviour of austenitic stainless steels under CO2-based environments is investigated. An oxidising facility capable of simulating AGR-type environment is developed. It is shown that the oxidational behaviour of the AISI 316 and 310 stainless steels follow Wagnerian parabolic kinetics. The behaviour of the AISI 321 stainless steels is shown to be non-parabolic and logarithmic in nature. Such behaviour is shown to be due to the replacement of the Cr203, oxide by an iron-rich nodular oxide. The nodular oxide is shown to be double layered in structure and consisting of Fe203 and complex spinel oxides. Reaction-rate constants and Activation energies are determined for parabolic oxidation. Two models based upon electronic transport and elemental diffusion are proposed. It is shown by the electronic model that a three staged oxidation process leads to the anomalous nodular oxidation. The diffusion model predicts alloy-oxide and alloy sub-surface elemental concentrations during oxidation. Tribological behaviour of the AISI 316 and 310 steels is studied under conditions of room and high temperatures and environments of air and CO,. The wear of the AISI 316 steels is shown to be transitional, changing from mild to severe wear about a transition load. For the AISI 310 steels no such transitional behaviour is observed; a severe mode of wear is encountered for all conditions. High friction coefficients are observed within a load dependent friction region and low friction coefficient within a load independent friction region. The dominant mild wear mechanisms are shown to be shearing, delamination, ploughing and oxidation. Rhombohedral, Cubic, Spinel and Wustite are shown to be the major constituents of the mild wear debris. Severe adhesive wear is shown to be the dominant mechanism producing severe wear. A-contact mechanical model is proposed to understand the mechanisms of shearing and delamination and the major stress components σz,σr, σe and ℑ max are determined. Heat flow analysis predict relatively low numbers of contacting asperities and low contact temperatures of ~100°C, Reaction-rate activation energies are predicted to be lower than obtained by static oxidation experiments.