The Oxidation and Carburisation of a 20/25/Nb Steel in Carbon Dioxide,in Carbon Monoxide and in Carbon Dioxide-Carbon Monoxide Mixtures

Abstract

The oxidation and carburisation of a 20% Cr/25% Ni/Nb austenitic steel has been investigated between 600° and 850° in carbon dioxide, carbon monoxide and their mixtures at pressures within the range 0·04–760 cm.

Under all conditions the oxide formed markedly inhibits the attack of the steel. In carbon dioxide, the major reaction is M + CO₂ = MO + CO with the reaction 2M + CO₂ = 2MO + C contributing less than 7% to the total weight gain. All the carbon deposition appears to take place within the first few hours of the oxidation. Detailed studies show that the deposited carbon diffuses into the steel.

In carbon monoxide, the reaction is essentially M + CO = MO + C, the carbon passing through the oxide film into the steel. Carbon is also deposited in varying amounts on the surface of the oxide by the reaction 2CO = C + CO₂. This carbon is present as a filamentary growth and diffuses only slowly into the steel.

In carbon dioxide-carbon monoxide mixtures containing up to 75 vol.-% carbon monoxide, the steel reacts mainly with the carbon dioxide at 650° and 725°. At 800°, however, significant reaction occurs between the steel and the carbon monoxide.

The distribution of the carbon which enters the steel depends upon the temperature of oxidation. Below 750° the carbon concentration steadily decreases as the penetration into the steel increases. At 750° and above, the carbon concentration-penetration curves are anomalous: a maximum carbon concentration is obtained within the body of the steel.