Effect of CO2 on the Strength and Ductility of Type 304 Stainless Steel at Elevated Temperatures

Abstract

Abstract One of the important problems in determining the usefulness of stainless steels as cladding materials in high-temperature gas-cooled reactors is the effect of chemical reactions between clad and coolant on the cladding strength and ductility. The mechanism by which the coolant (CO2 in this investigation) affects the strength properties of Type 304 stainless steel are being investigated in the range 1300 to 1700 F (704–927 C). Creep- and stress-rupture results obtained on sheet materials in wet and dry CO2 and argon are compared. The effect of annealing in CO2 on the tensile strength and ductility is also reported. The question of whether the strengthening observed in CO2 was due to oxidation or carburization was investigated. Experiments on the effect of various partial pressures of oxygen in argon showed that the creep rate was minimum at approximately 10 ppm. The creep rate in CO2 at equivalent stress and temperature was one-third the minimum rate observed in oxygen. Chemical analyses, metallography, and experiments with isotopic carbon showed that carburization occurred in pure flowing CO2 in the temperature range studied. From this evidence it was concluded that the strengthening observed in CO2 was primarily due to carburization. The creep- and tensile-fracture strains were adversely affected by exposure to CO2 with the magnitude of the effect dependent on the time and temperature of exposure.