The Influence of Nitrogen and Certain Other Elements on the Creep-Rupture Properties of Wholly Austenitic Type 304 Steel

Abstract

Experimental heats of Type 304 steel, compositionally balanced to be wholly austenitic and thus simulate the material used to produce seamless superheater tubing, were prepared to determine the influence of selected elements on the creep-rupture properties of this steel and thereby assess their possible contribution to the improvement in the elevated temperature properties which has been noted over the past years. Nitrogen is shown to increase the rupture strength at 1200 deg F of the wholly austenitic laboratory heats to a pronounced extent. Furthermore, almost the full effect of nitrogen was obtained after heat-treating at temperatures as low as 1750 deg F. Rupture strengths also increased with increasing carbon content although heat-treatment above 1750 deg F was necessary to obtain the maximum strengthening effect from the higher levels of this element. Nitrogen was a somewhat more effective strengthener than carbon, the rupture strengths correlating with (%C) + 1.25 × (%N). Small amounts of titanium, aluminum, boron, copper, and molybdenum had no or at most only minor effects on the properties. More than 0.1 percent manganese was necessary for good rupture properties but otherwise had little effect at levels up to at least 1.5 percent. Published data for commercial heats fitted the correlation developed from the laboratory heats. The generally higher level of the rupture strength data for Type 304 austenitic steel published since the early 1950’s appears to be closely related to higher levels of nitrogen. The data indicate that it is extremely important to control nitrogen content to obtain expected creep-rupture properties at 1200 deg F.