A Study Designed to Explain the Creep-Rupture Strength of Type 321 (“18Cr-8Ni+Ti”) Superheater Tubing

The creep-rupture strength at 1200 deg F of Type 321 (“18 – 8 + Ti”) superheater tubing was shown in the preceding paper (ref. [1]) to be very low when made by the usual cold-working methods and heat-treated at 1600 deg F and to increase with temperature of heat-treatment in accordance with the amount of TiC dissolved. There did not, however, appear to be an adequate explanation of the reasons why the solution of TiC resulted in such pronounced increases in strength. Without an adequate understanding for the causes of weakness and strength, the metallurgy of Type 321 superheater tubing would remain empirical and, therefore, subject to the possibility of unexpectedly poor properties under the many variations of manufacturing, fabrication, and service conditions. Accordingly, research was undertaken to determine the way solution of TiC increased creep-rupture strength.