Fabrication and Annealing Factors Affecting Grain Size of 18Cr8Ni-Ti Superheater Materials in Steam Boilers

Abstract

The effect of cold deformations as encountered in tube and superheater fabrication and of temperature and time of annealing on the grain-size characteristics of 18Cr8Ni-Ti is demonstrated by laboratory experiments with material from six heats of steel. It is shown that cold-drawn tubing retains a relatively uniform small grain size at annealing temperatures up to about 1900 F to 1950 F and that above this temperature individual grains begin to grow at an accelerated rate, leading to a mixed grain-size structure. Annealing times between 5 and 15 minutes caused only insignificant differences in the over all grain size, but extension of exposure to 30 minutes produced a noticeably larger grain structure. Small cold deformations as may be introduced into the material by tube straightening can, when followed by a final anneal, cause excessive localized grain enlargements. Observations pointed to the possibility that materials with high Ti/C ratios may retain a predominantly small grain size at annealing temperatures as high as 2050 F.