Date of Award
Master of Science
Materials Science and Engineering
Carl D. Lundin
Hahn Choo, Carl J. McHargue
Steam reforming of hydrocarbons is an important process for the production of hydrogen for industrial needs, such as ammonia synthesis. Due to the high temperature conditions (700 °C–900 °C), reformer furnace components require materials with excellent creep properties and thus highly alloyed austenitic stainless steels are typically employed. For reformer outlet manifolds, a cast, heat-resistant stainless steel with the composition 20Cr-32Ni-1Nb (ASTM A351 Grade CT15C) is widely used. However, after service exposure this alloy exhibits problems with liquation cracking in the base metal heat-affected zone (HAZ) during repair welding. In the work presented herein, two heats of material from centrifugally-cast manifold components were evaluated to quantify the potential susceptibility to HAZ liquation cracking. The weldability of the 20Cr-32Ni-1Nb materials was evaluated using the Gleeble® hot ductility test to determine the on-heating and on-cooling ductility (percent reduction in area) at various temperatures after exposure to a simulated welding thermal cycle. The as-received materials and selected hot ductility samples were characterized using optical light microscopy (OLM) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDXS) to correlate the hot ductility behavior with microstructural characteristics.
Both 20Cr-32Ni-1Nb heats showed similar hot ductility behavior when tested (i) on-heating, and (ii) on-cooling from the measured zero ductility temperature (ZDT) of 1302 °C (2375 °F). The hot ductility curves revealed that both materials exhibited a poor recovery of on-cooling ductility (Class C3 based on the Nippes criteria) after exposure to the ZDT, with a noticeable zero ductility range (ZDR) and a low ductility recovery rate (DRR) on the order of 20%. The microstructural evaluation revealed that the loss of on-heating and on-cooling ductility was a result of liquation along the interdendritic boundaries. EDXS analysis did not reveal the presence of significant amounts of Ni-Nb-Si enriched phases adjacent to the niobium carbides. The observed liquation along the interdendritic boundaries was attributed to constitutional liquation of niobium carbides which were present in the boundary regions. Based on these findings, the two 20Cr-32Ni-1Nb heats are sensitive to HAZ liquation cracking when exposed to a thermal cycle as would be encountered in repair welding.
Bohling, John William, "Evaluation of the Potential for Weld-Related Cracking in Cast 20Cr-32Ni-1Nb Heat-Resistant Stainless Steel. " Master's Thesis, University of Tennessee, 2016.