CREEP FAILURE OF A CHROMIZED WATERWALL TUBE IN A SUPERCRITICAL BOILER

摘要

In Spring, 2000, as part of a continuous mitigation program for circumferential cracking and waterwall wastage, 88 tubes, in panel sections of 22 tubes, were installed into the centerwall of a supercritical boiler. The replacement tubes were low alloy (1 1/4 Cr-1/2 Mo) steel chromized to a nominal depth of 12-20 mils (0.3-0.5 mm). After only three years of operation, one tube failed due to creep. Four months later, a second creep failure occurred in the same centerwall panel and four other tubes were removed due to visual variations in dimension. Two months later, two more tubes were removed with bulges. Two months following that, during the scheduled overhaul, seven other tubes were removed due to the formation of bulges and extensive creep damage was found in three of the four panels comprising the chromized tube section. Metallurgical evidence indicates tube metal temperatures in excess of 1000℉. Creep testing was performed on samples taken from this unit and its identical sister unit. The first data shows the creep properties to fall near the minimum band for this Cr-Mo alloy, but the testing remains ongoing.