Failure Analysis of Superheater Boiler Tube SA 213 T12

Abstract

Superheater tubes SA213 T12 (OD = 57 mm, WT = 6 mm) has been in service for 11 years in a supercritical coal fired boiler (315 MW). Tubes operating temperature are 430°C and pressure is 11 MPa. Failure occurred on tube no. 20, row 2. Three tubes specimens from the same row (failed tube, tube no. 3, and no. 4), were drawn for laboratory examinations. Vickers microhardnesses of failed and no. 3 tubes are the same (140 VHN), while tube no. 4 hardness is 170 VHN. These hardness values are much lower compared to the new tube (192 VHN). The lower hardness value of failed tube is due to complete decomposition of pearlite colonies in inner and outer side of failed tube to ferrite and spheroidized cementite. Failure of tube is characterized by thin lip rupture which indicated that the tube circumferential deformation occur gradually following spheroidization process. Failure by intergranular crack occurred along grain bondaries when the stress on the tube wall (hoop stress) exceeding material strength at service temperature. Tube no. 3 and 4 are partially spheroidized, mostly on the innerside. The severity of microstructure damage is related to the flow direction of flue gas, from tube no. 1 to no.2, no.3 and so on. Tube no. 1 was survived because it was protected by a front cover that will limit heat transfer from the flue gas. The previous failure analysis of similar boiler tube revealed that the hardness of the bulging tube is about 140 VHN which is the same as the hardness of the failed tube (no. 2) and no.3. Tube no. 3 is in the early stage of damage and shall be replaced because the hardness is much lower than 170 VHN. The effect of flue gas flow direction not only affects the hardness and microstructure but also deposit thickness. Deposit thickness in tube no. 2, 3, and 4 are 149, 123, and 100 µm respectively. Thicker deposit in tube no. 2 is also contributed to premature failure of this tube compared to the others. The local temperature of tube wall will be higher because thick deposit will reduce heat transfer between flue gas and steam inside the tube.