Graphitization damage on seamless steel tube of pressurized closed-loop of steam boiler

Abstract

The pressurized closed-loop of steam boiler has been in operation for five years has experienced leakage problems at the bottom of the reducer and at the center of the tube on weldment. The metallographic analysis found that the leakage was due to the degradation of microstructure as a result of long-term overheating. The degraded microstructure of the tube boiler material dominated by graphitization. The graphitization normally applicable at 450°C, but with high-pressure water flow (90 bar) in the combination of internal stress on the tube material due to the welding process as well as the formation of the reducer, making the boiler operating temperature is sufficient to produce graphitization. The leakage occurred when the chain and accumulated graphitization happen because the cavities that covered the graphite facilitates the cracking propagation due to the presence of stresses.