Microstructure Analysis And Fe Modelling For Creep Failure Prediction And Fitness-For-Service Assessment Of Superheater Tubes

Abstract

This study aimed to perform a Fitness-For-Service (FFS) Assessment and investigate the root cause of failure of Grade 14CrMo3 steel seamless tubes typically used in superheaters in power generation plants. Thickness and hardness measurements were taken from the samples, and microstructural analyses were performed using scanning electron microscopy and X-ray diffraction. The results showed the presence of vanadium (V) and sulfur (S) elements on the tubes' external surface (fireside) which is indicative of fuel ash corrosion. The formation of low melting point salts such as Na2SO4, NaVO3, Na2O, and V2O5 and degradation of the protective oxide layer led to loss of tube wall thickness. On the steam side of the tubes, the formation of an iron oxide layer and the presence of water in the steam due to the improper function of the steam drum created an insulated zone leading to the formation of localized hot spots, creep microvoids, and spheroidization of carbides. In addition, a thickness reduction of 18% resulted in a considerable increase in hoop stresses having a detrimental effect on the remaining creep life. To explain the creep damage mechanism and determine the remaining creep life, the Larson-Miller criteria and API 579-1/ASME FFS-1 guidelines were utilized. The effects of the reduction in wall thickness were considered by performing a 3D finite element analysis. The results showed that a temperature increase of only 50C could decrease the life of the tubes from 30 years to less than a year.