Microstructure Analysis and Finite Element Modelling for Creep Failure Prediction and Fitness-For-Service Assessment of Superheater Tubes

Abstract

This study aimed to perform a fitness-for-service assessment and investigate the root cause of failure of Grade 14CrMo3 steel seamless tubes typically used in superheaters in power generation plants. For this purpose, samples were taken from in-service superheater tubes in a 320 MW power plant. Thickness and hardness measurements were taken from the samples, and microstructural analyses were performed using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The results showed the presence of vanadium (V) and sulfur (S) elements on the tubes' external surface (flue gas facing-side), which is indicative of fuel ash corrosion. The formation of low melting point salts, such as Na2SO4, NaVO3, Na2O, and V2O5 (particularly between 10 and 2 o'clock positions) 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 (particularly between 12 and 2 o'clock positions) 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 fitness-for-service-1 guidelines were utilized. The effects of the reduction in wall thickness were considered by performing a three-dimensional finite element analysis. The results showed that a temperature increase of only 50 °C (from 480 °C) could decrease the life of the tubes from 30 years to less than a year.