Morphologies of oxide growth and exfoliation in superheater and reheater tubing of steam boiler

Abstract

The intent of this paper is to present the current state of knowledge concerning the development of the oxide morphologies associated with scales grown in steam on alloys used in the superheater and reheater circuits of conventional fossil and heat recovery steam generator plants, and the influence of those morphologies on the scale exfoliation behaviour. Scale development is described for three classes of alloy: standard 1 – 9 wt%Cr ferritic steels, using T22 as an example; the ferritic-martensitic steel T91; and the 300-series austenitic steels, represented by TP304 and TP347, for which coarse-and fine-grained versions are examined. Detailed information on oxidation mechanisms is combined with extensive knowledge of alloy behaviour in boiler service in an effort to allow engineers and materials scientists to gain an understanding of similarities and differences, as well as to appreciate the likely contributions of major parameters that influence oxide growth and exfoliation behaviour.     

Numerical simulation and life prediction on T22 boiler tubes with steam-side and fire-side oxide scale

In this study based on Larsen-Miller empirical formula, two simulation models of T22 boiler tube of a super-heater are analyzed by Finite Element Method (FEM). Model one only considers the steam-side oxide scale, and model two considers both the steam-side and the fire-side oxide scale. The results show that the average temperature and Von Mises stresses of the tube increase gradually due to the existence of the steam-side and fire-side oxide scale, in which the effect of fire-side oxide scale is more remarkable. Based on the simulation results, Kachanov-Rabotnov creep damage model was employed to assess the creep life of the tube. It was found out that at the initial stage of the service time the fire-side oxide scale can protect the tube and delay the creep damage obviously, and the critical creep damage of the tube is only about 0.5 before failure occurs.