Papers by Author: Ian G. Wright

Abstract: Long-term cyclic oxidation behavior was compared for commercial FeCrAl alloys and model Fe-Al and FeCrAl alloys, and their coefficients of thermal expansion (CTE) were measured. For single-phase disordered (ferritic) Fe(Al) alloys, the CTE increased only slightly with Al content and was similar to that of FeCrAl alloys. More significant CTE increases were observed at ≥20%Al, as intermetallic phases, Fe3Al and FeAl, formed. As expected, the intermetallic compositions showed increased oxide spallation rates during cyclic oxidation at 1100° and 1200°C. However, after extensive spallation and loss of Al from the substrate, the compositions of Fe3Al and FeAl specimens entered the ferritic phase field, and the amount of scale spallation decreased. Among commercial oxide dispersion strengthened (ODS) FeCrAl alloys, a composition containing Mo (ODM751) exhibited the lowest thermal expansion and showed the slowest degradation rate in long-term testing at 1100°C. The concept of minimizing CTE as a route for alloy development was investigated.

Abstract: In this study, the foundation is being developed for the numerical simulation of the processes that determine the oxide scale exfoliation behavior of the steam-side surfaces of superheater and reheater tubes in a steam boiler. Initially, the assumptions concerning the base state for calculating oxide strains also were critically examined. The state of stress-strain of an oxide growing on the inside surface of an externally-heated tube was considered for the conditions experienced in a boiler during transition from full- to partial-load operation. Since the rate at which the oxide grows is an important consideration, it was necessary to determine the appropriate temperature to use in the oxidation rate calculations. The existence of a temperature gradient through the tube, and the cyclic nature of the boiler operation (temperature and pressure) were considered; the growth temperature of the oxide was taken to be the oxide surface temperature. It was determined that the commonly-used approach for accounting for geometrical effects when calculating stress-strain development in a growing oxide scale of using the analogy of an infinitelylong flat plate gave sufficiently different results than when using a cylindrical geometry, that the latter was adopted as the preferred calculation procedure. Preliminary calculation of strains developed in multilayered oxides formed on alloy T22 as a function of boiler operating conditions indicated the magnitude of the strains in each layer; the large strain gradients between the layers inferred the importance of the detailed scale morphology in determining the mode of exfoliation.

Abstract: In the high temperature oxidation of metallic alloys oxide scale strains and in particular critical scale strains play a key role with regard to scale adherence. Scale spallation/exfoliation is a vital issue for not only long term alloy performance but also steel sheet production. In many cases materials selection is based on thermodynamic considerations and short term laboratory data, not taking into account changes in the oxidation mechanisms resulting from stresses induced by thermal cycles, oxide growth, specimen or component geometry, or other operational factors. This paper presents a very concise summary of the present knowledge in the form of an approach to a comprehensive scale failure model which is based on a number of microscopic and macroscopic system parameters. This failure model is part of an on-going work which aims at a computer-assisted assessment of oxide scale mechanical reliability.