Steam-side oxidation and the resultant exfoliation of iron-based scales cause unplanned shutdowns at coal-fired power generation facilities. Exfoliation mitigation is currently limited to frequent unit cycling to minimize the volume of exfoliated scale and upgrading a plant with a “blow down” system. This paper discusses the rate of steam-side oxidation on Type 304H stainless steel (304H) tube after shot peening the internal surface with commercially available techniques. Shot peening the ID of Type 304H austenitic stainless steel superheater tubes has been shown to improve the overall oxidation resistance in steam. Decreasing the oxidation rate directly impacts the exfoliation rate. The adherent spinel scales are thinner and more robust than non-shot peened tubes of he same alloy. Most of the improved oxidation resistance can be attributed to the presence of a spinel oxide layer combined with a continuous chromia layer formed near the steam-touched surfaces. The presence of a continuous chromia layer greatly reduces the outward diffusion of iron and minimizes the formation of iron-based scales that exfoliate. This work showed that a uniform cold-worker layer along the tube ID has a profound effect on oxidation resistance. Incomplete coverage allows oxidation to proceed in the non-hardened regions at a rate comparable to the oxidation rate on unpeened Type 304H.
Coal-fired power plants provide nearly half of the electricity to the power grid in the United States1. Plant maintenance outages are often planned events that have a known financial impact on the utility company. Extended plant outages have been experienced due to excessive steam-side oxidation of superheater tubes and subsequent exfoliation of iron-based oxides. Exfoliated scale collects in lower tube bends and creates blockages. The blockages can go undetected until short-term overheating results in unexpected failures. Investigators report an improvement in oxidation and exfoliation resistance in cold-worked austenitic stainless steel exposed to steam.2-5 Most of the available literature discusses laboratory-produced ingots with controlled chemistry, grain size, and degree of cold work. The supporting theory describes a mechanism by which chromium diffusion is improved by increasing the grain boundary density at the free surface. The greater availability of chromium along the oxidation front allows a robust, continuous film of chromium-rich spinel and chromium oxide to form and thicken faster than materials with larger grains. This paper discusses the rate of steam-side oxidation on Type 304H stainless steel (304H) tube samples after shot peening the internal surface with commercially available techniques. Three shot sizes (0.020 inch, 0.035 inch, and 0.25 inch) and two peening techniques were examined. An untreated section of 304H tube was used for the control sample. The samples were characterized in the as-treated condition prior to steam exposure using the following techniques; residual stress depth by x-ray diffraction, Knoop and Vickers micro hardness profiles, grain size, chemical analysis by energy dispersive spectroscopy, and microscopic documentation using an inverted light metallograph and a scanning electron microscope. The samples were exposed to superheated steam at 1150oF under 1 atmosphere of pressure for 1000 hours, 5000 hours, and 10,000 hours.
