Steam Oxidation Resistance of Shot Peened Austenitic Stainless Steel Superheater Tubes

Coal-fired power plants provide nearly half of the electricity to the power grid in the United States. Plant maintenance outages are often planned events that have a known financial impact on a utility company. Unexpected plant outages and have been experienced due to excessive steam-side oxidation of superheater tubes and subsequent exfoliation of iron-based oxides. The internal scales formed on these tubes are composed of at least two different types of oxide.  The scale that forms immediately adjacent to the fluid is primarily magnetite.  The inner oxide adjacent to the base metal is an alloy spinel oxide.  During cool down of the unit for an outage, the difference in thermal expansion of the outer scale and the base metal causes the magnetite to spall. Exfoliated scale collects in lower tube bends, creates blockages, and inhibits steam flow. The blockages may go undetected until unexpected failures occur due to overheating in the downstream tubing.

Investigators report an improvement in oxidation and exfoliation resistance in cold-worked austenitic stainless steel exposed to steam. 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 current work compares the rate of steam-side oxidation on Type 304H stainless steel (304H) tube after shot peening the internal surface with commercially available techniques. Three shot sizes (0.020”, 0.035”, and 0.25”) and two peening techniques were examined. An untreated section of 304H was used as a 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 1150^oF under 1 atmosphere of pressure for 1000, 5000, and 10,000 hours. This paper compares the oxidation resistance by presenting the microscopy results, scale thickness measurements, and chromium distribution following each exposure interval.