Steam oxidation of 80Ni-20Cr high-velocity oxyfuel coatings on 9Cr-1Mo steel: Diffusion-induced phase transformations in
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I. INTRODUCTION
INCREASING the steam temperature for the boiler and the turbine components of a power plant, which enhances the thermal efficiency, has necessitated the development of advanced ferritic heat-resistant steels with sufficient creeprupture strength and steam-oxidation resistance at high temperatures. Tempered martensite with 9 to 12 wt pct Cr strengthened with minor alloying elements has attracted much interest for application in high-temperature plants, such as coal-fired ultrasupercritical (USC) power plants at temperatures higher than 600 °C.[1] The improvement of creep strength has been reported for the 9Cr steel by optimizing the minor alloying elements, such as W, Mo, and B, to obtain a desirable precipitation and solid-solution strengthening.[2,3,4] However, the steam-oxidation resistance of such alloys is not satisfactory. The addition of 3 pct Pd to the 9 pct Cr–based steel remarkably improved the oxidation resistance, but the cost of Pd is restricting its use in actual plants.[5] In the recent past, thermal-spray technology has been extensively used in high-temperature applications such as aerospace and power generation.[6,7,8] The thermal-spray process has the potential to overcome the steam oxidation of USC boiler components, since it can offer a thick film and flexibility to deal with on-site modifications.[9–15] In our earlier studies, HVOF coatings of 80Ni-20Cr and 50Ni-50Cr on modified 9Cr-1Mo steel showed the excellent steam-oxidation resistance in the tested duration of 1000 hours. The formation of chromium oxide on the top surface of the coating protected the material against steam oxidation.[9–12] The coating adhesion to the substrate is of crucial importance for the present application; otherwise, the scale growth will be accelerated in the areas of delaminated coating due to an anomalous temperature rise. A delaminated scale or coating can damage the turbine in the down-stream as well. T. SUNDARARAJAN, Researcher, Steel Research Center, S. KURODA, Director, Thermal Spray Group, and F. ABE, Heat Resistance Design Group, are with the National Institute for Materials Science, Tsukuba, 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted August 5, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
The factors affecting the adhesive strength of the coatings are mainly (1) the generation of the residual stress, (2) thermal shock, and (3) diffusion behavior. In our earlier studies, we have reported the excellent adhesive strength of 80Ni-20Cr coatings after aging and thermal-cycling tests.[14,15] The aged and thermal-cycled specimens showed increased adhesive strength compared to the as-coated condition. The increased adhesive strength of the 80Ni-20Cr coatings on the modified 9Cr-1Mo steel was attributed to the diffusion of Ni from the coating to the substrate and the diffusion of Fe from the substrate to the coating. On the contrary, diffusion across the 50Ni-50Cr coating/substrate interface was essentially blocked by the formation of a continuous chromium carbid
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