HVOF Coating Case Study for Power Plant Process Control Ball Valve Application Luc Vernhes, David A. Lee, Dominique Poirier, Duanjie Li, and Jolanta E. Klemberg-Sapieha (Submitted February 12, 2013; in revised form July 23, 2013) This case study is the result of an investigation on HVOF 80/20 Cr 3 C 2 -NiCr coating failure of on-off metal-seated ball valve (MSBV) used in supercritical steam lines in a power plant and solution. HVOF 80/20 Cr 3 C 2 -NiCr coating is used to protect thousands of MSBVs without incident. However, in this case, the valves are challenged with exposure to rapid high-pressure and -temperature variations resulting in a unique situation where the coating experiences cracking and cohesive failure. It was found that carbide precipitation is a major factor causing embrittlement of the coating. Once the coating toughness and ductility is reduced, thermal, mechanical, and residual stresses can initiate and propagate cracks more easily, causing coating failure when exposed to thermal shock. To alleviate the above mentioned issues, possible coating alternatives were then evaluated. Keywords chromium carbide, decarburization, failure mechanism, high temperature application, power plant, self-fluxing alloys 1. Introduction Valves are being used on drain and vent lines to extract large quantity of condensate during power station start-up to get dry superheated steam rapidly. During normal operation, drain and vent valves must remain steam-tight to prevent energy loss and maximize plant efficiency. Over the last decade, metal-seated ball valves (MSBVs) have been the industry standard for this application, pro- viding maintenance-free, tight, reliable shut-off. MSBVs considered in this study are of a floating ball design with a fixed seat. Ball and seat are manufactured from forged Inconel Ò 718 PH and coated with a HVOF 80/20 Cr 3 C 2 - NiCr coating. The primary function of the HVOF coating is to increase the load carrying capacity of the Inconel 718 surface*. Indeed, it has been demonstrated that the deposition of such thick coating will not only reduce the wear rate but also distribute the peak stresses generated by localized loading from the surface to the base material and increase the galling threshold resistance (resistance to adhesive wear and material transfer) (Ref 1, 2). 80/20 Cr 3 C 2 -NiCr coating composition is also specially adapted to offer resistance to the high-temperature, oxidizing conditions experienced by the valve (Ref 3, 4). A major US-based power company had new challenges with their supercritical steam drain and vent lines in one of their plants. This particular fossil-fuel power station runs continuously from May to October because of high power demand to run air-conditioning systems. For the balance of the year, the plant only runs when the demand called for more power during peak usage times. Consequently, the operation of the plant changed from base-load to peaking. Every time the plant shuts down or start-up, MSBVs are frequently operated and exposed to super- critical steam, going up to 15 MPa (2200 psi) and 675 °C (1250°F) (Ref 5). HVOF 80/20 Cr 3 C 2 -NiCr coating has failed after 1 year and less than 500 mechanical cycles in service on balls exposed to supercritical steam, with deterioration extend- ing to regions where there is no contact between ball and seats. Coating was applied with a Diamond Jet Ò HVOF system using propylene as fuel. Visual examination of damaged components revealed minor frictional wear and typical stress/fatigue pattern (Fig. 1). The intent of this failure analysis is first to provide valuable information to understand failure mechanism and to help in preventing this type of failure. The second objective of this study is to evaluate alternative coatings to alleviate those issues. Section 2 describes the failure analysis, including experimental techniques, results, and discussion. Section 3 presents test results of potential solutions. Section 4 concludes. Originally presented at the 2012 International Thermal Spray Conference and expanded from the original presentation. Thermal Spray 2012: Proceedings of the International Thermal Spray Conference, Air, Land, Water, and the Human Body: Thermal Spray Science and Applications, Houston, Texas, USA, May 21-24, 2012. Luc Vernhes, Velan, Montreal, QC Canada; David A. Lee, Ken- nametal StelliteÔ, Goshen IN; Dominique Poirier, National Research Council of Canada, Boucherville, QC Canada; and Duanjie Li and Jolanta E. Klemberg-Sapieha, Department of Engineering Physics, Polytechnique Montreal, Montreal, QC Canada. Contact e-mail: luc.vernhes@velan.com. *In this article, load carrying capacity refers to localized load as discussed by Holmberg et al. (Ref 2). This has to be distinguished from the structural load carrying capacity. JTTEE5 22:1184–1192 DOI: 10.1007/s11666-013-9978-8 1059-9630/$19.00 Ó ASM International 1184—Volume 22(7) October 2013 Journal of Thermal Spray Technology Peer Reviewed