Investigations on the Behavior of HVOF and Cold Sprayed Ni-20Cr Coating on T22 Boiler Steel in Actual Boiler Environment
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JTTEE5 21:144–158 DOI: 10.1007/s11666-011-9698-x 1059-9630/$19.00 ASM International
Investigations on the Behavior of HVOF and Cold Sprayed Ni-20Cr Coating on T22 Boiler Steel in Actual Boiler Environment Niraj Bala, Harpreet Singh, Satya Prakash, and J. Karthikeyan (Submitted April 13, 2011; in revised form September 9, 2011) High temperature corrosion accompanied by erosion is a severe problem, which may result in premature failure of the boiler tubes. One countermeasure to overcome this problem is the use of thermal spray protective coatings. In the current investigation high velocity oxy-fuel (HVOF) and cold spray processes have been used to deposit commercial Ni-20Cr powder on T22 boiler steel. To evaluate the performance of the coatings in actual conditions the bare as well as the coated steels were subjected to cyclic exposures, in the superheater zone of a coal fired boiler for 15 cycles. The weight change and thickness loss data were used to establish kinetics of the erosion-corrosion. X-ray diffraction, surface and crosssectional field emission scanning electron microscope/energy dispersive spectroscopy (FE-SEM/EDS) and x-ray mapping techniques were used to analyse the as-sprayed and corroded specimens. The HVOF sprayed coating performed better than its cold sprayed counterpart in actual boiler environment.
Keywords
cold spray, erosion-corrosion, HVOF
1. Introduction Thermal spraying has been widely used to apply protective coatings to parts and to repair worn out large diameter shafts in turbines and pumps. In the conventional thermal spraying processes such as flame spraying, arc spraying and plasma spraying, it is important that the material melts completely. With high velocity oxy-fuel (HVOF) spraying, a partial melting is sufficient to achieve a high-quality coating. The reason for this is that the particle velocities upon impact on the substrate are considerably higher in HVOF spraying. In the new process, cold spraying, this development toward higher particle velocities with simultaneously reduced and controlled heat input of the spray material has been rigorously continued. Here the gas temperature is well below the melting temperature of the material, so that the particles in the jet no longer melt (Ref 1). In recent years, HVOF spraying, a novel variation of combustion spraying has had a dramatic influence on the field of thermal spray. Due to the high velocity and high impact of the sprayed powder, the coating produced is less porous and has higher bond strength than that produced Niraj Bala, Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab India; Harpreet Singh, Indian Institute of Technology Ropar, Rupnagar, India; Satya Prakash, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India; and J. Karthikeyan, ASB Industries, Barberton, OH. Contact e-mails: [email protected] and nirajbala1@rediffmail. com.
144—Volume 21(1) January 2012
by other methods (Ref 2). The HVOF spray process represents the state-of-the-art for thermal spray metallic coatings and
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