Microstructural Characterization and Properties Evaluation of Ni-Based Hardfaced Coating on AISI 304 Stainless Steel by
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NTRODUCTION
AISI 304 stainless steel is widely used as engineering materials due to its high strength and good corrosion resistance properties.[1] However, AISI 304 stainless steel undergoes extensive wear and erosion when applied as components in petro-chemical and marine atmosphere. Furthermore, the pitting corrosion resistance of this steel is very poor.[2] Wear is essentially a surface dependent degradation which may be improved by suitable surface treatment.[3] Borides are the hard phase which improves the wear resistance when are dispersed in metallic matrix.[4] However, the presence of borides in the metallic matrix makes the matrix brittle.[4] Hence, if it is dispersed only on the surface, it improves the wear and erosion resistance without sacrificing the bulk properties.[3] Dispersion of hard borides and the control of its distribution are difficult to achieve and the techniques applied for deposition include thermal spraying, electro-deposition, plasma transferred arc welding and laser surface alloying.[3,5,6] In situ dispersion of borides in steel, aluminum and titanium matrix by laser surface alloying was reported to improve the hardness and wear resistance properties of the substrate significantly.[7–10] High velocity oxy-fuel coating (HVOF) is a PRASHANT SHARMA, Senior Research Fellow, and JYOTSNA DUTTA MAJUMDAR, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, WB, India. Contact e-mail: jyotsna@metal. iitkgp.ernet.in Manuscript submitted February 15, 2012. Article published online September 8, 2012 372—VOLUME 44A, JANUARY 2013
thermal spray deposition based technique where, the precursor powder is melted using oxy-acetylene flame and subsequently, sprayed over the surface of the substrate at a very high velocity higher than the velocity of sound (>600 m/min).[11] Development of dense deposited layer (>97 pct of theoretical density), a higher bond strength, refinement of microstructure and retention of metastability are the notable advantages associated with HVOF coating technique.[11] In the past, HVOF spray deposition has been applied on steel based substrate to improve its wear and corrosion resistance.[12–17] Planche et al.[13] compared the characteristics and coating properties of Ni-7.5Cr-1.7B-3.6Si-2.5Fe alloy developed by flame spraying, plasma spraying and HVOF spraying route and concluded that hardness, porosity or Young’s modulus of the coating are best in HVOF spray deposition mainly due to the highest in-flight velocities of the particles which favors dense coatings. Lin et al.[14] developed Ni-17Cr-3B-4Si-4Fe1C coating on AISI 304 stainless steel substrate by HVOF spraying and achieved a significant improvement in slurry erosion resistance. Sidhu et al.[15] successfully deposited Ni-15.3Cr-3.1B-4.8Si-4.2Fe-0.6C coating on Fe and Ni-based substrate and found to improve hardness and hot corrosion property significantly. Carrasquero et al.[16] showed that the microstructure of Ni7.6Cr3.6Si2.4Fe1.6B0.25C coating developed
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