Wear Behavior of Spark Plasma Sintered Nanostructured Modified 9Cr-1Mo Steel

Abstract

Ferritic-martensitic (FM) steels are well known for the combination of properties such as high strength and good ductility. Modified 9Cr-1Mo is a popular FM steel widely used as a high temperature (250- 400?C) tubing material in power generation industry. This modified 9Cr-1Mo steel can be effectively used as a material for rings and races of hybrid bearings operating at 350-500?C. Refining the grain size to nanometer scale substantially improves the mechanical properties of steel. The nanostructured modified 9Cr-1Mo steel is expected to retain high hardness and wear resistance at elevated temperatures. In this work nanostructured modified 9Cr-1Mo steel is processed by spark plasma sintering of nanocrystalline steel powder. The nanocrystalline powder is produced by ball milling of as-received modified 9Cr-1Mo steel plate. A detailed x-ray diffraction analysis of a ball milled powder is performed to measure the crystallite size. The powder is spark plasma sintered at 500?C, 800?C and 1100?C for 5 min and 10 min holding time.The relative density more than 97% and the grain size < 1 &#956;m are observed for the samples sintered at 800 and 1100?C temperature. An increase of about 200% in microhardness is achieved for the sample sintered at 1100?C as compared to the as-received modified 9Cr-1Mo steel plate. A systematic wear analysis of the sintered samples and as-received plate is performed with alumina and silicon nitride balls. A detailed microscopic analysis of the wear mechanisms is performed. The order of magnitude lower wear rate is observed in the samples sintered at 800 and 1100?C as compared to as-received plate. The improvement in wear behavior can be attributed to the reduction in grain size of the sintered samples.