Austempered high silicon steel: investigation of wear resistance in a carbide free microstructure

The production of wood-pellets involves fractioning of the raw material, e.g. the sawdust and cutter shavings, into a homogeneous particle size before pressing through a ring die pelletizer. The fractioning takes place in a hammer mill, where wear on the cutting knives is considered a problem. The main material property requirements of the knives are high wear resistance and toughness. This research project examines the suitability of austempered high silicon steel to achieve these desirable properties. One specific steel, 55Si7, has been austempered to obtain a carbide free ausferritic microstructure and the mechanical properties, microstructure and wear resistance of this steel have been characterized. Microscopy and x- ray diffraction were used for microstructural characterization and examination of the stress-induced transformation of retained austenite into martensite. The mechanical properties were characterized via tensile testing, Charpy V-notch impact testing and hardness measurements; and the wear resistance has been investigated in a laboratory environment under non- lubricated sliding wear conditions. The wear resistance of austempered high silicon steel has been compared to other steels heat treated under different conditions to obtain different microstructures. In addition, a field test took place in a hammer mill for a direct comparison between the wear on four austempered high silicon knives and commercial knives made of case hardened low carbon steel. The austempered high silicon steel proved to have a uniform and high hardness combined with good impact toughness, plastic deformation before fracture, and high wear resistance. These desirable properties were achieved by austempering at low temperatures, resulting in a very fine microstructure and absence of carbides due to the presence of silicon.

Author: Lindstrom, Anders

Source: Lulea University of Technology

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