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The research is divided into four related sections of work. The first relates to the rim wall wear of the existing unlubricated steel, and polyethylene centre bearing components. Based on these findings, the second and third sections of work includes materials characterisation of alternative centre bearing surfaces - plasma nitrided molybdenum steel and stellite 6 laser clad layers, respectively. Finally, in the last section of work, the reciprocating pin-on-plate wear test method is used to evaluate the friction and wear of the existing and alternative centre bearing materials. The worn dimensions of the AISI 1053 steel, Hadfield steel, and polyethylene centre bearing components were determined. The wear of the high density polyethylene centre bowl liner was negligible. The rim wall wear of the unlubricated steel components was greatest in the longitudinal direction, whilst there was negligible wear in the lateral direction. The average wear depth rate for the AISI 1053 steel top centre was approximately twice that of the Hadfield steel centre bowl liner. The cross-sectional microhardness and microstructure of one worn AISI 1053 steel top centre and two worn Hadfield steel centre bowl liners were determined. The worn Hadfield steel centre bowl liners showed significant near surface work hardening. The wear mechanism for the AISI 1053 steel top centre was plastic strain accumulation in conjunction with low cycle fatigue. The quench and tempered AISI 4016 molybdenum steel samples were plasma nitrided at 450, 500, 550 and 580 C using 75% N2: 25% H2 mixture gas for 5 hours. The microstructures of the coatings were determined using optical microscopy, and scanning electron microscopy. The treated samples were characterised using x-ray diffraction and vi microhardness. The optimum condition for this material was achieved at the temperature of 500 0C. Stellite 6 multi-track layers were laser clad onto mild and AISI 4016 steel substrates with a continuous wave Nd:YAG laser at 1800 W laser source power using four different processing speeds: 600, 900, 1200, and 1500 mm/min. The laser power, defocused laser spot size, and powder feed rate were held constant. The clad samples were characterised using optical microscopy and scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS). Microhardness profiles of the clad layers and heat affected zones were determined. For both substrates the optimum processing speed is between 600 and 900 mm/min. Wear testing of Hadfield pin - AISI 1053 steel plate, Hadfield pin - untreated AISI 4016 steel plate, HDPE pin – Hadfield steel plate, Hadfield pin - plasma nitrided AISI 4016 steel (500 °C) plate, and Hadfield pin – laser clad Stellite 6 (600 mm/min) plate material pairs was conducted using the pin-on-plate reciprocating wear test method. The wear test conditions provided a good simulation of the rim wall operating conditions for the Hadfield steel pin – plasma nitrided AISI 4016 steel (500 °C) plate and Hadfield steel pin – laser clad Stellite 6 (600 mm/min) plate material pairs. The Hadfield steel pin – nitrided AISI 4016 steel (500 °C) plate material pair had the lowest wear under these wear test conditions, whilst it’s co-efficient of friction of 0.57 would make it suitable for use in lightly loaded (50 ton wagon mass) 3-piece freight bogies. |
