Dissertations/Theses - Department of Materials & Metallurgical Engineering

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Effects of copper and nickel on the structure and properties of heat treated AI-6Si-0.5Mg alloy
(Department of Materials and Metallurgical Engineering (MME), 2014-05) Abul Hossain; Kurny, Dr. ASW
The purpose of this research was to investigate the influence of copper and nickel on Al-6Si- 0.5Mg alloy. Copper in the range of 0.5 - 4 wt%, individual nickel 2wt% and combined 2wt%Cu and 2wt% Ni have been added to Al-6Si-0.5Mg alloy by melt processing technique. The cast alloys were subjected to homogenization at 500oC for 24 hours. The homogenised alloys were solution treated for 2 hours at 540oC and quenched in ice-salt-water solution. The evolution of microstructure and mechanical properties during heat treatment was studied. The temperature ranges of interest were; solution treatment at 540°C, natural ageing at room temperature, and artificial ageing at 100-400°C. The changes in dendritic composition and eutectic morphology due to solution treatment were quantified by optical and scanning electron microscopic analysis. For the ageing investigations, characterisation of mechanical properties was done by hardness, resistivity and tensile testing. Artificial and isochronal ageing was conducted in an electric furnace. Isochronal ageing was conducted at 100, 150, 175, 200, 225, 250, 300, 350 and 400oC for 1 hour. For all alloys, the hardness increase was remarkable in the temperature range of 200-250oC and maximum hardness was found at ~225oC. The electrical resistivity of the alloys was found to decrease significantly at 225oC. The isothermal ageing was carried out at 150, 200, 225, 250 and 300oC for time ranging from 15 minutes to 360 minutes. For all alloys under investigation, significant improvement of hardness was achieved by ageing for ~60 minutes at 225oC. Optical Microscopes (OM) and Scanning Electron Microscopes (SEM) were used for the microstructural investigation. The morphology of the intermetallic phases were observed in the as-cast and the heat treated samples. The Cu and Mg-rich intermetallics completely dissolved but the Ni-rich intermetallics did not dissolve completely during solutionising. The eutectic Al- Si phases were modified after homogenization and solution treatment. The aged samples were tested for tensile properties. Tensile tests were performed at a constant strain rate of 10-3s-1 for all ageing conditions. In addition, the peakaged samples (1hr at 225oC) were tested at three different strain rates of 10-2s-1, 10-3 s-1 and 10-4s-1. Tensile strength was found to increase with ageing temperature, the maximum being attained at peak aged condition (1 hr at 225oC). The additions of Cu and/or Ni resulted in an increase in tensile strength and maximum strength was found for 2 wt% Cu addition. The strain rates affected the tensile properties
Effects of inclusions on the quality of steel
(Department of Materials and Metallurgical Engineering (MME), 3009-06) Abul Hossain; Kurny, Dr. ASW
Mini steel of mills or plants in Bangladesh melt scraps in Induction Furnaces. Very little refining the liquid steels is possible in these induction furnaces. In many plants the molten steels are cast into what are usually known as pencil ingots. These ingots are cast in vertical moulds resulting in the formation of pipe at the top. Such steel plants that produce pencil ingots do not have refining (ladle refining) facilities. In absence of proper refining and quality control, these ingots usually contain slags, non-metallic inclusions and inhomogeneities and are usually of poor quality. As a result deformed bars produced from pencil ingot contain significant amount of slags and inclusions and show sev.ere segregation effect and inhomogeneity in the microstructure. These defects affect the properties ofthe reinforcing bars. Since these pencil ingots are produced in small lots and with little or no metallurgical control, the products obtained from different heats using apparently similar raw materials usually give different amount of inclusions and these give uncertain physical and mechanical properties. As a result the finished products of these pencil ingots are in general of inferior quality and give substandard reinforcing bars. Many of the steel plants do not use ladle refining furnace (LRF) for refining the liquid steel or do not do the ladle refining of the melt properly. The billets as welL as the . reinforcing bars produced from such unrefined or improperly refined melts contain slags, . inclusions and inhomogeneity in the microstructure .. The deformed bars thus produced show inferior properties. Proper refining in LRF of induction melted. assorted .scrap can give fairly clean and . refined liquid steels. The billets and the reinforcing bars produced from such refined melts are generally free from inclusions and slags. Still better quality is obtained if such refined steel can be cast continuously in billet form. Metallographic study of the reinforcing bars produced from properly refined continuously cast billet show uniform grain size, no heterogeneity in the microstructure, little or no . slag and little inclusions and thus have better properties. The following is summary of the results obtained through this investigation. .:. Pencil ingots contain higher and larger sized inclusions. During hot rolling some of the inclusions (>40~m) are plastically deformed or fractured and act as a critical flaw size to cause failure . •:. Reinforcing bars produced from such pencil ingots showed low yield strength, tensile strength, percentage of 'elongation and reduction in area and in general fail in bend tests . •:. Billets cast without any ~efining give poor physical properties but sometimes satisfactory results . •:. Billet processing through ladle refining fu.rDace ensures smaller sized «20 ~) and lower number of inclusions .The reinforcing bars from such properly refined continuously cast billets give superior mechanical properties such as yield strength, tensile strength, ductility, formability and micro.structural requirements.