Master of Engineering - Research
Faculty of Engineering
Muzaka, Khairul, Studies of metal-hydrogen interaction using EDAMM and hydrogen permeation studies in pipeline steels, Master of Engineering - Research thesis, Faculty of Engineering, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3584
Part A. The first part of the work was focused on synthesis of the metal hydride using the electric discharge assisted mechanical milling (EDAMM). EDAMM is a new method used to synthesis metal hydride that was developed at university of Wollongong by A/Prof. Andrzej Calka. The reaction of metal and hydrogen to form metal hydride was observed. Different parameters were used to synthesis metal hydride. A variety of samples such as: Ti, Zr, W, Al, Mg, Fe, V, Nb were prepared and processed by the EDAMM method under hydrogen environment. After EDAMM processing, the phases present were analyzed by X-ray diffraction (XRD) and carbon, hydrogen, nitrogen (CHN) analysis was used to determine hydrogen content.
Zirconium hydride (ZrH1.8) was formed from the reaction of zirconium and hydrogen using this technique containing 1.24 wt%H. Vanadium formed new phase using this method with 0.23 wt%H. This phase could be metal hydride but because of lack of database in the traces software, the new phase could not be determined. For Fe, Ti and Nb there was shifting of peaks in XRD patterns suggesting the formation of solid solution after milling in hydrogen atmosphere. These elements contained 0.23 wt%, 0.31 wt% and 0.29 wt% of hydrogen respectively. The XRD patterns of W, Al and Mg XRDs showed no reaction, possibly due to the low decomposition temperature of Al and Mg metal hydride, and very high temperature decomposition for W above the spark milling temperature. This study was able to show that the EDAMM technique is suitable to synthesis metal hydrides that have high decomposition temperatures.
Part B. The second phase of the work dealt with hydrogen permeation studies in X70 pipeline steels using the electrochemical method developed by Devanathan and Stachurski. Samples from centreline and edge regions of hot rolled strips of standard (1.1 wt. % Mn) and medium Mn (0.5 wt. % Mn) X70 steel as well as normalized X70 transfer bar were examined. The effect of increasing grain size on the diffusion of hydrogen in X70 steel was also investigated. The diffusivity of hydrogen was correlated to the grain size, dislocations, microstructure and inclusions/precipitates present in the X70 pipeline steel samples.
The average grain size of the microstructure plays an important role in the diffusion process. The variation in diffusion coefficient with grain size showed a maximum value for a grain size of 46 μm. At grain sizes lower than this value, there was increased trapping at nodes and triple junctions whereas at higher grain sizes, there was a reduction in diffusion due a reduction in grain boundary surface area per unit volume.
For the X70 steel, the microstructure of the normalized transfer bar containing dislocation-free, coarse equiaxed ferrite grains was the least effective in trapping hydrogen and therefore exhibited the highest hydrogen diffusion coefficient. On the other hand, the hot rolled standard Mn strip having finer elongated grains and a larger amount of strain in the material than the normalized transfer bar displayed the lowest diffusivity. The as received transfer bar samples which had bainitic microstructures showed intermediate values of diffusivity.
The hot rolled medium Mn strip displayed the diffusion coefficients lower than that of the standard Mn strip, due to the contribution to trapping from a finer grain size as well as higher area fraction of carbo-nitride precipitates and inclusions compared to the X70 strip. Moreover, the medium Mn strip exhibited a uniform microstructure and consequently similar diffusion coefficients for the edge and centerline regions. On the other hand, the finer grains of the edge region of the X70 strip resulted in a lower diffusion coefficient.