Browsing by Author "Ball, A"

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    Laboratory simulation of metal dusting corrosion
    (1997) Vaughan, Andrew James Laburn; Vaughan, Andrew James Laburn; Ball, A; Ball, Anthony
    A laboratory carburising furnace in which metal dusting conditions are simulated has been designed and constructed. This furnace has been used to simulate and study the metal dusting corrosion of four iron-based alloys viz. 9Cr Mo 45 steel, Incoloy 800H, AISI 310 stainless steel and Chromanite - an experimental high-nitrogen Cr-Mn stainless steel (HNSS). Tests conducted on the carburising furnace show that the rig is capable of heating a flowing gas environment to temperatures of 800°C in the horizontal ceramic tube. The design allows the testing of up to thirty-six test specimens in a constanttemperature test zone. Systems for the safe heating and disposal of gases such as hydrogen and carbon monoxide have been incorporated into the design. Four twenty-four hour exposures were performed on specimens of the CrMo steel and as received samples of AISI 310. This was followed by a series of seven week-long exposures of Incoloy 800H, AISI 310 and the high nitrogen stainless steel (HNSS). These specimens were tested in an annealed and polished condition in order to increase their susceptibility to metal dusting. In a third test series, specimens of these three alloys were tested in an annealed and abraded condition in order to determine the effect of grain size and surface roughness on metal dusting resistance. Exposures of the CrMo specimens resulted in general metal loss and massive carbon deposition after the first exposure of 24 hours. Filamentous carbon deposits containing metal particles showed that metal dusting corrosion of the specimens had taken place. The as received AISI 310 specimens showed no signs of metal dusting attack over the same exposure time. This was attributed to a protective surface chromia layer that prevented carburisation of the specimens. During the second test series, specimens of Incoloy 800H in the sensitised condition showed a high susceptibility to metal dusting. Carburisation of the matrix carburisation was accompanied by large carbon protrusions growing from the specimens' surfaces. Large pits were observed on the specimens after five weeks. Sensitised AISI 310 specimens also showed signs of metal dusting but at a slower rate. The difference in performance between these two alloys was attributed to the difference in alloying contents, notably chromium, nickel and silicon. The HNSS specimens showed a high resistance to carburisation, carbon deposition and metal loss during the first six weeks. Small amounts of carbon deposition and pitting were observed after the seventh exposure. The good resistance to metal dusting of this alloy was attributed to its alloying contents, which included chromium, manganese, sulphur and nitrogen. The results of the third test senes showed that resistance to metal dusting was significantly improved by increasing the surface roughness and decreasing the grain size of the specimens. A new alloy, Fe-25Cr-12Ni-9Mn-4.5Al-2Si-0.5N is proposed for fabrication and exposure to metal dusting environments to evaluate its suitability for use in industrial applications. It is also recommended that further work be carried out in evaluating the effect of increasing the nitrogen, chromium and manganese contents of the Fe-18Cr- 9Mn-0.5N alloy that performed well in this project. Investigations into the effect of aluminising and nitriding components should also be carried out.
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    The erosion of materials
    (1998) Feng, Zheng; Feng, Zheng; Ball, A; Ball, Anthony
    Solid particle erosion tests of glass, stainless steel, WC-Co and sintered alumina, have been performed with seven erodents using a range of particle diameters D (63 μm - 1000 μm), velocities V (33 m.s⁻¹ - 131 m.s⁻¹ ) and impact angles α (30° - 90°). The seven erodents are steel shot, glass beads, silica, alumina, tungsten carbide, silicon carbide and diamond· particles. In addition, the target materials have been subjected to cavitation erosion using a conventional ultrasonic horn in distilled water. Systematic studies of the influence of the impact variables on the erosion rate have been made. Scanning electron microscopy of the eroded surfaces and the erodents after impact has been performed. Empirical correlations between erosion rate and the parameters of erosion and the erodents were obtained and are discussed in terms of the modes and mechanisms of erosion. A semi-quantitative theoretical model has been developed to explain the empirical correlations for brittle and ductile materials. The mode of erosion of glass impacted by irregularly shaped particles is associated with the formation and interaction of lateral cracks over all impact velocities and angles used in this study. The erosion of glass by spherical particles is determined by particle size, impingement velocity and angle. An erosion map, in which the erosion of glass is mapped against velocity and particle size, has been constructed to categorise the types of damage observed in glass for impingement angles between 90° and 30°. The erosion. behaviour of 304 stainless steel is associated with cutting or ploughing and plastic accumulation processes. The erosion of WC-Co is associated with a combination of ductile and brittle modes of erosion. The erosion of alumina is brittle and associated with intergranular spallation and grain-crushing. An analysis of the results reveals that for the brittle materials, glass and alumina, the erosion rate is determined by kinetic energy, particle size and the relative hardness and toughness of the erodents. However, for ductile materials, the shape and kinetic energy of erodents are the most important factors determining the erosion rate. There is no significant effect of hardness and toughness of erodents on erosion. Surprisingly, the erosion resistance of the softer 304 stainless steel is better than that of alumina and WC-Co when hard erodents are used at impact angle greater than 40°. On the other hand the erosion resistance of the harder WC-Co and alumina is better than that of 304 stainless steel for softer erodents like silica erodents. Glass always exhibits poor erosion resistance. In cavitation erosion, stainless steel exhibits better cavitation erosion resistance than glass, alumina and WC-Co. The cavitation erosion resistance of WC-Co is dependent upon the cobalt content. An attempt to rationalise the results in terms of mechanisms has been made. Both solid particle and cavitation erosion rate for the as received glass is higher than that for the tempered glass due to introduction of residual compressive stresses into the surface by the tempering process. Particularly, it reveals that compressive stresses are more efficient in preventing the formation and propagation of Hertzian cracks. These findings will assist in the choice and design of materials that undergo both particle and cavitation erosion under specified conditions.