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No description available.

An investigation has been undertaken to examine the effect of microstructure on the mechanical properties of FV520B, a precipitation hardenable martensitic stainless steel. This high performance grade of stainless steel was heat treated to three commercially available material specifications, namely the peak hardened, standard and softened overaged conditions. These three precipitation hardened conditions were found to exhibit a range of tensile properties. In order to determine the role of the microstructure, a full materials characterisation programme was performed. The investigative techniques used to characterize the microstructures, were Transmission Electron Microscopy (TEM); Analytical Scanning Electron Microscopy (ASEM); optical microscopy; dilatometry and X-Ray Diffraction (XRD). The microstructural phases and features identified were measured and quantified wherever possible. The effect of the material microstructure and environment on the fatigue properties of FV520B have been investigated. Fatigue tests were performed under uniaxial loading conditions at a stress ratio R (omin/omax) of -1. The tests were undertaken using highly polished specimens to determine the fatigue strength of the three precipitation hardened conditions. The test conditions employed were air and a corrosive 3.5% sodium chloride environment, at pH2 and ambient temperature. The role of the microstructure and the effectiveness of the tensile strengthening mechanisms on the fatigue and corrosion fatigue strength have been discussed. Using SEM, the fatigue crack nucleation mechanisms prevalent within the three microstructures in air and the chloride environment have also been identified. For the peak hardened material, nonmetallic inclusions dominated the fatigue crack nucleation process in air and chloride environments. For the softened overaged condition, multiple site nucleation due to slip band cracking was the prevalent mechanism especially at higher nominal stress amplitudes. The tolerance of this high strength material to small defects at higher stress levels and the actual size of the critical microstructural defects initiating failure have also been highlighted. The microstructure has been shown to strongly influence the processes of fatigue crack nucleation, Stages I and II crack propagation and the concept of the microstructure acting as barriers and providing resistance to crack growth have been discussed. The effectiveness and the size of these microstructural barriers to crack growth have been considered. This discussion has led to the proposal of a model that facilitates flow stress and fatigue lifetime predictions as a function of the quantity of a key microstructural phase. The key microstructural phase, namely reverted austenite affected both the tensile and fatigue properties of FV520B as a function of the heat treatment. The standard overaged material was found to exhibit the greatest resistance to fatigue crack propagation.

No description available.

No description available.

No description available.

The ability of metals and alloys to form and retain protective oxide scales is crucial to their stability at elevated temperatures for extended times. Hence the identification of factors that promote or limit the integrity of oxides on high temperature materials has been the subject of intensive investigations. In the present study the mechanical properties of this chromium-rich scale on 304 stainless steel foil has been investigated in relation to the deformation rates in the substrate. It was shown that heavy cold working (up to 90%) delays the onset of breakaway oxidation and results in a very adherent scale. The cracking behaviour of the scale was found to be strain rate and temperature dependent under slow strain rate conditions when the substrate deforms by creep. No strain rate dependence was observed over the temperature range 700-900°C when faster strain rates (> 10⁻⁵ sec⁻¹) were applied. The transition between these two responses was found to vary only slightly with temperature between 5.0x10⁻⁵ sec⁻¹ and 7.8x10⁻⁵ sec⁻¹, increasing as the temperature is raised. A new method has been described for determining the fracture behaviour of oxide scale by estimating the composite defect size. From a knowledge of the onset of scale cracking, determined in situ using the acoustic emission technique, it was possible to correlate the measured intercrack spacing with the fracture toughness from which the tensile properties of the scale can be evaluated.

A dissertation submitted to the University of the Witwatersrand in fulfillment of the degree of Master of Science, Engineering. Johannesburg, 1995. / Some authors define weldebility as the ability of a material to maintain its integrity, that is, its microstructure, corrosion and mechanical properties after welding. If such a stringent criteria is used to adequately describe the weldability of a material, then there would be but a few alloys which could be deemed weldable. As such this definition was found to be much to restrictive, and the author, has defined the term 'weldability' as the ability of a material to retain its corrosion and mechanical properties, such that the integrity of an as-welded structure under a particular service environment would be maintained to within acceptable limits. With this definition in mind the weldability of experimental low-nickel duplex stainless steels containing chromium, manganese and nitrogen were assessed. This assessment was based on the corrosion and mechanical properties of these alloys which were manual metal arc welded and plasma arc welded using a range of heat inputs. The results of these tests showed that the wrought and spun cast alloys are readily weldablility for the range of heat inputs considered. As a comparison to the welding charactristics of the high nitrogen alloys, a commercially available high carbon containing spun cast duplex stainless steel, MP36, alloy was used as a reference material. This alloy proved invaluable in that, not only was it possible to show that this alloy faked poorly in comparison with the high nitrogen containing alloys, but the effect of nitrogen as an interstitial element in welded stainless steels could be compared with that of carbon. The results of the latter evaluations showed that nitrogen, unlike carbon, does not have a detrimental effect on the corrosion or mechanical properties of these alloys, provided it is kept below the solubility limit of 'the stainless steel. The appearance of the microstructures of the high nitrogen containing alloys and the reference alloy in the solution annealed condition, were maintained after welding, with no rampant ferrite phase growth or extensive precipitation in the heat affected zone for the range of {Abbreviated Abstract. Open document to view full version} / MT2017

University of Macau / Faculty of Science and Technology. / Department of Electromechanical Engineering

"Case hardening, a process which includes a wide variety of techniques, is used to improve the wear resistance, by diffusing carbon (carburization), nitrogen (nitriding) and/or boron (boriding) into the outer layer of the steel at high temperature, and then heat treating the surface layer to the desired hardness without affecting the softer, tough interior of the part. In this research, a nitrogen-hydrocarbon gas mixture was used as the process atmosphere for carburizing steels. It can offer a cost and part quality alternative to the conventional endothermic atmosphere and vacuum processes. It can hold the promise for matching the quality of work parts processed in vacuum furnace, i.e. eliminating the intergranular oxidation which normally occurs in the endogas atmosphere. The process control of nitrogen-hydrocarbon atmosphere is also investigated in the research. Modified shim stock method is used to measure the carbon pickup and constant carbon flux modeling tool is used afterwards to predict the carbon profile. With minimum modification, commercially available equipment or sensors can be used to monitor non-equilibrium process atmosphere. Gas nitriding was also studied. For nitriding, the kinetics of the nitriding process with hydrocarbon gases addition and electric arc discharge activation of the nitrogen diluted ammonia atmosphere were investigated. Prior to and during the nitriding, hydrocarbon gases were reacted with metal surface and removed oxidation layers, which can accelerate nitriding process. Overall, nitriding with this unique gas mixture provides an alternative to a long-hour pure ammonia nitriding with more efficient energy utilization. The main objective of this project is to develop the conventional, atmospheric-pressure, low-cost surface hardening treatments for the case hardening of carbon, alloy and stainless steel. The possibility of plasma activation of atmosphere and metal surface to shorten processing time and save energy and time is investigated in this research. The process atmosphere is safer, more efficient, less toxic and less flammable. "