The imminent emergence of the hydrogen fuel industry has resulted in an urgent mandate for very specific material testing. Although storage of pressurized hydrogen gas is both practical and attainable, demands for increasing storage pressures (currently around 70 MPa) continue to present unexpected material compatibility issues. It is imperative that materials commonly used in gaseous hydrogen service are properly tested for hydrogen embrittlement resistance. To assess material behavior in a pressurized hydrogen environment, procedures were designed to test materials for susceptibility to hydrogen embrittlement. Of particular interest to the field of high-pressure hydrogen in the automotive industry, austenitic stainless steels SUS 316 and 316L were used to validate the test programs. Tests were first performed in 25 MPa helium and hydrogen at room temperature and at -40°C. Tests in a 25 MPa hydrogen atmosphere caused embrittlement in SUS 316, but not in 316L. This indicated that alloys with higher stacking fault energies (316L) are more resistant to hydrogen embrittlement. Decreasing the test temperature caused slight embrittlement in 316L and significantly enhanced it in 316. Alternatively, a second set of specimens was immersed in 70 MPa hydrogen at 100°C until reaching a uniform concentration of absorbed hydrogen. Specimens were then loaded in tension to failure to determine if a bulk saturation of hydrogen provided a similar embrittling effect. Neither material succumbed to the effects of gaseous pre-charging, indicating that the embrittling mechanism requires a constant supply of hydrogen at the material surface rather than having bulk concentration of dissolved hydrogen. Permeation tests were also performed to ensure that hydrogen penetrated the samples and to develop material specific permeation constants. To pave the way for future work, prototype equipment was constructed allowing tensile or fatigue tests to be performed at much higher hydrogen pressures. To determine the effect of pressure on hydrogen embrittlement, additional tests can be performed in hydrogen pressures up to 85 MPa hydrogen. The equipment will also allow for cyclic loading of notched tensile or compact tension specimens for fatigue studies. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Previous events have demonstrated the vulnerability of reinforced concrete infrastructure to blast loading. In buildings, ground-story columns are key structural components, and their failure can lead to extensive damages which can cause progressive collapse. To prevent such disasters, the steel reinforcement in such columns must be properly detailed to ensure sufficient strength and ductility. The use of modern concrete materials such ultra-high performance concrete (UHPC) is one potential solution to improve the blast performance of columns. UHPC shows high compressive strength, high tensile resistance and superior toughness, properties which make it ideal for use in the blast-resistant design of columns. The combined use of UHPC and high-performance steels can potentially be used to further enhance the blast resistance of columns. This thesis presents an experimental and analytical study which investigated the use of high-performance materials to increase the blast capacity and ductility of reinforced concrete columns. As part of the experimental study, a total of seventeen columns were tested under simulated blast loading using the University of Ottawa Shock-Tube. Parameters investigated included the effect of concrete type (NSC and UHPC), steel reinforcement type (normal-strength, high-strength or highly ductile), longitudinal reinforcement ratio, seismic detailing and fiber properties. The test program included two control specimens built with normal-strength concrete, five specimens built with UHPC in combination with high-strength steel, and ten columns built with highly ductile stainless steel reinforcement. Each column was subjected to a series of increasing blast pressures until failure. The performance of the columns is investigated by comparing the displacements, impulse capacity and secondary fragmentation resistance of the columns. The results show that using high-performance steels increases the blast performance of UHPC columns. The use of sufficient amounts of high-strength steel in combination with UHPC led to important increases in column blast capacity. The use of ductile stainless steel reinforcement allowed for important enhancements in column ductility, with an ability to prevent rupture of tension steel reinforcement. The study also shows that increasing the longitudinal reinforcement ratio is an effective means of increasing the blast resistance of UHPC columns The thesis also presents an extensive analytical study which aimed at predicting the response of the test columns using dynamic inelastic, single-degree-of-freedom (SDOF) analysis. A sensitivity analysis was also performed to examine the effect of various modelling parameters on the analytical predictions. Overall, it was shown that SDOF analysis could be used to predict the blast response of UHPC columns with reasonable accuracy. To further corroborate the results from the experimental study, the thesis also presents an analytical parametric study examining the blast performance of larger-scale columns. The results further demonstrate the benefits of using UHPC and high-performance steel reinforcement in columns subjected to blast loading.

The stress corrosion cracking (SCC) of components which are fabricated from austenitic stainless steel has been observed in the primary circuit of pressurised water reactors (PWR). In recent years it has become an increasing concern that cold work can induce susceptibility to SCC in these materials, even when exposed to good-quality flowing coolant. Laboratory studies which were launched in response to this observation have confirmed that SCC susceptibility is enhanced by cold work. The intention of this study is therefore to investigate the link between the effects of cold work on the material and the susceptibility to SCC. The investigation has been conducted on a grade 304 austenitic stainless steel. Characterisation of the microstructure and mechanical properties has been carried out in the annealed condition, and following cold rolling to a reduction in thickness of 20 %. The cold rolled material has then been subjected to SCC tests in simulated PWR primary circuit coolant. Two types of test were utilised: slow strain rate tests (SSRTs) were carried out in order to investigate the initiation of cracks from a smooth surface and constant load tests using pre-cracked specimens were used to investigate the crack propagation behaviour. In both types of test the SCC produced was predominantly intergranular. The SSRTs revealed that the most susceptible grain boundaries separated grains which had dissimilar deformation microstructures (one grain deformed heavily by planar bands, the other more homogenously). It was also observed that initiation could occur on a grain boundary which is adjacent to an annealing twin. In both microstructural configurations the susceptibility is likely to be due to the deformation incompatibility across the failed boundary, possible indicating that shear at the boundary is important for the initiation of cracking. The crack propagation behaviour of the rolled material was particularly anisotropic; regardless of the loading direction (specimens were manufactured to allow loading along the rolling, transverse and normal plate directions) cracking was observed to occur parallel to the rolling-transverse plane. The origin of this behaviour was explored in terms of preferential alignment of the deformation microstructure and the anisotropic mechanical properties of the rolled plate. Limited transgranular cracking was also observed, which occurred along oxidised deformation bands. The results overall indicate that heterogeneous deformation between different regions of the material, and preferential alignment of the deformation microstructure are important with respect to the SCC susceptibility of the rolled material.

The thesis is focused on evaluation and comparison of welds made by welding technologies using laser and plasma. For the purpose of comparing those technologies, were chosen austenitic stainless steel x5CrNi18-18 and ferritic stainless steel X6Cr17. These materials were chosen for their diversity in properties gained by high temperatures that go along with most welding processes. First part of the thesis is focused on description of welded materials and technologies of laser and plasma welding. In following experimental section, the thesis is focused on evaluation of welded samples, made for this purpose. Samples were examined for metallography, specifically macrostructure and microstructure. Followed by tensile test of mechanical properties and microhardness test.

The master’s thesis deals with corrosion resistance of cast duplex steels. In the first part the corrosion mechanisms and their individual types are described, the second part characterizes the types of stainless steels in terms of their properties, use, structural components, and their impact on mechanical and corrosion properties of steels. Individual chapter is dedicated to duplex steels. In the experimental part the resistance of 1.4517 steel against pitting corrosion depending on the alloy contents and the PREN value was investigated. Four specimens graded according to PREN were cast. Supplemented with sample stirrer blade which was affected by corrosion during service they were subjected to corrosion test according to ASTM G48-11 standard in 6% FeCl3 solution. Cast specimens were subjected to tensile test and impact test.

Robotization of industry is very important in these days. It is possible to replace human workers with robots and because of that make manufacturing faster. This master’s thesis includes feasibility study of robotic welding for stainless steel part guide rail. Quality of robotic welding was evaluated on testing pieces. Robotic welding was 88,5 % faster than current welding method. Costs of manufacture of one piece were 40 % cut.

The susceptibility to solidification cracking of unstabilized and stabilized ferritic stainless steels was investigated using self-restrained Houldcroft, Modified Varestraint-Transvarestraint (MVT), and hot tensile testing. Five experimental steel grades comprising an unstabilized, two mono stabilized (Ti or Nb), and two dual stabilized (Ti + Nb), and two commercial unstabilized and a dual stabilized (Ti + Nb), and another dual stabilized containing-Mo alloy (nine different alloys in total) were used in this study. Seven steel grades comprising an unstabilized, two mono stabilized (Ti and Nb) respectively, three dual stabilized (Ti + Nb) and a dual stabilized containing Mo were used for the self-restrained Houldcroft method. Autogenous gas tungsten arc welding at a speed of 6 mm/s, 3 mm/s, and 1 mm/s was done. The unstabilized ferritic stainless steel was resistant to solidification cracking. Ti addition to ferritic stainless steel resulted in a minor increase to susceptibility to solidification cracking. Nb in ferritic stainless steel increased solidification cracking. The addition of Ti and Nb resulted in a decreased susceptibility to solidification cracking compared to an alloy containing only Nb. The weld metal microstructures were a mixture of columnar and equiaxed grains. The interdendritic crack surfaces were enriched in Nb, Ti, Mn, Si, Al, Mn, and Mo. The MVT test was used for the test of an unstabilized, a Nb stabilized and two (Ti + Nb) dual stabilized ferritic stainless steels. Two different welding speeds of 6 mm/s and 3 mm/s using autogenous gas tungsten arc welding were employed. The high content (Ti + Nb) steel at a welding speed of 3 mm/s had the greatest sensitivity to solidification cracking. The Nb stabilized steel at both welding speeds (6 mm/s and 3 mm/s) and high content (Ti + Nb) steel at a welding speed of 6 mm/s showed intermediate sensitivity to solidification cracking. The unstabilized and low content (Ti + Nb) grades were the least sensitive to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. Three experimental Ti-, Nb-, and dual Ti + Nb stabilized ferritic stainless steels were used for hot tensile testing using a Gleeble-1500D thermo-mechanical machine at testing temperatures of 1200°C, 1250°C, and 1300°C. The dual stabilized ferritic stainless steel showed a high and fairly constant hot ductility with an increasing testing temperature. The Ti stabilized alloy revealed a slightly lower ductility compared to the dual stabilized steel but much higher ductility than the Nb stabilized ferritic stainless steel. The SEM images of the intergranular cracking showed interdendritic morphologies. EDX analysis showed the elements Al, Mn, Ti, Si, Ni, S, Nb, and Ni to be associated with the fractured surfaces. The hot tensile test results were inconclusive, due to the small number of samples and an acquisition frequency that was too low. The MVT test was better than the self-restrained Houldcroft, and the self-restrained Houldcroft was better than the hot tensile tests in quantifying the susceptibility of a specific ferritic stainless steel alloy to solidification cracking. The cracking response of Houldcroft seemed to be dominated by welding speed. Cracking response of MVT test seemed to be dominated by the Nb content. The effect of Nb and Ti on the susceptibility cracking could be explained in terms of the effect of these two alloying elements on the difference between the liquidus and the solidus. Nb was found to segregate strongly to the grain boundaries (low k value) which resulted in a significant increase in the difference between the liquidus and the solidus. This difference increased BTR which results in a high susceptibility to solidification cracking. Ti has a higher k value and segregates less than Nb during solidification. / Thesis (PhD)--University of Pretoria, 2018. / Materials Science and Metallurgical Engineering / PhD / Unrestricted

The oxide scale created during manufacturing of stainless steel is often removed by a chemical pickling with mixed acid. Various pre-treatments to pickling are also applied to increase the efficiency of the oxide scale removal. Shot blasting is one such pre-treatment, which operates to remove a certain amount of oxide and also to generate cracks and openings for the subsequent pickling. In this work, three materials, AISI 2205, 430 and 304 have been blasted and later exposed to mixed acid. This was conducted to quantify the effect shot blasting has on the subsequent pickling of stainless steel. Three shot products and two blasting parameters were investigated and varied. Shot blasting significantly reduced the oxide scale but was unable to remove the chromium oxide layer closest to the steel surface. No significant difference was observed when comparing the result from the three shot products after blasting. Increasing the particle velocity or the coverage rate further enhanced the pickling but reduced the amount of oxide removed during blasting. These effects are however relative small, so in practise relatively mild conditions seems preferable for economic reasons. Oxide scale containing hematite was difficult to remove by both shot blasting and pickling.

The purpose of this research is to identify the effects of the carbon-infiltrated carbon nanotube (CICNT) growth process on the material properties of 316L stainless steel, particularly those properties which are essential for biocompatibility. Physically altering the micro-topography of a surface can dramatically affect its capacity to support or prevent biofilm growth. Growing CICNTs on biomedical materials is one approach which has demonstrated success at preventing biofilm growth. Unfortunately, the high temperature and carbon-rich gas exposure required for this procedure has proven to have deleterious effects. Rusting has been observed on samples that have been coated with CICNTs and then placed in culture media. A proper understanding of this rusting phenomenon, along with an exploration of other material properties which could be affected by the procedure, is a necessary prelude to further development of this novel antibacterial method. This thesis proposes a kinetic model derived from Fick's Second Law to predict the growth of chromium carbide as a function of temperature and time. Chromium carbide formation is shown to be the mechanism of corrosion, as chromium atoms are leeched from the the material, preventing the formation of a passivating chromium oxide layer that protects iron oxide from forming. The model is validated using experimental methods, which involve immersion in culture media, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and double loop electrochemical potentiokinetic reactivation (EPR) testing. This thesis further explores how the CICNT growth procedure affects the mechanical properties of 316L stainless steel as a function of temperature, time exposure to ethylene gas flow, and sample geometry. It is shown that the CICNT growth procedure effectively carburizes the stainless steel surface. Tensile tests demonstrate that the carburized surface leads to brittle failure for thin samples that have a relatively small ductile interior. This thesis also examines the adhesion and wear of the CICNTs on the surface of the 316L stainless steel. Tape tests and torsional shearing show strong adhesion between the CICNTs and the metal substrate. External fixator pin drilling also shows remarkably good wear properties for the CICNT surface. The changes in mechanical properties and the overall adhesive performance must be considered and properly managed by biomedical engineers hoping to use CICNT coatings.

A literary study has been made to find parameters affecting dephosphorization of stainless steel. Ways to lower phosphorus content without major loss of chromium in order to decrease the production costs of Sandvik AB’s stainless steel. The study was conducted with respect to oxidizing dephosphorization and parameters affecting dephosphorization are carbon and chromium content, temperature and slag properties. It was revealed that higher carbon content and basicity is beneficial to the dephosphorization process. However, the choice of temperature, chromium content and flux were dependant on which way dephosphorization was approached. A method of refining chromium in slag using an electric arc furnace (EAF) was also discovered. This made it possible to extract 97% of all chromium in slag. The conclusions are that in order for Sandvik to successfully remove phosphorus, scrap metal with higher carbon content should be used, together with a basic CaO slag with constituents of e g Li2O3 and CaF2. A deslagging step can be added to the argon oxygen decarburizer (AOD) process in order to remove the phosphorus bound to the slag, before decarburization.