This thesis was written on behalf of SOMAS instruments AB. SOMAS develops, manufactures and markets valves. The mission was to reduce the wear between the disc edge and the seat in a butterfly valve that are used at high temperatures. This study investigated the possibility of using a surface treatment or coating that can reduce wear or if a new base material is better to use than the current steel 316 material. Possible materials, surface treatments and coatings were investigated and compared in a material study. In discussion with the company, four test pairs were chosen; • Steel 316 - Steel 316 • Steel 316 - Nitronic 60 • Nitronic 60 - Nitronic 60 • Steel 316 - Steel 316 with a surface coating of Tribaloy T-400 The wear behavior of the selected materials was investigated using a test rig at Karlstad University based on the method of block-on-ring. The machine spins a cylinder that corresponds to the seat against a block which in this case corresponds to the disc edge. The tests were carried out at room temperature, 250 ˚C and 500 ˚C. The cylinder was spinning at a speed of 100 rpm while the block pressed against with a load of 50 N. A profilometer, micro-hardness tester and scanning electron microscope (SEM) were used to investigate the wear. A comparison was made based on the volume of material removed from the blocks, the maximum wear depth of the blocks and the maximum wear depth of the cylinder. Wear mechanisms, chemical composition and hardness profiles were used to explain differences in results. The results were compared with the steel 316 to steel 316 solution. The Tribaloy T-400 showed good properties to reduce the wear at room temperature. Block made of Tribaloy T-400 showed no wear, instead steel from the cylinder had been adhered to the blocks. At higher temperatures, the amount of adhered material on the tribaloy T-400 increased, resulting in more wear at the surface of the cylinder than at room temperature. The wear on the cylinder was comparable to the wear of a steel cylinder that slid against a steel block. Nitronic 60 against nitronic 60 exhibited a significant reduction in wear compared with steel to steel at room temperature. At higher temperatures, nitronic 60 against nitronic 60 was the test pair that exhibited least wear. Nitronic 60 is recommended for further examination as a material in both the disc edge and the seat in a butterfly valve to be used at both room temperature and elevated temperatures. / Detta examensarbete skrevs på uppdrag från SOMAS instruments AB. SOMAS utvecklar, tillverkar och marknadsför ventiler. Uppdraget gick ut på att minska nötningen mellan spjällkanten och sätet i en vridspjällsventil som ska användas i höga temperaturer. Denna studie undersökte möjligheten det att använda en ytbehandling eller ytbeläggning som kan minska nötningen eller om ett nytt basmaterial är bättre att använda än det nuvarande materialet stål 316. Möjliga material, ytbehandlingar och ytbeläggningar undersöktes och jämfördes i en materialstudie. I diskussion tillsammans med företaget så valdes fyra stycken testpar; • stål 316 - stål 316 • stål 316 - nitronic 60 • nitronic 60 - nitronic 60 • stål 316 - stål 316 med en ytbeläggning av tribaloy T-400 Nötningsbeteendet hos de valda materialen undersöktes med hjälp av en testrigg på Karlstad universitet som bygger på metoden ”block-on-ring”. Maskinen snurrar en cylinder som motsvarar sätet mot ett block som i detta fall motsvarar spjällkanten. Testerna utfördes i rumstemperatur, 250 ˚C och 500 ˚C. Cylindern snurrade med en hastighet av 100 rpm medan blocket tryckte mot med en last på 50 N. En profilometer, mikrohårdhetstestare och svepelektronmikroskåp användes för att undersöka nötningen. En jämförelse gjordes baserad på volymen av det bortnötta materialet från blocken, maximala nötningsdjupet på blocken och maximala nötningsdjupet på cylindern. Slitagemekanismer, kemisk sammansättning och hårdhetsprofiler användes för att kunna förklara skillnader i resultaten. Resultaten jämfördes mot stål 316 mot stål 316. Tribaloy T-400 visade upp bra egenskaper för att kunna minska nötningen i rumstemperatur. Block gjorda av Tribaloy T-400 uppvisade ingen nötning, istället hade material från stål cylindern adderats på blocken. Vid högre temperaturer så ökade mängden adderat material på tribaloy T-400 vilket resulterade i att ytan på cylindern uppvisade mer nötning än i rumstemperatur. Nötningen på cylindern kunde jämföras med nötningen hos en stålcylinder som glidit mot ett stål block. Nitronic mot nitronic uppvisade vid rumstemperatur en stor minskning av nötning jämfört med stål mot stål. Vid högre temperatur var nitronic 60 mot nitronic 60 det test par som uppvisade minst nötning. Nitronic 60 rekommenderas för vidare undersökning som material i både spjällkanten och sätet i en vridspjällsventil som ska användas i både rumstemperatur och i höga temperaturer.

A typical hard coating on metal cutting inserts used in for example turning, milling or drilling operations is TiAlN. At elevated temperatures, TiAlN exhibits a well characterized spinodal decomposition into coherent cubic TiN and AlN rich domains, which is followed by a transformation from cubic to hexagonal AlN. Using in-situ synchrotron x-ray radiation, the kinetics of the second transformation was investigated in this thesis and the strong temperature dependence on the transformation rate indicated a diffusion based nucleation and growth mechanism. The results gave additional information regarding activation energy of the transformation and the critical wavelength of the cubic domains at the onset of hexagonal AlN. After nucleation and growth, the hexagonal domains showed a striking resemblance with the preexisting cubic AlN microstructure. During metal cutting, the tool protecting coating is subjected to temperatures of ~900 ºC and pressure levels in the GPa range. The results in this thesis have shown a twofold effect of the pressure on the decomposition steps. Firstly, the spinodal decomposition was promoted by the applied pressure during metal cutting which was shown by comparisons with annealed samples at similar temperatures. Secondly, the detrimental transformation from cubic to hexagonal AlN was shown to be suppressed at elevated hydrostatic pressures. A theoretical pressure/temperature phase diagram, validated with experimental results, also showed suppression of hexagonal AlN by an increased temperature at elevated pressures. The spinodal decomposition during annealing and metal cutting was in this work also shown to be strongly affected by the elastic anisotropy of TiAlN, where the phase separation was aligned along the elastically softer <100> directions in the crystal. The presence of the anisotropic microstructure enhanced the mechanical properties compared to the isotropic case, mainly due to a shorter distance between the c-AlN and c-TiN domains in the anisotropic case. Further improvement of the metal cutting behavior was realized by depositing individual layers with an alternating bias. The individual bias layers exhibited microstructural differences with different residual stress states. The results of the metal cutting tests showed an enhanced wear resistance in terms of both crater and flank wear compared to coatings deposited with a fixed bias.

No description available.

Includes bibliographical references. / The thermal conductivity of titanium aluminide and several ruthenium-aluminium alloys has been studied from room temperature up to 500°C. Ruthenium aluminide is a B2-type intermetallic which is unusual and of special interest because of its toughness, specific strength and stiffness, oxidation resistance and low cost. The possible use of ruthenium aluminide in high temperature industrial applications required an investigation of the thermal properties of this compound. Apparatus, capable of measuring thermal conductivity at elevated temperatures has been designed and constructed. This study represents the first experimental results for the thermal conductivity of ruthenium aluminide alloys. The electrical resistivity of the intermetallic compounds has been measured using apparatus based on the Van der Pauw method. The Weidman-Franz ratio of the ruthenium aluminide alloys has been calculated and this indicates that the primary source of heat conduction in these alloys is by electronic movement and that the lattice contribution is minor. The electrical and thermal properties of ruthenium aluminide are shown to be similar to that of platinum and nickel aluminide. This has important implications for the use of these alloys in high temperature applications.

Bibliography: pages 212-223. / The performance of developmental alloyed steels with improved abrasive-corrosive wear resistant properties has been evaluated. The synergistic effect of abrasion and corrosion in the accelerated wear of steels is examined and the main parameters identified. A model of the process is proposed. The model is used to develop the optimum abrasive-corrosive wear resistance in steels for applications in the gold mines of South Africa. A wide range of engineering steels, both commercially available and experimental, has been evaluated in laboratory simulated abrasive and abrasive-corrosive wear tests. An appraisal of the wear tests and the applicability of the results to in-service conditions has led to the development of an additional abrasive-corrosive wear test. It has been established that both the microstructure and chemical composition determine the resistance of a material to wear. Control of the microstructure by alloying and heat treatment is attempted in order to optimise the abrasive-corrosive wear resistant properties for each class of microstructure whilst maintaining adequate formability and weldability. Abrasion of a metal surface has been shown to accelerate the rate of corrosion. Three categories of corrosion behavior are defined. A model of the abrasive-corrosive wear process is proposed to account for the behavior. The model adequately predicts the outcome to a change in system parameter, namely: an increase in the corrosivity of the water, an increase in the frequency of abrasive events, a change in the chemical composition and the degree of passivity inherent in the material. Recommendations are made to maximize the abrasive-corrosive wear resistant properties without resorting to expensive highly alloyed steels. To satisfy the needs of the mining industry, two microstructures of note are identified: a metastable austenitic (TRIP type) steel and a 0.25% carbon lath martensitic alloyed steel. A basic chemical composition is proposed with each microstructure. The austenitic steel is shown to achieve its abrasion resistance through the high degree of work hardening it undergoes during abrasion and the high ultimate strength of the strained material. The lath martensitic steel has the necessary strength to toughness ratio for good abrasion resistance. A 20% degree of work hardening in conjunction with a bulk hardness in excess of 500 HV is prescribed for superior abrasion resistant properties in the wear system of the mines. The life time cost of the martensitic alloyed steel recommends it for applications in the gold mines of South Africa.

Includes bibliographical references. / The stress corrosion cracking susceptibility of an experimental high nitrogen Cr-Mn stainless steel, known as Cromanite ™, and conventional AISI 304 stainless steel were investigated in order to compare their stress corrosion performance in solutions where AISI 304 stainless steel is known to be susceptible. Slow strain rate tests (SSRT) were performed on solution treated specimens a t30°C in aerated aqueous sodium chloride (NaCI) solution containing hydrochloric acid (HCI) of varying concentration at open circuit potentials. Static tests in the form of bent-beam tests were performed on both solution treated and aged specimens in 3M NaCI solution containing 0.05 M HCI. Potentiodynamic scans and Tafel plots were used to assess corrosion behaviour and corrosion rate respectively, while the electrochemical potentiokinetic reactivation (EPR) method was used to quantify the degree of sensitisation for the materials. The SSRT revealed poor corrosion behaviour of Cromanite TM in the presence of hydrochloric acid. Whilst AISI 304 could be examined for stress corrosion cracking at HCI concentrations up to 0.5 M HCI, Cromanite ™ exhibited corrosion rates which were too fast to permit assessment of stress corrosion susceptibility at HCI concentrations of 0.15 M or above. SCC started in a salt solution containing 0.05M HCI for AISI 304 while Cromanite TM cracked in both salt solution (3M NaCI) and in 0.05 M HCI +3M NaCI. The bent-beam test performed on solution treated specimens revealed no evidence of cracking for both alloys after 100 days of exposure; however, Cromanite ™ suffered substantial mass loss after this period. While aged Cromanite TM suffered intergranular cracking after only 25 days in the test solution, no cracking was observed for the aged AISI 304 after 75 days.

The electrical properties of platinum, gold-palladium and a selection of alloys from the ruthenium-aluminium system have been studied at high temperatures (up to 1000°C). The majority of the ruthenium-aluminium alloy compositions studied lie near or in the ruthenium aluminide phase field. Ruthenium aluminide is a B2 structure intermetallic which is suited to high temperature applications because in addition to a high melting point (2060°C), oxidation resistance to 1200°C and high temperature strength, it is also relatively ductile at room temperature. The possibility of high temperature electrical applications required an investigation of the electrical properties of ruthenium-aluminium alloys as compared to platinum and gold-palladium. Two sets of apparatus, capable of measuring the resistivity and thermo-e.m.f to high temperatures, were constructed and used to obtain the first experimental results for the electrical properties of ruthenium-aluminium alloys. Chemical analysis of these alloys has been performed for the first time, and together with energy dispersive spectroscopy, has revealed a composition at which there is a resistivity minimum and a positive thermo-e.m.f maximum, which appears to be associated with the formation of the ordered ruthenium aluminide phase. The resistivity and the temperature dependence of resistivity of some ruthenium-aluminium alloys are similar to that of platinum, the least resistive of the three materials investigated.

Bibliography: pages 103-107. / A study has been made of the structure and abrasive wear resistance of two austempered commercial spheroidal cast irons. Heat treatments have been carried out for different times between 2 and 120 minutes for a range of austenitising temperatures between 850°C and 950°C and austempering temperatures between 250°C and 450°C. The morphology and constitution of the resulting dual phase ferrite/austenite structure has been examined using optical and scanning electron microscopy and x-ray analysis. The maximum quantity of retained austenite in the structure has been shown to vary up to 50% and to be strongly dependent on heat treatment parameters and the composition of the iron. Laboratory abrasive wear testing has been carried out on these austempered irons and compared with the results of similar tests on a range of abrasion resistant carbon steels. All the austempered irons were found to have better abrasion resistance than proprietary abrasion resistant steels. These austempered irons derive their outstanding properties from the morphology of the dual phase ferritic/austenitic matrix coupled to the high work hardening characteristics brought about by the stress induced austenite to martensite transformation during abrasion. The influence of microstructure and mechanically induced transformation has been studied as a function of austempering temperature and time.

Includes bibliographical references (pages 90-94). / A series of alumina ceramics and silicon carbide (SiC) particulate composites were evaluated in terms of their erosive and abrasive wear behaviour under different conditions, with the aim of reducing wear damage in power plants. The alumina ceramics tested ranged in composition from 90% alumina to 97% alumina content. A nitride fired and an oxide fired SiC particulate composites were also tested for comparison. The impact angle, impact velocity, as well as particle size and type were varied for solid-partide erosion, whereas effects of the applied load, abrasive speed and type of abrasive were studied for abrasive wear. The target materials were also evaluated in terms of morphology and mechanical properties including hardness, flexural modulus and flexural strengths. The erosion rates of the tested alumina ceramics increase with an increase in the impact angle, reaching a maximum at 90°. The high purity 96% alumina dry-pressed body has the best erosion resistance at most impact angles, while the 92% alumina dry pressed body has the worst erosion resistance. The erosion rates also increased with an increase in particle impact velocity, resulting in a velocity exponent (n) value of 1.5. A decrease in the erosion rate was observed for both an increase in particle size range and a decrease in erodent partide hardness. At all angles of impact, solid partide erosion of the target materials is dominated by intergranular fracture and surfaces are typically characterized by erosion pits. The five alumina target materials also show a marked increase in erosion rates when the test temperature is increased from ambient to 150°C. The abrasive wear rates for the materials increased with both applied load and abrasive speed, owing to increased tribological stresses at the contacting asperities. There is also a general trend of increasing abrasion resistance with increasing alumina content. Severe wear, characterized by fracture and grain pullout, is the dominant mechanism of material removal during abrasive wear. This was accompanied by the formation of grooves on the wear surfaces. Although this study was successful in terms of material selection for wear damage reduction in power plants, it also highlighted significant factors and modifications that might need to be considered in future studies.

Nine different ceramic and ultrahard materials have been eroded by four different erodents under standardised experimental conditions. The target materials range from the soft stabilised zirconias to the very hard polycrystalline diamond composites. The four erodent particles used were: soft, friable silica, sharp alumina, tough silicon carbide and very hard, synthetic industrial diamonds. The steady state erosion rates of the different target/erodent combinations were measured. The erosion for each combination was studied by examining the target surfaces at progressive stages of erosion and the erodent particles after impact. Special attention has been paid to the morphology of impact sites, the amount of material lost and the mechanisms of erosion. The ceramic materials eroded by an elastic/plastic process: irreversible deformation is followed by lateral fracture. The ultrahard materials erode by a complex process involving deformation and extrusion of the softer phases and fracture and loss of the harder crystallites. The relative hardness of the target and erodent is a determining factor in erosion. When the hardnesses are similar, the ease of initiation and propagation of lateral fracture determines the rate of material removal. When the erodent particles are much harder than the target material, initiation of fracture is inevitable and the ease of crack propagation determines the rate of material removal. This is controlled by the sharpness of the particles and the microtoughness of the target material. The microtoughness is a function of grain size, porosity and defect density. Melting features, seen in many of the eroded surfaces, are thought to arise from a combination of plastic deformation and frictional heating under high contact stresses. The amount of heat dissipated is determined by the erodent and target hardness and friction coefficients. The rate at which the heat diffuses away from the impact zone is determined by the thermal diffusivities, heat capacities and densities of the target and the erodent.