No / M3/2 sintered high speed steel and composite materials processed by initial admixing of 5 wt.% TiC (to decrease wear) and 5 wt.% MnS (to minimise friction) powders, singly and in combination, were assessed in pin-on-disc tribometers specially constructed to simulate use in the automotive valve train. Pins were of the sintered materials and the mating tribological material discs of T1 high speed steel. For comparison with existing conventional materials, identical tests were performed with discs and pins of two types of spheroidal cast iron. Testing at 110 °C, employing a few drops of fresh Shell Helix Standard SAE: 15W-40, API: SJ/CF oil, in daily segments of 5000 m of sliding distance was carried out until the break of the boundary film and appearance of the early signs of seizure. Accordingly the conditions were initially elastohydrodynamic, then mixed lubrication, then boundary, and finally decaying boundary. In comparison with the baseline cast iron system, the friction, wear and lifetime performance of all the high speed steel systems was markedly superior. MnS further lowered the coefficient of friction and TiC increased the load carrying capacity of M3/2 steel. The lifetime, test distance until seizure, was the most discriminating parameter between the high speed steel systems, being 1.5¿3 times longer for the unmodified M3/2 than the composites and 10 times longer than that of the cast irons system.

One of the most important additives in High Speed Steels (HSS) is cobalt, mainly for its effect on the hot properties. Based on statistic data about the increased price of cobalt and its negative effect on human health, an ethical and financial barrier in the steel industry have occurred. In order to solve the problem, it is of great importance to examine the future cobalt price and accessibility, as well as examine the possibility of finding alternative substitutes to cobalt. The purpose of this project was therefore to examine alternatives to cobalt as an alloying element in HSS. A qualitative literature study was performed by analyzing the economy of cobalt, studying the main reasons for cobalts tendency to improve the hot properties of the steel and finding alternative elements to replace, or at least reduce, cobalt in HSS without degrading the hot properties. Cobalt is used both in the chemical and metallurgical business. But the demand of cobalt is largely driven by chemical purposes with the focus on its rechargeable battery applications. The analysis shows that there is nothing pointing at a significant decrease of the price of cobalt. Lithium ion batteries stands for about 50% of current cobalt supply, which is why the price has surged the recent years. The market for electric vehicles and rechargeable batteries has skyrocketed. To decrease the price of cobalt, a substitute for cobalt in rechargeable batteries would need to be found, which is not very likely for the time being. The effect of cobalt in HSS is mainly on the red hardness and tempering resistance. Cobalt increases the bonding strength in the steel matrix and changes the microstructure of the finer secondary carbides. Also the growth rate and coalescence rate of the carbides decreases. This causes the red hardness and the tempering resistance to increase. To replace cobalt, several alternative alloying elements have been researched. Among the most promising are niobium, nitrogen and aluminium, where niobium were found to be of most interest, due to the broad support of relevant articles in the field of powder metallurgical processing. The positive effect of niobium could be regarded as three-fold. The first contribution is the refinement of grain size and homogeneity of the primary carbides, which increases the overall hardness. The second effect is that the addition of niobium shifts the phase equilibria in such a way that the precipitation of primary carbides mainly will be in the form of hard and stable NbC. The majority of the other alloying elements will hence be precipitated as secondary carbides during tempering. The final effect is an increase in secondary hardness, as a consequence of the large amounts of vanadium and smaller amounts of niobium that is being precipitated during tempering to the secondary carbides. This enables a high matrix hardening potential in the optimal state of tempering.

The recent development of tool steels and high-speed steels has led to a significant increase in alloy additions, such as Co, Cr, Mo, N, V, and W. Knowledge about the phase relations in these multicomponent alloys, that is, the relative stability between different carbides or the solubility of different elements in the carbides and in the matrix phase, is essential for understanding the behaviour of these alloys in heat treatments. This information is also the basis for improving the properties or designing new alloys by controlling the amount of alloying elements. Thermodynamic calculations together with a thermodynamic database is a very powerful and important tool for alloy development of new tool steels and high-speed steels. By thermodynamic calculations one can easily predict how different amounts of alloying elements influence on the stability of different phases. Phase fractions of the individual phases and the solubility of different elements in the phases can be predicted quickly. Thermodynamic calculations can also be used to find optimised processing temperatures, e.g. for different heat treatments. Combining thermodynamic calculations with kinetic modelling one can also predict the microstructure evolution in different processes such as solidification, dissolution heat treatments, carbide coarsening, and the important tempering step producing secondary carbides. The quality of predictions based on thermodynamic calculations directly depends on the accuracy of the thermodynamic database used. In the present work new experimental phase equilibria information, both in model alloys containing few elements and in commercial alloys, has been determined and was used to evaluate and improve the thermodynamic description. This new experimental investigation was necessary because important information concerning the different carbide systems in tool steels and high-speed steels were lacking. A new thermodynamic database for tool steels and high-speed steels, TOOL05, has been developed within this thesis. With the new database it is possible to calculate thermodynamic properties and phase equilibria with high accuracy and good reliability. Compared with the previous thermodynamic description the improvements are significant. In addition the composition range of different alloying elements, where reliable results are obtained with the new thermodynamic database, have been widened significantly. As the available kinetic data did not always predict results in agreement with new experiments the database was modified in the present work. By coupling the new thermodynamic description with the new kinetic description accurate diffusion simulations can be performed for carbide coarsening, carbide dissolution and micro segregation during solidification. / QC 20100929

Orientador: Eduardo Carlos Bianchi / Resumo: Aços-rápidos são materiais que exibem elevada resistência ao desgaste abrasivo, aliada a uma tenacidade relativamente alta, propriedades estas que os tornam adequados para se fabricar ferramentas de corte. Grande parte de suas propriedades se deve a presença de carbonetos na microestrutura. Ao mesmo tempo que estas propriedades mecânicas são favoráveis para a utilização como ferramentas, tornam-se desafios na sua fabricação. O processo de retificação é empregado nas últimas etapas de fabricação de ferramentas de corte como machos e brocas e a presença dos carbonetos afetam a eficiência dos rebolos. Neste contexto, este trabalho visa investigar o quanto a diferença de microestrutura de aços-rápidos classe AISI M3:2, obtidos de diferentes fornecedores, influencia na retificação, levando em consideração a integridade superficial. Os materiais, aqui nomeados como M-A, M-B e M-C, foram avaliados sob três aspectos: características da microestrutura, resistência à abrasão e integridade da superfície após a retificação. Da microestrutura os carbonetos tipo MC e M6C, foram descritos quanto à forma e a distribuição, utilizando para isso MEV e EDS. A resistência à abrasão dos materiais foi medida recorrendo ao método de ensaio tribológico pino-lixa. Após esta caracterização, foram realizados ensaios de retificação tangencial plana com rebolo de Carboneto de Silício (SiC) em várias penetrações de trabalho (entre 10 µm e 30 µm). As superfícies das amostras foram avaliadas mensurando a rug... (Resumo completo, clicar acesso eletrônico abaixo) / High-Speed Steels are materials that exhibit high abrasive wear resistance coupled withrelatively high toughness, properties that make them suitable for making cutting tools. Much ofits properties are due to the presence of carbides in the microstructure. While these mechanicalproperties are favorable for use as tools, they impose challenges in their manufacture. Thegrinding process is employed in the final stages of the cutting tools manufacturing, such as tapsand drills and the presence of carbides affects the efficiency of the grinding wheels. In thiscontext, this work aims to investigate how the microstructure difference of class AISI M3: 2steel, obtained from different suppliers, influences the grinding, taking into consideration thesurface integrity. The materials, here named M-A, M-B, and M-C, were evaluated under threeaspects: microstructure characteristics, abrasion resistance, and surface integrity after grinding.From the microstructure, carbides type MC and M6C were described regarding the shape anddistribution, using for this purpose SEM and EDS. The abrasive wear resistance of the materialswas measured using the pin-abrasive tribological test. After this characterization, flat tangentialgrinding tests were performed, using silicon carbide grinding wheel (SiC), in various workdepths (between 10 μm and 30 μm). The ground samples surfaces were evaluated by measuringthe roughness parameters, evaluated by SEM, and the microhardness profil / Doutor

<p>In powder metallurgy research field, Direct Metal Laser Sintering (DMLS) and Metal Powder Starch Consolidation (MPSC) are relatively new rapid forming techniques to fabricate complex and near net-shaped components. The working principles of DMLS are to melt and fuse metal powder layer by layer in computer controlled systems to pile up components like three dimensional printing. It has been for instance extensively used for mould inserts, die parts, and functional metal prototypes. Another, less explored method, starch consolidation is a pressureless direct casting method which consists principally of mixing powder slurry, casting into moulds, consolidation, drying, and sintering. With a strong focus on both methods, the study here combines several strong material technology sectors; powder, rapid forming, mechanical property testing and surface technology. It covers the processing chain from green body preparation, optimization of</p><p>sintering, nitriding, post sinter heat treatment, to modeling and assessment of material behaviour for end-user applications. An iron based powder and a high vanadium high speed steel powder with low and high carbon contents were used in the DMLS and MPSC processes, respectively. The overall aim of the study is to synthesize near net-shaped powder-based components, to characterize pores and microstructure, and to establish a fundamental understanding of failure mechanisms of powder based materials in bending fatigue, thermal fatigue and wear.</p><p>The study showed the DMLS and MPSC technologies could produce shaped components with a multi-phased structure, controllable nitriding depth and high relative densities in a range of 97 - 99.7 %. Materials' heterogeneity and porosity have detrimental influence on mechanical properties, especially on crack initiation and subsequent propagation.</p>

In powder metallurgy research field, Direct Metal Laser Sintering (DMLS) and Metal Powder Starch Consolidation (MPSC) are relatively new rapid forming techniques to fabricate complex and near net-shaped components. The working principles of DMLS are to melt and fuse metal powder layer by layer in computer controlled systems to pile up components like three dimensional printing. It has been for instance extensively used for mould inserts, die parts, and functional metal prototypes. Another, less explored method, starch consolidation is a pressureless direct casting method which consists principally of mixing powder slurry, casting into moulds, consolidation, drying, and sintering. With a strong focus on both methods, the study here combines several strong material technology sectors; powder, rapid forming, mechanical property testing and surface technology. It covers the processing chain from green body preparation, optimization of sintering, nitriding, post sinter heat treatment, to modeling and assessment of material behaviour for end-user applications. An iron based powder and a high vanadium high speed steel powder with low and high carbon contents were used in the DMLS and MPSC processes, respectively. The overall aim of the study is to synthesize near net-shaped powder-based components, to characterize pores and microstructure, and to establish a fundamental understanding of failure mechanisms of powder based materials in bending fatigue, thermal fatigue and wear. The study showed the DMLS and MPSC technologies could produce shaped components with a multi-phased structure, controllable nitriding depth and high relative densities in a range of 97 - 99.7 %. Materials' heterogeneity and porosity have detrimental influence on mechanical properties, especially on crack initiation and subsequent propagation.