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.

Made available in DSpace on 2016-12-08T17:19:39Z (GMT). No. of bitstreams: 1 Resumo.pdf: 35261 bytes, checksum: 5c14dd5939481fe398edfb044903a091 (MD5) Previous issue date: 2011-07-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This paper presents a study on the linear thermal expansion of foundry sands used as aggregates in foundry cores with resin phenolic-urethane/Cold Box. The work examines the correlation between thermal expansion of the sand and hot deformation tendency of the core during the casting process. The hot deformation tendency has important technological significance, because it affects the dimensional quality of the casting. It was tested three silica sands, one chromite sand, two types of based aluminum silicates sands and two types of blended minerals. The sands were characterized for particle size distribution, loss on ignition, AFS clay content, density, permeability, pH, acid demand, specific surface area, coefficient of angularity, chemical analysis, XRD, shape and surface of the grains and linear thermal expansion. The dilatometer test was done with loose unbonded sands. The sand was placed into an alumina sample holder and inserted into the dilatometer. The hot deformation tendency of the core was performed with two tests: first, using an experimental iron casting and second a hot distortion laboratory test, in which the test specimen was subjected to heating by a flame. It was verified that the hot deformation tendency is directly correlated to the thermal expansion of the sand. The higher the thermal expansion of the sand used to make cores the more susceptible it is to hot deformation. / O presente trabalho apresenta um estudo sobre a influência da expansão térmica linear das areias de fundição utilizadas para a confecção de machos com resina fenólicauretânica/ Cold Box sobre a tendência à deformação a quente dos machos durante o processo de fundição das peças. A tendência à deformação a quente dos machos tem grande importância tecnológica, porque isto afeta diretamente a qualidade dimensional das peças fundidas. Foram ensaiadas três areias de sílica, uma areia de cromita, duas areias à base de silicato de alumínio, dois compostos de minerais. As areias foram caracterizadas quanto à distribuição granulométrica, perda ao fogo, teor de argila AFS, densidades solta e relativa, permeabilidade, pH, demanda ácida, superfície específica, coeficiente de angularidade, análise química, difratometria de raios-X, formato e superfície dos grãos e expansão térmica linear. O ensaio de dilatometria das areias foi realizado em corpos de provas não ligados com aglomerantes, ou seja, a areia foi acondicionada em um porta-amostra de alumina e este foi inserido no dilatômetro. Foram executados dois ensaios de tendência à deformação a quente dos machos: o ensaio de corpo de prova fundido e o ensaio de distorção a quente em laboratório, onde o corpo de prova (macho) é submetido a um aquecimento por chama. Verificou-se que a tendência à deformação a quente dos machos tem correlação direta com a expansão térmica da areia. Quanto maior a expansão térmica da areia utilizada para a confecção do macho mais suscetível ele estará à deformação a quente.