Nuevos materiales tipo cermet de matriz Fe: estudio de la composición, microestructura y propiedades

En la industria de la fabricación de herramientas de corte el material más utilizado en los últimos años ha sido el carburo cementado o metal duro, WC‐Co; sin embargo, debido a su alto coste y a su toxicidad, en la actualidad existe un interés por buscar alternativas a su utilización. Los cermets base TiCN podrían ser una opción competitiva debido a su superioridad en propiedades como dureza y resistencia al desgaste y a la oxidación; no obstante los metales que se utilizan convencionalmente como matriz son Ni y Co, los cuales presentan los mismos problemas de coste y toxicidad. En este trabajo se propone la utilización de Fe como matriz metálica alternativa en cermets base TiCN, la desventaja que presenta la utilización de Fe es la baja mojabilidad sobre las partículas de TiCN durante la sinterización en fase líquida, lo que dificulta su procesado. En cermets convencionales de matriz Ni y Co, adiciones de elementos de aleación o compuestos como Cr, Mo, Mo₂C y WC pueden mejorar el procesado y propiedades de los cermets debido a cambios en la mojabilidad de la fase líquida. En este trabajo se propone estudiar si estos elementos o compuestos juegan el mismo papel cuando la matriz es Fe. Para ello se han elegido dos aceros comerciales como matriz base Fe: el acero inoxidable (grado 430) que contiene Cr y el acero rápido (grado M2) que contiene W, Mo y Cr, y se han comparado con la matriz de referencia de Fe. Para estudiar el papel de los elementos de aleación y del contenido de carbono en la formación de fase líquida y en su mojabilidad sobre la fase cerámica elegida (TiCN) se han utilizado dos herramientas fundamentales: la medida del ángulo de contacto entre la matriz metálica y el refuerzo cerámico y su evolución con el tiempo y la temperatura, y el cálculo de los diagramas de fase de los sistemas estudiados en función del contenido de carbono. Los resultados obtenidos han servido de base para explicar las propiedades de los cermets tras la sinterización. Debido a los altos valores de densidad y dureza alcanzados por el sistema M2/TiCN, reflejo de la buena mojabilidad e interacción entre sus fases, se ha estudiado este sistema con mayor profundidad. El estudio del sistema M2/TiCN incluye la influencia del contenido de carbono en las transformaciones de fase y en los mecanismos de sinterización, responsables de los cambios en la microestructura y en las propiedades finales de los cermets. Una de las ventajas del uso de Fe como matriz es la capacidad de ser endurecido mediante tratamiento térmico. Por ello se ha realizado un estudio de las condiciones de tratamiento (austenización seguido de temple y doble revenido) para alcanzar un aumento de la dureza de la pieza. Además se ha estudiado el comportamiento frente a oxidación tanto del cermet en estado sinterizado como tras ser sometido a tratamiento térmico. La importancia de conocer este aspecto radica en la potencial aplicación de este material como herramienta de corte y conformado, operaciones que conllevan disipación de energía en forma de calor. El comportamiento a oxidación del cermet base Fe se compara con el de dos materiales comercialmente utilizados para tal fin, un acero rápido y un carburo cementado. Otro aspecto que se ha que se ha abordado en este estudio es el afino de la microestructura con el fin de alcanzar mejores propiedades mecánicas. Para ello se han utilizado técnicas alternativas como la etapa de mezcla mediante molienda mecánica y la consolidación de polvos mediante Spark Plasma Sintering. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------The most commonly used material in the cutting tool industry in the last years has been the cemented carbide or hard metal, WC‐Co. However, there is an interest to look for alternative materials due to its high cost and toxicity. TiCN based cermets could be a competitive option due to their good properties such as hardness, wear and oxidation resistance, nevertheless the metals which are conventionally used as metallic matrix are Ni and Co, which show the same problems of cost and toxicity as cemented carbide. In this work the use of Fe as alternative metallic matrix is proposed. The disadvantage in the use of Fe is its low wettability on TiCN particles during liquid phase sintering, which hinders the processing of the cermet. It is well known that in cermets with Ni and Co as matrixes alloying elements or compounds such as Cr, Mo, Mo₂C and WC can improve the wettability by decreasing the contact angle between the matrix and the reinforcement. The role of these elements and compounds in an iron matrix cermet has been studied in this work. To perform this research two commercial steels have been chosen as metallic matrix: stainless steel (430 grade) which contains Cr and high speed steel (grade M2) which contains W, Mo and Cr. The results obtained are compared with those of the reference cermet with plain Fe as matrix. The influence of alloying elements and carbon content in the liquid phase formation has been investigated. For this purpose two main tools have been used: the measurement of the evolution of the contact angle at high temperature between the metal matrix and the ceramic reinforcement; and the calculation of the phase diagrams of the studied cermets as a function of carbon content. The results have provided a basis for explaining the properties of cermets after sintering. The sintered cermet M2/TiCN shows high density and hardness due to the high wettability and good interaction between both phases. This system has been analyzed in depth. The study includes the influence of carbon content in the transformations during heating which are the responsible for the changes in the microstructure and the final properties of the cermets. One advantage of using Fe as a matrix is the ability to be hardened by heat treatment. Therefore the heat treatment conditions (austenitizing followed by hardening and double tempering) to increase the hardness of the sample have been established. In addition, the oxidation behavior of the cermet as sintered and after heat treatment has been studied. The main reason for investigating the oxidation behavior of these materials is that their potential application is in cutting tools manufacturing. Because of that, its oxidation behavior has been compared to that of two commercial materials, a high speed steel and a cemented carbide. Finally the refinement of the microstructure in order to improve the mechanical properties has been investigated. To obtain a fine and homogeneous microstructure alternative processing methods such as consolidation by Spark Plasma Sintering and mixing by mechanical milling have been chosen

High Temperature Transformations in a Steel-TiCN Cermet

The influence of the carbon content on the microstructure, phase transformation and hardness of an iron-based cermet is studied. The cermet is constituted by a high-alloyed steel as matrix, and TiCN particles (50 vol.%) as reinforcement. The material is produced by conventional powder metallurgy techniques, that is, uniaxial pressing and sintering, and the carbon content is varied from 0 wt.% to 1 wt.%. The aim of the research is the understanding of the transformations undergone by the material with increasing C amounts when temperature is increased. For this purpose, the cermet is studied by mechanical spectroscopy (MS) and differential thermal analysis (DTA) and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and hardness measurements. The equilibrium phase diagram calculated by ThermoCalc software contributes to explain the differences found on phase transformations with respect to the C content of the cermet.The authors would like to acknowledge the financial support from the Spanish Ministry of Science and Innovation through the R&D Project MAT2009-14448-C02 and Regional Government of Madrid through the program ESTRUMAT-CM (Ref. S2009/MAT-1585)Publicad

Estudio de la intercara en materiales compuestos tipo cermet para el diseño de matrices metálicas alternativas

La ponencia presentada en: XI Congreso Nacional de Materiales Compuestos, celebrado los días 6, 7 y 8 de julio de 2015, en Móstoles (España).Uno de los grandes temas de investigación actual en el campo de los materiales compuestos tipo cermet base TiCN es la búsqueda de matrices metálicas alternativas a las convencionales las cuales cuentan en su composición Ni y/o Co de manera mayoritaria. El objetivo principal que debe cumplir el cermet con una matriz metálica alternativa es alcanzar propiedades similares e incluso superiores a las obtenidas con matrices convencionales utilizando materiales más económicos e inocuos. Además, siendo un material compuesto, a la hora de diseñar la composición de una nueva matriz metálica se debe tener en cuenta cómo es su interacción con el refuerzo cerámico. Este concepto cobra una mayor importancia en este tipo de materiales debido a que su fabricación se lleva a cabo mediante sinterización en fase líquida y las propiedades finales del material compuesto dependerán de la mojabilidad y solubilidad entre ambas fases. Algunas investigaciones sobre cermets reforzados con partículas de TiCN y matriz convencional han concluido que carburos como WC ó Mo2C tienen una gran influencia en la sinterabilidad de estos materiales compuestos porque mejoran la mojabilidad de la matriz metálica sobre el refuerzo cerámico. El objetivo principal de este trabajo es el estudio sistemático de la influencia de estos carburos en la sinterabilidad de un cermet base TiCN con Fe como matriz metálica. Para ello se ha estudiado la solubilidad entre ambas fases estudiando su intercara tanto de forma experimental como mediante herramientas de simulación termodinámica y cinética utilizando el software Dictra. Gracias a los resultados obtenidos se crea una metodología de trabajo que permite conocer la influencia de la composición en la solubilidad entre las dos fases siendo una herramienta útil en el diseño de nuevas matrices metálicas alternativas en cermets.Los autores agradecen la financiación recibida para la realización de este trabajo al MINECO (proyecto MAT2012-38650-C02-01) y a la Comunidad de Madrid por el programa MULTIMAT-CHALLENGE, ref. S2013/MIT-2862.Publicad

Influence of carbon content on the sinterability of an FeCr matrix cermet reinforced with TiCN

The influence of carbon content on an iron-chromium cermet composite reinforced with Ti(C,N) (50 vol.%) has been studied. A thermodynamic simulation was performed using the ThermoCalc software to calculate the phase diagram of the composite. The results were validated by a thermal study performed using differential thermal analysis (DTA), and cermet samples with C percentages between 0 and 1 wt.% added to the steel matrix were prepared using a conventional powder metallurgy process. The sintered samples were characterised by measurements of density and hardness, microstructural analysis using scanning electron microscopy (SEM), microanalyses using energy dispersive X ray spectroscopy (EDX) and X-ray diffraction (XRD). The results obtained show a significant influence of the C percentage on the solidus temperature, which influences sintering behaviour, leading to changes in the Ti(C,N) particles' shape and composition, due to changes in the stoichiometry of the Ti(C,N). This influence is reflected in the cermet microstructure and hardness. The results are discussed with reference to the DTA and thermodynamic studies.The authors would like to acknowledge the financial support of the Spanish Ministry of Science and Innovation through R&D Project MAT2009-14448-C02 and the Regional Government of Madrid through the programme ESTRUMAT-CM (Ref. S2009/MAT-1585).Publicad

Estudio de fatiga térmica de cermets base hierro

Ponencia presentada en: XI Congreso Nacional de Materiales Compuestos, celebrado los días 6, 7 y 8 de julio de 2015, en Móstoles (España).En el presente trabajo se analiza el comportamiento a fatiga térmica de materiales compuestos de matriz férrea y refuerzo de TiCN desde dos puntos de vista: la influencia de la temperatura máxima alcanzada durante la fatiga térmica y la influencia del número de ciclos de calentamiento y enfriamiento. Además, se compara el comportamiento de estos materiales con el de un acero de herramientas de uso convencional. Este estudio del comportamiento frente a cambios cíclicos de temperatura para ambos materiales se realiza en base a su resistencia a la oxidación y se compara con la oxidación estática que se produce a temperatura elevada constante. Para ello las probetas se han sometido a diferentes ciclos de fatiga térmica, alcanzando temperaturas máximas de 1000 ºC durante un máximo de 100 ciclos; posteriormente se ha caracterizado tanto su superficie como su sección transversal utilizando diferentes técnicas: medida de cambio de masa, DRX, SEM, EDX y microdureza.Los autores agradecen la financiación recibida para la realización de este trabajo al MINECO (proyecto MAT2012-38650-C02-01) y a la Comunidad de Madrid por el programa MULTIMAT-CHALLENGE, ref. S2013/MIT-2862Publicad

Microstructure Evolution in a Fast and Ultrafast Sintered Non-Equiatomic Al/Cu HEA

One of the attractive characteristics of high entropy alloys (HEAs) is the ability to tailor their composition to obtain specific microstructures and properties by adjusting the stoichiometry to obtain a body-centered cubic (BCC) or face-centered cubic (FCC) structure. Thus, in this work, the target composition of an alloy of the FeCrCoNi family has been modified by adjusting the Al/Cu ratio in order to obtain a BCC crystalline structure. However, processing conditions always play a key role in the final microstructure and, therefore, in this work, the microstructure evolution of FeCrCoNiAl1.8Cu0.5 HEA sintered by different powder metallurgy (PM) techniques has been investigated. The techniques used range from the conventional PM sintering route, that uses high heating rates and sintering times, going through a fast sintering technique such as spark plasma sintering (SPS) to the novel and promising ultrafast sintering technique electrical resistance sintering (ERS). Results show that the increase in the processing time favours the separation of phases and the segregation of elements, which is reflected in a substantial change in the hardness of the alloy. In conclusion, the ERS technique is presented as a very promising consolidation technique for HEA.The authors gratefully acknowledge the financial support of MAT4.0-CM project funded by Madrid region under programme S2018/NMT-4381. J. Cornide also acknowledges funding from the Spanish Ministry of Science and Innovation (IJCI-2017-31348)

High-entropy alloys fabricated via powder metallurgy. A critical review

High-entropy alloys (HEAs) have attracted a great deal of interest over the last 14 years. One reason for this level of interest is related to these alloys breaking the alloying principles that have been applied for many centuries. Thus, HEAs usually possess a single phase (contrary to expectations according to the composition of the alloy) and exhibit a high level of performance in different properties related to many developing areas in industry. Despite this significant interest, most HEAs have been developed via ingot metallurgy. More recently, powder metallurgy (PM) has appeared as an interesting alternative for further developing this family of alloys to possibly widen the field of nanostructures in HEAs and improve some capabilities of these alloys. In this paper, PM methods applied to HEAs are reviewed, and some possible ways to develop the use of powders as raw materials are introduced

Influence of heat treatment on the high temperature oxidation mechanisms of an Fe-TiCN cermet

In this study, the oxidation behaviour of an iron matrix cermet containing 50 % vol. Ti(C,N) was investigated before and after heat treatment by oxidation tests performed in static air at temperatures between 500 °C and 1000 °C. The oxidation mechanism for this type of composite materials was established and it was found that the heat treated material presents lower mass gain than the as-sintered material at the early stages of the oxidation, due to the volatilization of oxides. The oxidation behaviour of the sintered cermet was compared to those obtained for two commercial materials used in the fabrication of cutting tools, a highspeed steel and a cemented carbide, revealing a higher oxidation resistance for the iron matrix cermet due to the formation of a self-protective layer during the oxidation process.The authors acknowledge the funding received for this work from the Spanish Government through the R&D projects MAT2009-14448-C02-02 and MAT2012-38650-C02-01 and the Regional Government of Madrid through the ESTRUMAT program (S-2009/MAT–1585)

Microstructural development and mechanical properties of iron based cermets processed by pressureless and spark plasma sintering

Iron-based cermets are an interesting class of metal-ceramic composites in which properties and the factors influencing them are to be explored. In this work the metal matrix contains Cr, W, Mo and V as alloying elements, and the hard phase is constituted by 50 vol% of titanium carbonitride (TiCN) particles. The work studies the influence of the C content and the processing method on the sinterability, microstructure and hardness of the developed cermet materials. For that purpose, cermet samples with different C content in the matrix (0 wt%, 0.25 wt%, 0.5 wt%, 1.0 wt%) were prepared by conventional pressureless sintering (CPS) and, in order to achieve finer microstructures and to reduce the sintering time, by spark plasma sintering (SPS). The density and hardness (HV30) of the processed materials was evaluated, while their phase composition and microstructure was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The equilibrium phase diagram of the composite material was calculated by ThermoCalc software in order to elucidate the influence of the carbon content on the obtained phases and developed microstructures.The authors would like to acknowledge financial support from the Spanish Ministry of Science and Innovation through the R&D Project MAT2009-14448-C02 and the Comunidad de Madrid through the program ESTRUMAT-CM (reference S2009/MAT- 1585). K. Vanmeensel thanks the Research Fund Flanders (FWO) for his postdoctoral fellowship.Publicad

Influence of carbon content on microstructure and properties of a steel matrix cermet

There is a marked correlation between the composition, microstructure and properties in TiCN-based cermets. In the case of using iron alloys as metallic matrix the carbon content is of particular significance, as not only influences the stoichiometry of ceramic phase but also induces phase transformations in the steel matrix. However, such influence has been less studied in steel matrix cermets than in conventional Ni or Co ones, so the aim of this work is to contribute to the study of the influence of carbon content on the microstructure and properties of a steel matrix cermet containing fixed quantities of alloying elements. Samples were prepared by powder metallurgy and characterized combining different techniques as Transmission Mössbauer spectroscopy (TMS), X Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), to explain differences found in hardness and toughness.Peer ReviewedPostprint (author's final draft