Publication: Aceros inoxidables pulvimetalúrgicos obtenidos por mezclas de polvos atomizados en agua y en gas para aplicaciones a alta temperatura
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2008
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2008-06-24
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Abstract
En este trabajo de investigación se han desarrollado aceros inoxidables pulvimetalúrgicos (P/M) partiendo de diferentes mezclas de polvos atomizados en agua (aa) y atomizados en gas (ag). Se han realizado mezclas de aceros dúplex P/M con los polvos ferríticos AISI 434L aa y distintos porcentajes de polvos austeníticos AISI 316 ag. Se han preparado también aceros P/M ferríticos puros, mezclando polvos AISI 434L aa o AISI 430L aa, con diferentes cantidades de polvos AISI 430L ag. Todos los materiales se han conformado mediante compactación uniaxial de simple efecto y luego se han sinterizado, a diferentes temperaturas, en condiciones de alto vacío. Se ha conseguido optimizar de forma significativa la densidad final de estos aceros, cuya potencial aplicación final implicaría la exposición a altas temperaturas. Mediante la adición de polvo ag a un polvo, más convencional aa, se han obtenido una serie de materiales con menor grado de porosidad a medida que se ha incrementado la cantidad de polvo ag. La utilización de partículas de tan diferente tamaño y morfología ha dado lugar a un mejor empaquetamiento de los polvos en el compacto en verde y a una mayor superficie para la difusión durante la sinterización, mejorando decisivamente sus propiedades físicas y mecánicas. Igualmente, la mejora de la densidad ha contribuido a optimizar el comportamiento frente a la corrosión de estos aceros. Con la utilización de polvo ag se ha incrementado de forma muy significativa la resistencia frente a la corrosión acuosa en disoluciones NaCl y KClO4 0,1M. Esta mejora se debe a la reducción de la porosidad y, por lo tanto, del área real activa en contacto con el medio agresivo. Por otro lado, en condiciones de alta temperatura (800 y 900 ºC), se ha observado que el cálculo de la ganancia de masa no es concluyente en el comportamiento de aceros inoxidables con porosidad variable en estas condiciones agresivas. Por eso, ha sido necesario caracterizar los óxidos formados de manera detallada mediante microscopia electrónica de barrido (MEB) y difracción de rayos X (DRX). Además, su cinética de oxidación se ha estudiado mediante análisis termogravimétrico. Se han conseguido aceros P/M en los que se forman capas continuas y adherentes de óxidos protectores de cromo a alta temperatura. En conclusión, se han fabricado y caracterizado nuevos tipos de aceros P/M con mejores propiedades, mediante una ruta de procesado que se puede implementar fácilmente en la industria. Por un lado, se han logrado aceros dúplex sinterizados con buenas propiedades mecánicas y un buen comportamiento frente a la corrosión acuosa y a alta temperatura. Por otro, también se ha obtenido aceros ferríticos con mejores propiedades frente a la oxidación que los P/M más tradicionales. ____________________________________________
In this work of investigation, powder metallurgical (P/M) stainless steels have been developed using different mixes of water atomized powders (wa) and gas atomized powders (ga). P/M duplex steel mixes have been prepared with ferritic, wa AISI 434L powders and austenitic ga AISI 316L powders at different percentages. P/M pure ferritic stainless steels have also been manufactured, mixing wa AISI 434L powders or wa AISI 430L powders with different amounts of ga, AISI 430L powders. All the materials have been compacted in an uniaxial, single-effect press and then sintered at different temperatures in a ultra-high vacuum furnace. The final density of these steels (whose potential final application would imply high temperature exposures) has been optimized in a meaningful way. Adding ga powder to the more traditional wa powder, it has been obtained a series of materials whose porosity decreases as the amount of ga powder increases. The use of particles with such a different size and morphology have caused better packing of the powders in the green compact and higher surface for diffusion during sintering, improving decisively its physical and mechanical properties. Moreover, the improvement of the density has contributed to the optimization of the corrosion behaviour of these steels. The use of ga powder has caused a marked increase of the resistance to the aqueous corrosion in NaCl and KClO4 0,1M solutions. This improvement is due to the reduction of the porosity, and so, of the real active area in contact with the aggressive medium. On the other hand, at high-temperature conditions (800 and 900 ºC), it has been observed that the mass gain measurements are not a good indicator about the behaviour of stainless steels with different porosity levels in such aggressive conditions. For this reason, it has been necessary a careful characterisation of the oxides by scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The oxidation kinetics of the steels has also been studied through thermogravimetric tests. P/M steels where continuous and adherent chromia scales are formed at high temperature have been obtained. In conclusion, new P/M steels, with better properties and that can be obtained through a processing route easily implementable in the industry, have been manufactured and characterized. On one hand, sintered duplex steels have been obtained with good mechanical properties and good aqueous and high-temperature corrosion behaviours. On the other hand, ferritic steels with higher oxidation resistances than those of more traditional P/M steels have also been obtained.
In this work of investigation, powder metallurgical (P/M) stainless steels have been developed using different mixes of water atomized powders (wa) and gas atomized powders (ga). P/M duplex steel mixes have been prepared with ferritic, wa AISI 434L powders and austenitic ga AISI 316L powders at different percentages. P/M pure ferritic stainless steels have also been manufactured, mixing wa AISI 434L powders or wa AISI 430L powders with different amounts of ga, AISI 430L powders. All the materials have been compacted in an uniaxial, single-effect press and then sintered at different temperatures in a ultra-high vacuum furnace. The final density of these steels (whose potential final application would imply high temperature exposures) has been optimized in a meaningful way. Adding ga powder to the more traditional wa powder, it has been obtained a series of materials whose porosity decreases as the amount of ga powder increases. The use of particles with such a different size and morphology have caused better packing of the powders in the green compact and higher surface for diffusion during sintering, improving decisively its physical and mechanical properties. Moreover, the improvement of the density has contributed to the optimization of the corrosion behaviour of these steels. The use of ga powder has caused a marked increase of the resistance to the aqueous corrosion in NaCl and KClO4 0,1M solutions. This improvement is due to the reduction of the porosity, and so, of the real active area in contact with the aggressive medium. On the other hand, at high-temperature conditions (800 and 900 ºC), it has been observed that the mass gain measurements are not a good indicator about the behaviour of stainless steels with different porosity levels in such aggressive conditions. For this reason, it has been necessary a careful characterisation of the oxides by scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The oxidation kinetics of the steels has also been studied through thermogravimetric tests. P/M steels where continuous and adherent chromia scales are formed at high temperature have been obtained. In conclusion, new P/M steels, with better properties and that can be obtained through a processing route easily implementable in the industry, have been manufactured and characterized. On one hand, sintered duplex steels have been obtained with good mechanical properties and good aqueous and high-temperature corrosion behaviours. On the other hand, ferritic steels with higher oxidation resistances than those of more traditional P/M steels have also been obtained.
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Pulvimetalurgia, Aceros inoxidables, Ensayo de materiales