Vanadis 4 steel is a tool steel processed by powder metallurgy (PM). Its main alloying elements are Cr, V and Mo. Through the application of a design of experiments with six factors and eight experiments, the parameters of the process related to the thermal treatment of these steels are deliberately varied. Those thermal parameters related to the destabilisation of austenite were analysed: the cooling method in quenching, tempering and the application of an ionic nitriding treatment. Through XRD, the percentage and types of precipitated crystalline phases were determined, and, through SEM-EDX, the microstructure was revealed. At the same time, through a pin-on-disc test, those factors with a significant influence on resistance to wear were determined. It has been concluded that, in order to increase resistance to wear, treatments of destabilisation of the austenite at 900 °C with oil quenching, tempering at 550 °C for 4 h and a subsequent treatment of plasma nitriding would be very favourable. This tempering favours a second destabilisation of the austenite and its transformation into martensite, as well as the joint precipitation of type M7C3 and MC carbides. The thickness of the nitrided layer exceeds 100 microns and generates a fully adhesive wear mechanism.

Work-rolls manufactured through the Indefinite Chill Double Poured (ICDP) method present an exterior work layer manufactured in a martensitic white cast iron alloyed with 4.5 %Ni, 1.7 %Cr, and 0.7 %Nb (wt.%). In its microstructure, there are abundant carbides of the type M3C and MC, which give high resistance to wear, and graphite particles which improve the service behaviour of the rolls against thermal cycling. The core of the rolls is manufactured in grey cast iron of pearlitic matrix and spheroidal graphite. These work-rolls are used in the finishing stands in Hot Strip Mills for rolling slabs proceeding from continuous casting at 1200 °C. Through the application of a Design of Experiments (DoE), an attempt has been made to identify those manufacturing factors which have a significant effect on resistance to wear of these rolls and to find an optimal combination of levels of these factors which allow for improvement in resistance to wear. To increase resistance to wear, it is recommended to situate, simultaneously, the liquidus temperature and the percentage of Si in the respective ranges of 1250–1255 °C and 1.1–1.15 (wt.%). Higher liquidus temperatures favour the presence of the pro-eutectic constituent rather than the eutectic constituent. The outer zone of the work layer, in contact with the metal sheet, which is being rolled, does not show the graphitising effect of Si (0.8–1.15 wt.%). On the contrary, it confirms the hardening effect of the Si in solid solution of the ferrite. The addition of 0.02% of Mg (wt.%) and the inoculation of 6 kg/T of FeB tend to eliminate the graphitising effect of the Si, thus favouring that the undissolved carbon in the austenite is found to form carbides in contrast to the majority formation of graphite.

ABSTRACT:The aim of this work was the microstructural optimization of cast irons with nodular graphite for the manufacture of wind turbine hubs, paying preferential attention to the geometry and distribution of graphite spheroids to ensure the required mechanical properties for this application. The target was pursued based upon microstructure-properties correlation, in an environment of great competitiveness and exigency marked by current international standards. The methodology followed consisted of the generation of knowledge from tailor-made industrial castings, followed by the analysis of their microstructures, in order to extract valuable conclusions for the production process through the use of statistical analysis. The approach method employed was a Fractional Design of Experiments (DOE) with 7 factors, 16 experiments and resolution IV. The samples from each experiment were cubes of identical geometry, and designed to match a surface-to-volume module equal to 4 cm (1.57 in) found as the highest values in real hubs of 3 MW power wind turbines. It is concluded that the use of nodulizers with traces of lanthanum favour the reduction of the volume fraction of pearlite, although La has proved not to promote the spherical shape of primary graphite. The negative effect of preinoculants containing SiC on the spheroidal morphology of graphite has also been verified, and also that lowMn bearing scrap favours graphite formation and the reduction of the volume fraction of pearlite, in spite of being a carbide forming element. The whitening effect of Mn was minimized with low carbon equivalent melts. RESUMEN:Optimización microestructural de fundiciones grises dúctiles no aleadas, con matriz ferrítica, empleadas en la fabricación de bujes de aerogeneradores. El objetivo de este trabajo fue la optimización microestructural de fundiciones grises con grafito esferoidal y matriz ferrítica, empleadas en la fabricación de bujes de aerogeneradores, prestando una especial atención a la geometría y distribución de los esferoides de grafito para asegurar el cumplimiento de las propiedades mecánicas requeridas para esta aplicación. En un entorno de gran competitividad y exigencia, marcado por los estándares internacionales que establecen las propiedades mecánicas mínimas que deben cumplir los materiales empleados, el cumplimiento de este objetivo se llevó a cabo correlacionando microestructura y propiedades. Para ello, la metodología de investigación consistió en la generación de conocimiento a partir de coladas industriales "a medida", seguido de un análisis de la microestructura, con el fin de extraer conclusiones importantes, de aplicación práctica en el proceso de fabricación, mediante el 2 • J. Asensio-Lozano et al.Revista de Metalurgia 54(2), April-June 2018, e118. ISSN-L: 0034-8570. https://doi.org/10.3989/revmetalm.118 empleo de técnicas estadísticas. La herramienta estadística empleada fue un Diseño de Experimentos Fraccional (DOE), con 7 factores, 16 experimentos y resolución IV. Las muestras empleadas en cada experim...