Microstructural optimization of unalloyed ductile cast irons with a ferritic matrix used in the manufacture of wind turbine rotors

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 pre-inoculants containing SiC on the spheroidal morphology of graphite has also been verified, and also that low-Mn 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

Erosive wear resistance regarding different destabilization heat treatments of austenite in high chromium white cast iron, alloyed with mo

With the aim of improving erosive wear resistance in hypoeutectic white cast irons with 18% Cr and 2% Mo, several samples of this grade were subjected to different heat treatments at 1000 °C to destabilize the austenite. The dwell times at this temperature varied from 4 to 24 h and the samples were cooled in air or oil. The existing phases were identified and quantified by applying the Rietveld structural refinement method. The results were correlated with the hardness of the material and with the microhardness of the matrix constituent. The greatest resistance to erosive wear was achieved in those samples that had a higher percentage of secondary carbides. The longer the dwell time at the destabilization temperature of austenite, the greater the amount of precipitated secondary carbides. However, the percentage of dissolved eutectic carbides is also higher. These eutectic carbides were formed as a result of non-equilibrium solidification. Low cooling rates (in still air) can offset this solution of eutectic carbides via the additional precipitation of secondary carbides in the 600−400 °C temperature range. A sharp decrease is observed in the percentage of retained austenite in those treatments with dwell times at 1000 °C equal to or greater than 12 h, reaching minimum values of around 2% volume. The percentage of retained austenite was always lower after oil quenching and the hardness of oil quenched samples was observed to be greater than those quenched in air. In these samples, the maximum hardness value obtained was 993 HV after a 12 h dwell, which result from the optimum balance between the percentages of retained austenite and of precipitated carbides

Enhanced fracture strength in the working layer of rolls manufactured in Ni-hard cast iron alloyed with Mo, Nb and Mg

One of the main in-service failure mechanisms of the work-rolls used in hot strip mill finishing stands is surface spalling. The indefinite chill double-poured rolls usually comprise of a peripheral working layer made of crushed Ni-hard cast iron and a grey cast iron core, mostly pearlitic matrix with spheroidal graphite. To enhance its wear resistance, the working layer can be alloyed with Mo and Nb. The possible cracking and spalling of the surfaces of these work-rolls is strongly influenced by the presence of carbides and the continuity of their network. The flexural and impact toughness tests are reliable testing methods to assess these properties. The aim of this paper is to identify those manufacturing factors that have a significant effect on the flexural strength and toughness of this material, correlating the results with the volume fraction of precipitated carbides. It is worth highlighting, among the analysed factors are the liquidus temperature, the %Si, the use of an inoculant with traces of Lanthanum, and inoculation with different amounts of FeB, SiCa and Mg. Inoculation with SiCa is found to have a positive effect on the toughness of the material, breaking up the continuity of the carbide network, while FeB is found to act as a heterogeneous nucleant for NbC precipitation. However, high FeB contents reduce flexural strength and do not have a significant effect on the hardness of the material. To enhance the fracture toughness of the working layer, a liquidus temperature in the 1270–1275 °C range is recommended, as well as inoculating the ladle with Mg, 3 kg/T FeB and 0.6 kg/T SiCa

Aplicación del ajuste de Rietveld para correlacionar la evolución microestructural de fundiciones blancas con 18 y 25% en Cromo, templadas en aceite y sucesivos revenidos, con el comportamiento frente al desgaste abrasivo y esfuerzos de flexión

Through the application of Rietveld’s structural refinement method, the phases present in oil quenched and successively tempered treatments at 500 ºC of hypoeutectic white cast irons containing 18 and 25%-wt. Cr were identified. The former were correlated to the behavior shown under abrasive wear testing and bending testing too. The fraction of retained austenite after quenching for both highly alloyed irons was low. The alloy containing 18%-wt. Cr showed the highest fraction of carbides after quenching, and yet it was observed that secondary carbides seemed to develop preferentially in former destabilized proeutectic austenite as compared to former destabilized eutectic austenite. On the other hand, the highly alloyed cast iron with 25%-wt. Cr exhibited a more homogeneous phase resulting from austenite transformation after tempering. It was also observed that the cast iron containing 18%-wt. Cr showed partial solution of M7C3 carbides and a transformation of mixed secondary carbides of the M7C3 y M2C types. The fraction of martensite from quenching was detected to have a positive response to the resistance to abrasive wear, and illustrating of the inadequacy of quenching in oil for this sought property. The alloy bearing 25%-wt. Cr was found to have the highest martensite fraction after quenching, and thus a higher abrasive wear resistance. After a double tempering treatment, under bending testing, it displayed the highest rupture stress and strain, yielding a fragile fracture with facets of the transgranular mode. It was however observed the presence of regions depicting ductile fracture associated to double tempered martensite

Optimization of graphite morphology in mottled Nihard cast irons inoculated with feb and manufactured by centrifugal casting

The present research was focused on the identification of manufacturing factors that have an active influence on the graphite phase formation in Nihard cast irons inoculated with FeB, constituting the outer layer of duplex work rolls. These rolls are used in the finishing stands of hot-strip steel mills where the following are desired: (a) between 2.5 and 4 vol % of graphite; (b) homogeneous graphite distribution across the layer section; and, (c) a reasonable high number of graphite particles across the layer. The research methodology that followed consisted of the application of a saturated design of experiments (DOE), with seven factors, eight experiments, and resolution III. The analyzed responses obtained by quantitative metallographic techniques were: the volume fraction of graphite, Vv; the number of counts per unit area of graphite, NA; and the graphite morphology across the layer thickness. Increasing the addition of FeB from 6 to 10 kg/T reduced the graphite volume fraction and the count number, but had no influence on its morphology. However, an increase of the liquidus temperature from 1225–1230 to 1250–1255 °C, and an increase in the amount of SiCaMn added to the ladle from 0.3 to 0.6 kg/T produced the desired compact graphite morphology

Identificación de factores con efecto significativo sobre fundiciones Nihard atruchadas empleadas en capas periféricas de cilindros de laminación dúplex

Se presentan los resultados de un proyecto de investigación cuyo principal objetivo es identificar, cuantificar y controlar los parámetros metalúrgicos de fabricación que tienen un efecto significativo sobre la microestructura en fundiciones Nihard atruchadas que son empleadas en las capas periféricas de trabajo en cilindros de laminación dúplex, fabricados por el método de doble colada mediante colada centrífuga vertical. Estos cilindros presentan un núcleo de fundición gris con grafito esferoidal en una matriz ferrito perlítica. La estrategia de investigación seguida ha sido secuencial, y el método seguido fue la aplicación de 3 diseños de experimentos. Dos de estos diseños resultaron saturados de resolución III, analizándose 7 factores en 8 experimentos. Un tercer diseño presentaba un menor grado de fraccionamiento con resolución IV, analizándose 7 factores en 16 experimentos. Dentro de los niveles definidos para los parámetros metalúrgicos de fabricación estudiados se concluye que la inoculación con FeB favorece la precipitación del grafito con morfologías compactas y pequeños tamaños uniformemente distribuidos, aunque con un efecto blanqueante aumentando la fracción en volumen de ledeburita. Una disminución del carbono equivalente favorece la precipitación del grafito con morfología pseudo esferoidal, aumenta la tenacidad del material y su resistencia bajo esfuerzos de flexión. La inoculación con FeTi favorece la precipitación del grafito con morfologías laminares. La inoculación con SiCaMn favorece un aumento de la fracción en volumen de grafito precipitado

Optimization of the Mechanical Behaviour Under Tensile Stress of Spheroidal Cast Iron with Ferritic Matrix used in the Manufacture of Wind Turbine Hubs

The aim of this study is the microstructural optimization of the toughness behaviour at sub-zero temperatures and the enhancement of the ductile behaviour under tensile stress of spheroidal graphite cast irons with a ferrite matrix used to manufacture hubs for horizontal axis wind turbines. The approach employed was a fractional design of experiments with 7 factors, 16 experiments and resolution IV. The research was conducted at an industrial scale on cubes with the same mass coefficient as that of the industrial parts to be enhanced. The main conclusion worth highlighting is that the use of nodulizers with traces of lanthanum enhances the toughness of the material at sub-zero temperatures and improves the ductility of these cast irons at the cost of lowering their yield strength. However, this negative effect may be offset by a decrease in the carbon equivalent to eutectic levels

Influence of thermal parameters related to destabilization treatments on erosive wear resistance and microstructural variation of white cast iron containing 18% Cr. application of design of experiments and rietveld structural analysis

High-Cr hypo-eutectic white cast irons are used in very demanding environments that require high resistance to erosive wear. The influence on the microstructural variation and erosive wear resistance of several fundamental factors related to the thermal treatments of these cast irons was analysed by means of a fractional Design of Experiments (DoE). These factors included the ones related to the destabilization of austenite. The precipitated phases were identified by X-ray diffraction (XRD), while the Rietveld structural refinement method was used to determine their percentages by weight. Erosion wear resistance was calculated using the test defined by ASTM G76. It was concluded that the quench cooling medium does not significantly influence either erosive wear resistance or the proportion of martensite or retained austenite. The destabilization temperature is a key factor with respect to the percentage of retained austenite. In order to increase the amount of martensite and decrease the amount of retained austenite, temperatures not exceeding 1000 °C are required. An increase of 100 °C in the destabilization temperature can lead to a 25% increase in retained austenite. Moreover, tempering temperatures of around 500 °C favour an additional increase in the percentage of martensite. Erosive wear commences on the matrix constituent without initially affecting the eutectic carbides. Once the deterioration of the matrix constituent surrounding these carbides occurs, they are released. High tempering times provide an increase in resistance to erosive wear due to a second destabilization of austenite during the said tempering