Niobium in Cast Irons

This work reviews the effect of niobium on different cast irons. This alloying element has a high affinity by carbon forming carbides of the type MC; therefore, the main effect is to strengthen the matrix of the alloys due to a hard-phase dispersion mechanism. Even at low levels of niobium (<0.2%), MC carbides are found. The presence of such carbides increases hardness, yield and tensile strength in ductile irons, but the most important role is to increase wear resistance. Therefore, the use of niobium has been focussed mainly in alloyed white irons for wear resistance applications and in mottled cast iron used for rolls in hot rolling mills

High resolution observations of friction-induced oxide and its interaction with the worn surface

A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron,an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni3Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni3Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear

Casting Fe-Al-based intermetallics microalloyed with Li and Ag

The effect on the mechanical properties at room temperature of Li and Ag additions to the Fe–Al (40 at.%)-based alloy produced by conventional casting were evaluated in this work. Alloying elements were added into a previously molted Fe–(40 at.%) aluminum-based alloy, stirred, and then cast into sand molds to directly produce tensile specimens. To determine the mechanical properties, tensile tests and hardness measurements were performed. The additions of both Ag and Li showed an increase in ductility and tensile strength of the intermetallic alloys. In addition, hardness was substantially increased with the Li addition. Lithium additions promoted a solid solution hardening, whereas 3 at.% of Ag additions promoted ductility due to a microstructural modification and to the formation of a soft Ag 3 Al phase. Characterization by both optical and electronic microscopy, energy dispersive spectroscopy microanalysis, and x-ray diffraction supported the mechanical characterization.Peer ReviewedPostprint (author's final draft

Estudio a nivel laboratorio de la Degradación atípica en un refractario tipo SiO2 utilizado en hornos de inducción

En este trabajo de investigación se realizó un estudio de la degradación atípica que se produce en las paredes de los hornos de inducción. Para esto, se utilizó un material refractario base - SiO2 (98.8%) con dos escorias de composición química diferente y se analizó su comportamiento mediante las pruebas experimentales del método de copa o crisol y el de inmersión, así como por un modelado asimétrico del metal líquido en el horno de inducción. La escoria, la estructura del material refractario y el mecanismo de penetración de la escoria fueron caracterizados por microscopia óptica, utilizando la técnica de luz reflejada. Los resultados demostraron que, con valores altos de frecuencia (240 Hz), para la operación del horno, se obtiene una fuerza de Lorentz muy alta (14.26 N) pero con una velocidad del metal liquido baja (6 m/s) la cual tiene un vector de dirección perpendicular a las paredes del horno, provocando presión sobre la escoria incrustada en los poros del refractario favoreciendo el proceso de degradación del material

Estudio a nivel laboratorio de la Degradación atípica en un refractario tipo SiO2 utilizado en hornos de inducción

The refractory degradation on induction furnace walls at different points was studied. A SiO2 (98.8%) base refractory was analyzed with two different slag chemical composition through crucible and dipping laboratory degradation test and also applying an asymmetric modeling. The slag, refractory structure and slag penetration was analyzed by optical microscopy using reflecting light technique. The results demonstrated that slag penetration starts on the refractory pores produced by the sintering process and also by hollows generated by the came off grains. The modeling results evidenced that, with high frequency (240 Hz), a high Lorentz force (14.26 N) is originated but with a low liquid velocity (6 m/s) that have a perpendicular vector direction to the furnace walls, making pressure over the filtered slag.En este trabajo de investigación se realizó un estudio de la degradación atípica que se produce en las paredes de los hornos de inducción. Para esto, se utilizó un material refractario base - SiO2 (98.8%) con dos escorias de composición química diferente y se analizó su comportamiento mediante las pruebas experimentales del método de copa o crisol y el de inmersión, así como por un modelado asimétrico del metal líquido en el horno de inducción. La escoria, la estructura del material refractario y el mecanismo de penetración de la escoria fueron caracterizados por microscopia óptica, utilizando la técnica de luz reflejada. Los resultados demostraron que, con valores altos de frecuencia (240 Hz), para la operación del horno, se obtiene una fuerza de Lorentz muy alta (14.26 N) pero con una velocidad del metal liquido baja (6 m/s) la cual tiene un vector de dirección perpendicular a las paredes del horno, provocando presión sobre la escoria incrustada en los poros del refractario favoreciendo el proceso de degradación del material

Fatigue resistance improvement of a forging medium carbon steel using Mischmetal (rare earths) as inclusions (MnS) modifier element

For this work, hot-rolled plates of medium carbon steel (0.5%C) were prepared with different sulphur contents (0.023% to 0.065%). Two steels were alloyed with mischmetal (0.01%) as inclusions modifier to evaluate its effect on fatigue and tensile properties. A detailed study of the type, size, shape and distribution of the sulphide inclusions was carried out by optical and SEM microscopy in the as-cast and hot rolled conditions and correlated with their mechanical properties. The mischmetal treated steels developed a globular, small (~2µm) and well distributed MnS inclusions, producing an improvement in tensile strength (840 MPa and 19% elongation) and fatigue behavior (375 MPa), which was not evident on the untreated steels (741 MPa tensile strength and 300 MPa fatigue limit). The fatigue behavior was tested under the E466 standard, by using nine samples per composition in a R.R. Moore - rotating beam fatigue testing machine. The higher fatigue limit was obtained for the mischmetal-treated steel with 0.05%S. These results were discussed and correlated to the small size and globular shape of the MnS inclusions. However, it was observed that, additions of 0.01% of mischmetal are less effective as inclusion modifier when sulphur content is higher than of 0.05%, but it is still good enough to obtain a better fatigue behavior compared with the untreated steel containing 0.023% sulphur.En el presente trabajo, se analizaron placas de un acero de medio carbono (0.5%C) laminado en caliente, con diferentes contenidos de azufre (0.023% a 0.065%), así mismo, se realizaron adiciones de Mischmetal (tierras raras) a dos aceros como agente modificador de las inclusiones y de esta forma evaluar su efecto sobre la resistencia a la tensión y fatiga. Se llevó a cabo un estudio detallado sobre el tipo, tamaño, forma y distribución de las inclusiones de azufre por medio de microscopía óptica y microscopía electrónica de barrido en condiciones de colada y después de la laminación en caliente, finalmente, los resultados de la caracterización metalográfica fueron correlacionados con sus propiedades mecánicas. En los aceros tratados con tierras raras, las inclusiones presentes fueron de un tamaño de alrededor de 2 µm, uniformemente distribuidas y de forma globular, lo cual produjo un incremento en la resistencia a la tensión (840 MPa y 19% de elongación) y límite de fatiga (375 MPa) en comparación con los aceros sin adición de Mischmetal (741 MPa de resistencia máxima y 300 MPa de límite de fatiga). El ensayo de fatiga se realizó acorde con la norma ASTM E466 mediante el cual se ensayaron 9 muestras de cada composición en una máquina de flexión rotativa R.R. Moore. El límite máximo de fatiga se obtuvo en el acero con 0.05% de azufre tratado con Mischmetal. Estos resultados se atribuyen al menor tamaño y a la forma globular de las inclusiones. Sin embargo, se observó que el efecto de las adiciones de 0.01% de mischmetal como agente modificador de las inclusiones es disminuido cuando el contenido de azufre es mayor al 0.05%, aun así, es suficiente para obtener un límite de fatiga mayor comparado con el acero sin adición de mischmetal y 0.023% de azufre

Hot ductility behavior of high-Mn austenitic Fe-22Mn-1.5Al-1.5Si-0.45C TWIP steels microalloyed with Ti and V

This research work studies the influence of microalloying elements (Ti and V) and the solidification route on the hot ductility behavior of high-manganese TWIP steels. Uniaxial hot tensile tests in the temperature range of 700–1100 °C under a constant strain rate of 10-3 s-1 were carried out. Hot ductility as a function of reduction of area (RA) showed a significant improvement in the V-microalloyed TWIP steel, when compared to a non-microalloyed TWIP steel with a similar composition, in the intermediate temperature range of 800–900 °C. The highest value of 86% RA is attributed to the onset of dynamic recrystallization (DRX) near to the fracture tip. On the other hand, Ti addition to TWIP steel did not exhibit any improvement on the hot ductility, resulting in the worst hot ductility behavior, with a maximum value of 34% RA. The TWIP steels solidified in metallic ingot molds (MM) showed higher peak stress (sp) and ductility values than the sand mold (SM) cast ingots at low and intermediate temperatures, respectively, which is associated with the finer microstructure generated during solidification. Grain boundary sliding was recognized as the failure mechanism associated with second-phase particles precipitated at the grain boundaries, which play the role of nucleation and propagation sites of void-cracksPeer ReviewedPostprint (published version