Removal of non-metallic inclusions from molten steel by ceramic foam filtration

”Ceramic filters are routinely used in steel foundries to remove non-metallic inclusions from steel melt. Removal efficiency for both solid and liquid inclusions by magnesia-stabilized zirconia foam filters (10ppi) were evaluated and distribution of the captured inclusions through the filter thickness was also investigated. A mold design was developed using a commercial computational fluid dynamics software package to produce two castings that fill simultaneously, one with a filter and the other without a filter, from a single ladle pour, while also matching the fill rates and avoid turbulence and reoxidation during pouring. An industrial-scale experiment was also performed to investigate the distribution of captured inclusions through the filter thickness for higher inclusion loading compared to that of laboratory-scale experiments. Inclusion removal efficiency was observed to be strongly dependent on the initial inclusion concentration. Solid alumina inclusions are found to be captured within the filter at the metal-filter macropore interface. The concentration of the captured solid inclusions decreased exponentially from the entry to exit side of the filter, following first order capture kinetics. Liquid inclusions were captured within the micropores of the ceramic web structure and at the metal-filter macropore interface. The captured liquid inclusion concentration within filter micropores also followed an exponential trend for lower inclusion loading, whereas it became constant for higher inclusion loading due to complete saturation of the ceramic web micropores. Upon filter micropore saturation, continuous liquid inclusion films developed at the metal-filter macropore interface, increasing the possibility for the release of large liquid inclusions from the filter”--Abstract, page iv

Efficiency of Solid Inclusion Removal from the Steel Melt by Ceramic Foam Filter: Design and Experimental Validation

An investigation was performed to measure the efficiency of solid alumina inclusion removal by filtration during casting. A mold design was developed using modeling software to produce two castings that fill simultaneously, one with a filter and the other without a filter. The design avoided vortex formation and thus air entrainment, which helped to avoid reoxidation inside the mold cavity. Samples from these castings were analyzed utilizing an SEM/EDS system with automated feature analysis (AFA) to measure the efficiency of inclusion removal using a 20 ppi zirconia foam filter. This study also documents the occurrence of inclusion flotation and agglomeration in the ladle which, in turn, affects the removal efficiency of these inclusions by filtration in the mold

On the Effect of Hot Rolling on Inclusion Size and Distribution in a Cast AISI 1070 Steel Railroad Wheel

The goal of this work is to examine the effect of hot deformation on shrinkage porosity and nonmetallic inclusions in an AISI 1070 grade steel industrially produced wheel casting. Steel cleanliness is an important consideration as it influences the mechanical properties of the final product. A high density of porosity and inclusions have been shown to be detrimental for mechanical properties, especially during hot rolling. Using a laboratory-scale rolling mill, cast preforms were subjected to a 66% cumulative reduction to determine the effect of thermomechanical processing on void closure and inclusions that may produce anisotropy in mechanical properties. Quantitative automated feature analysis, AFA, of inclusion type, size, morphology, and distribution was conducted utilizing an Aspex PICA 1020 scanning electron microscope to determine differences in inclusions and shrinkage porosity in the as-cast and as-rolled conditions. The results were compared with previously reported impact toughness values which indicated a trend with MnS projected length and average impact toughness in the T-L orientation. Reduction in shrinkage porosity was also verified utilizing 3D micro-X-ray CT scans. The AFA results showed a decrease in shrinkage porosity from 177 ppm in the as-cast condition to less than 35 ppm after rolling. Pores were in general much smaller and widely distributed after hot rolling and this would suggest improved impact properties. Analysis of nonmetallic inclusions revealed three primary categories of inclusions that included MnS, Al2O3, and complex inclusions that mainly consisted of MnS with an Al2O3 core, with small quantities of mixed silicates of Mn and Al and calcium aluminates (CaAl2O4)

A NOVEL HYBRID METHOD FOR NON-TRADITIONAL MACHINING PROCESS SELECTION USING FACTOR RELATIONSHIP AND MULTI-ATTRIBUTIVE BORDER APPROXIMATION METHOD

Selection of the most appropriate non-traditional machining process (NTMP) for a definite machining requirement can be observed as a multi-criteria decision-making (MCDM) problem with conflicting criteria. This paper proposes a novel hybrid method encompassing factor relationship (FARE) and multi-attributive border approximation area comparison (MABAC) methods for selection and evaluation of NTMPs. The application of FARE method is pioneered in NTMP assessment domain to estimate criteria weights. It significantly condenses the problem of pairwise comparisons for estimating criteria weights in MCDM environment. In order to analyze and rank different NTMPs in accordance with their performance and technical properties, MABAC method is applied. Computational procedure of FARE-MABAC hybrid model is demonstrated while solving an NTMP selection problem for drilling cylindrical through holes on non-conductive ceramic materials. The results achieved by FARE-MABAC method exactly corroborate with those obtained by the past researchers which validate the usefulness of this method while solving complex NTMP selection problems

Ceramic Foam Filter Micropores as Sites for Liquid Inclusion Retention

In steel foundries, ceramic filters are often used to capture non-metallic inclusions. It is well documented that solid inclusions are captured and retained within the filter macropores at the metal-refractory interface. However, liquid inclusions appear to be captured and retained by two mechanisms: one within the filter web micropore structure and another as a liquid film at the metal-filter macropore interface. Experiments were carried out to study the removal of various non-metallic liquid inclusions by magnesia-stabilized zirconia filters. The results documented the effective removal of liquid inclusions in all experiments. Samples were extracted from filter element to investigate the inclusion attachment mechanism during deep bed filtration. Energy-dispersive X-ray spectroscopy and associated scanning electron microscopy was employed to evaluate the inclusion attachment mechanism with the ceramic filters. Liquid inclusion retention in the filter micropores followed an exponential trend from entry to exit side of the filter, until the micropores became completely saturated. After complete saturation, the retained inclusion distribution remained constant through the thickness of the filter. Filters were found to capture the liquid inclusions, drawing them into the open micropores due to inclusion-refractory favorable wetting conditions. Once the accessible micropores were fully saturated, liquid inclusion films developed at the metal-filter macropore interface, increasing the possibility for the release of large liquid inclusions from the filter

Effect of Physical State of Non-Metallic Inclusions on the Accumulation Within Magnesia-Stabilized Zirconia Foam Filters

In foundry steelmaking, filtration is a common practice to lower the concentration of non-metallic inclusions in steel castings. Removal of non-metallic inclusions reduces the scrap rate and improves machinability, casting appearance and mechanical properties. Non-metallic inclusions are captured by different types of ceramic filters, the choice of which depends on the specific application and location in the process. Ceramic foam filters are commonly utilized in multiple positions in the gating system of sand molds and are effective by a deep bed filtration mechanism. Inclusions can be formed during the melting, pouring and casting and are separated into two main categories of endogenous and exogenous inclusions. Exogenous inclusions come from sources outside the refining process, such as worn refractories, slag, sand, or by reoxidation of the melt and are often much larger than endogenous inclusions. Endogenous inclusions are formed as consequence of the steelmaking and refining process and can be modified at different stages of steelmaking operation for effective removal. Depending on the deoxidizer used, the physical state of the inclusions can be both solid and liquid. Filtration of both solid (alumina) and liquid (manganese silicate) inclusions have been reported by several authors, however, the differences in the capture method have not been well documented. The equilibrium reactions during deoxidization of molten steel using aluminum and silicomanganese can be represented by equations 1 and 2

Removal of Alumina Inclusions from Molten Steel by Ceramic Foam Filtration

The efficiency of removal of solid alumina inclusions by filtration and the distribution of inclusions captured through the thickness of the filter was investigated for an aluminum killed 316 stainless steel casting. A mold design was developed using modeling software to produce two castings that fill simultaneously, one with a filter and the other without a filter. The design was optimized to produce the filtered casting and unfiltered casting from a single ladle pour, while also matching the fill rates and avoiding turbulence and reoxidation during pouring. Samples from the filters and the castings were analyzed using an SEM with EDS and automated feature analysis to measure the efficiency of inclusion removal for a 10 ppi zirconia foam filter. Results showed that inclusion removal efficiency depends strongly on the initial inclusion concentration and that the alumina inclusions are captured within the filter at the filter web-steel interface. This study also documented that inclusion floatation inside the mold cavity plays a role in reducing the inclusion concentration in the casting. The distribution of alumina inclusions captured through the filter thickness was quantified using elemental mapping and the inclusion distribution was found to decrease exponentially, following first-order capture kinetics

Filtration Efficiency of Inclusions in Lightweight FeMnAl Steels

The efficiency of ceramic foam filters in removing different inclusion populations in a Fe-30Mn-9Al-1Si-0.9C-0.5Mo steel was investigated. A mold design was created utilizing fluid flow and solidification modeling software. The design utilized a common pouring cup attached to two different but balanced gating systems. One runner utilized a ceramic foam filter, while the other runner was unfiltered. Three molds were poured in sequence from a teapot-style ladle. Metallographic samples revealed extensive Al- and Mn-rich oxide bi-films in samples taken before the filter. Samples sectioned after the filter did not contain bi-films. AlN or complex AlN-MnS or AlN-MnO comprised more than 70% of all inclusions. Samples sectioned from the first two molds showed an inclusion removal efficiency of 38% and 39%, respectively. Larger inclusions greater than 3 µm were more efficiently filtered. The third mold with the greatest number of larger inclusions showed the highest inclusion removal efficiency of 55%

Filtration Efficiency of Inclusions in Lightweight FeMnAl Steels

This presentation was given at the Transaction of American Foundry Society Conference