No-Bake S-Containing Mold/DI Metal Interactions: Consequences and Potential Applications
The ductile iron liquid metal is very reactive with environmental gases and solid molds after a nodulizing treatment. Mold-metal reactions could alter the spheroidal graphite shape to vermicular or flake. In this research, the metal-mold reactions were enhanced by various sulfur additions to the no-bake mold inserts to study these phenomena and to artificially produce layered dual-graphite structure in the casting. Thermodynamic and solidification modeling together with sets of different experimental techniques were applied to a test casting with internal core passages representing the cylinder head of a diesel engine. The effects of insert surface shape and location in the mold; content and type of sulfur-containing agent; coating; pouring temperature and additional chilling were experimentally studied and discussed. These results could be used to better understand the metal-mold interaction, prevent surface defects and design dual-graphite structure processes.
Emission of BTEX and PAHs from molding sands with furan cold setting resins containing different contents of free furfuryl alcohol during production of cast iron
At present, furan resin is the largest selling no-bake system of moulding sands. The most commonly used furan no-bake binders (FNB) are condensation products of furfuryl alcohol (FA) urea, formaldehyde and phenol. They are generally cured by exposure to organic sulfonic acids. FNB provide excellent mold and core strength, cure rapidly and allow the sand to be reclaimed at fairly high yields, generally 75%-80%, especially in applications where due allowance is made for the need to keep total sulfur content below 0.1%. However, due to probable carcinogenic properties of furfuryl alcohol, the EU Directive limits the content of this substance (in a monomer form in resin) to 25%. The classification of furfuryl alcohol and the resulting furan resin products has changed from “harmful” to “toxic by inhalation”. The aim of this study was to determine the effect of free furfuryl alcohol content in the resin on the emission of harmful substances from the BTEX (Benzene Toluene Ethylbenzene & Xylene) and PAHs (polycyclic aromatic hydrocarbon) group exposed to high temperature and how it affects the emissions allowance of reclaimed sand in the matrix. Three resins from a leading manufacturer were examined, which contain a free furfuryl alcohol content of 71%-72%, about 50% and < 25%, respectively. The hardener for each resin was 65% aqueous solution of paratoluenesulfonic acid. Tests were carried out in semi-industrial conditions where liquid cast-iron was poured into sample sand mold at 1,350 °C. The matrix of the studied sands was reclaimed in the amount of 0, 50%, 100%, respectively. With the increase of free furfuryl alcohol content, the volume of evolved gases decreased. For all resins the main component from the BTEX group dominating in the emitted gases was benzene; however toluene also appeared in the amount of a few percentages. In contrast, ethylbenzene and xylenes occurred only in the gases emitted from resin-bonded sands with the largest furfuryl alcohol content. The increase in the share of reclaimed sands resulted in very significant increase in the volume of gas generated and participation of benzene in these gases. In terms of emission of compounds from the PAHs group virtually for all resins, the total content of these substances was comparable (within the limits of error). The main components were: fluoranthene, pyrene and benzo(a) anthracene.
A New Approach to Graphite Nucleation Mechanism in Gray Irons
A complex research program involving the Scanning Electron Microscopy (SEM) has been undertaken in order to achieve a more detailed understanding of graphite nucleation in un-inoculated and High Purity (HP)-FeSi, Ca-FeSi and Sr-FeSi treated irons at different residual aluminum(Al) levels (0.001…0.05% ) in iron melt. (Mn,X)S compounds, usually less than 5.0 µm in size, with an average 0.4-2.0 µm well-defined core (nucleus), were found to be important sites for graphite nucleation in gray irons. Al contributes to the formation of Al2O3-based sites which act as nucleation sites for (Mn,X)S compounds, even at a very low Al level (<0.003%) in iron melt. Increasing the residual Al content in iron melt helps the initiation of graphite nucleation at a lower undercooling degree. A 0.005-0.01% Al content in melt appears to be beneficial, without the detrimental effect on pinhole occurrence in gray irons. Calcium (Ca) is present in most (Mn,X)S compounds and at similar levels in the core and shell (higher level in Ca-FeSi treated irons), while strontium (Sr) was found mainly in the core and only in Sr-FeSi treated irons. A three-stage model for the nucleation of graphite is proposed: 1) small Al2O3-based sites are initially formed in the melt; 2) complex (Mn,X)S compounds nucleate on these microinclusions and 3) graphite nucleates on the sides of the (Mn,X)S particles which have a lower crystallographic misfit with the graphite. In inoculated irons, the (Mn,X)S compound is more complex, at a lower Mn/S ratio and a higher compatibility to graphite nucleation, especially as inoculating elements (Ca, Sr, etc.) contribute.