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Metal Casting Technologies : December 2010
METAL Casting Technologies December 2010 31 foundries now make use of liquid iron pre-conditioning before inoculation or nodularizing treatments . It is also well known that chemical analysis alone cannot predict the microstructure and properties of cast irons. To provide improved knowledge on how graphite may form during solidification the patented Adaptive Thermal Analysis System (ATAS) was developed and has now become widely used [9, 10]. The ATAS approach uses twin sample cups and is based on measurement of the so called "active carbon equivalent, ACEL". The ACEL describes the effects of C, Si, P, and of impurities like oxides that affect the first stages of solidification. The system then compares the measured ACEL with the specified target value for CEL to enable the exact amount of Si and or recarburiser that needs to be added to the iron for pre- conditioning and to hit the target CEL. For thermal analysis of a melt two samples are poured: one into a plain sample cup to solidify via the grey iron reaction and one into a Te coated cup to solidify as a white iron. The various features on each cooling curve are compared and analyzed using artificial intelligence by a linked computer which contains a built in up-datable data base. The system is adapted to suit a given operation and is set up for the particular liquid iron preparation (charging, melting, holding, etc.) in each foundry. Additional software can be used to optimize pre-conditioner additives, and subsequent Mg treatment and inoculation in producing FCD irons. Development of Compacted Graphite Irons Compacted Graphite (CG) Irons contain eutectic graphite whose morphology is intermediate between that of flake and nodular forms . Compacted graphite can be obtained when there is insufficient residual magnesium or cerium in the liquid iron to fully nodularize the graphite during solidification. A number of compacting treatments have been developed based on the use of Mg, Mg + Ti, and rare earths, etc. Compacted graphite grows in eutectic cells in which there is a continually branched graphite structure as in normal flake graphite irons, but the flakes in the compacted structure are shorter and thicker than those formed in normal grey irons and significantly their ends are not sharp but are rounded. Due to their rounded but still interconnected graphite structures CG irons have mechanical and physical properties in between those of flake graphite (FC) irons and ductile (FCD) irons. The tensile strength of CGI is less dependent on carbon equivalent than FC irons. Mechanical property requirements from the ISO 16112:2006 Standard cover grades with minimum tensile strength levels from 300 to 500MPa. In spite of their inferior mechanical properties FC irons are superior to FCD irons with respect to their founding characteristics, damping capacity, thermal conductivity and machinability. Damping capacity, the ability to dampen vibration and noise, and thermal conductivity are required properties for engine blocks and heads. With their intermediate graphite form CG irons not only retain the sufficient levels of damping capacity and thermal conductivity needed for these parts but also provide higher strengths and better mechanical and thermal fatigue resistance. CGI is able to withstand the higher peak combustion chamber pressures in modern engines as well as provide savings in wall thickness and weight. The founding behaviour of CG irons, like FC irons, also allows complex shaped castings to be confidently produced with sound sections. It has been known for some time that very precise metallurgical conditions, and hence exacting process controls, were required for commercial production of compacted graphite irons . An excessive amount of nodular graphite must be prevented in thin sections of castings, while in thicker sections flake graphite must be avoided. This requires a precise balance between degree of nucleation, cooling rate and the level of the compacting elements in the iron. Consequently, until recent advances in the use of thermal analysis for process control of compacting treatments , the commercial growth of CGI was slow with only limited automotive applications such as exhaust manifolds, piston rings, and brake parts, etc. CG iron can be produced by under-treatment of low S iron using conventional Mg containing modularizing alloys but the range of residual Mg contents over which it is possible to produce consistent microstructures is very narrow at only 0.015 to 0.020% Mg. CGI is therefore much more difficult to produce compared to ductile irons for which the window of residual Mg levels is much wider at between 0.025 to 0.06%Mg. It is extremely difficult to produce CGI castings using IT HAS BEEN KNOWN FOR SOME TIME THAT VERY PRECISE METALLURGICAL CONDITIONS, AND HENCE EXACTING PROCESS CONTROLS, WERE REQUIRED FOR COMMERCIAL PRODUCTION OF COMPACTED GRAPHITE IRONS.