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Metal Casting Technologies : September 2008
TECHNICAL FEATURE Production of compacted graphite irons Effect of Nitrogen Although it has long been known that the presence of around 0.01 – 0.015wt% Nitrogen in grey iron can promote compacted graphite structures the addition of Nitrogen is not used to produce CGI engineering castings. Nitrogen only has a strong compacting effect in heavy section castings such as ingot moulds, in small and medium section castings the compacting effect of Nitrogen is minor and inconsistent . A further problem is that whenever more about 0.01% Nitrogen is present there is a likelihood that castings will suffer from Nitrogen related gas and unsoundness defects. Use of Rare Earths The use of Rare Earth additions to produce compacted graphite structures was first reported by Morrogh and Williams following work on using additions of cerium rich mischmetall as potential nodularising treatments . As for magnesium treatment, it was found that, if there was insufficient residual cerium in the iron to fully nodularize the graphite, the eutectic graphite had an intermediate morphology which was first classified by Morrogh as “quasi-flake”. This term is no longer used to describe the graphite in CGI but it is still often used in industry to describe the graphite morphology in incorrectly treated Ductile (FCD) Irons which are not fully nodular. Rare earth treatments (to give residual Ce levels of 0.02 – 0.05%) have been used to produce CGI castings from very low sulphur (<0.01%) base irons . This method appeared to be only suitable for medium and thick section castings since it was difficult to avoid eutectic carbides in thin sections. The use of more powerful inoculation to avoid such carbides resulted in unacceptable proportions of nodular graphite. Such difficulties restricted the use of single Ce treatment. Use of Magnesium + Titanium Treatments Compacted graphite iron can be produced by under-treatment of low sulphur iron using conventional Mg containing nodularizing alloys. However the range of residual magnesium contents over which it is possible to produce satisfactory compacted graphite microstructures is very narrow at only 0.015 to 0.020% Mg. Consequently CGI is much more difficult to produce compared to ductile irons for which the window of residual Mg levels is much wider lying between 0.025 to 0.06%Mg. It is therefore extremely difficult to produce CGI castings using under-treatment with conventional Mg base treatment alloys. However when a small amount of Titanium (around 0.15- 0.3%) is present in the iron the upper limit of the range over which compacted graphite can be obtained is extended to 0.03%Mg. The effect of titanium is enhanced by the presence of cerium so when cerium is included with the Mg treatment the titanium level can be reduced to 0.1%. This discovery led to the development of commercial CGI treatment alloys for use as single ladle treatments to make the correct combined addition of Mg + Ti + Ce to the iron . The treatment procedures, including in the mould processing , are very similar to those used for conventional production of ductile irons. After the compacting treatment the iron must be inoculated but great care is needed since inoculation must prevent eutectic carbide formation without promoting the formation of nodular graphite. Although increasing nodularity raises mechanical properties it reduces thermal conductivity, damping capacity and machinability. For engine blocks up to 20% nodularity is acceptable but no flake graphite must be present. As in ductile irons treated iron must be poured as soon as possible to avoid the effects of fading. Loss of Mg and reduced nucleation would result in flake graphite and free carbide in the structure. Treatment with Mg-Ti-Ce alloy remained the only reliable production method for CGI until the development of the SinterCast Process during the late 1990s. Up to this point there was no great interest in producing CGI and its actual applications remained somewhat limited. The reluctance, on the part of both design engineers and foundries, to produce CGI was due to difficulties caused by the presence of titanium [5, 7, 15]. These problems include: ? Titanium increases dross formation during transfer and holding in the ladle and during pouring of moulds, and hence increases the chance of dross defects in castings. ? Titanium is an undesirable element in both FC and FCD irons so foundries would have to be extremely careful in separating returns to avoid contamination. Ti contamination in FC iron would cause undercooled graphite (plus a ferrite matrix). Ti contamination in FCD irons would result in sub-nodular graphite forms. ? Titanium considerably reduces machinability since it gives rise to hard Ti rich inclusions that can cause high wear rates on tooling. These inclusions can also nucleate cracks during cyclic stressing and thus reduce fatigue life. “TREATMENT WITH MG-TI-CE ALLOY REMAINED THE ONLY RELIABLE PRODUCTION METHOD FOR CGI UNTIL THE DEVELOPMENT OF THE SINTERCAST PROCESS DURING THE LATE 1990S.” 62 www.metals.rala.com.au