LAUSR

Silicon molybdenum (SiMo) cast iron and SiMo with aluminum additions (SiMoAl) are currently used to produce exhaust manifolds and turbocharger housings for automobiles. The effect of microstructure and alloying elements on SiMo iron has been extensively studied. A new ASTM standard (A1095) specification for high-silicon molybdenum ferritic iron castings has been developed including compacted, mixed, and spheroidal graphite shape. A number of patents and trademarks for SiMoAl cast iron for high-temperature applications have been published since the 1990s. Recently, the microstructure, hot oxidation resistance, and mechanical properties of SiMo and SiMoAl cast iron were published in the article by Ibrahim et al., referred to hereafter as article 6. This letter attempts to comment on article 6 in the aspects of microstructure, hot oxidation resistance, and mechanical properties of SiMo and SiMoAl iron. The first comment is concerned with the microstructure of SiMoAl iron. As well known, the presence of antispheroidizing element aluminum in cast iron can result in graphite shape deterioration. It appears that spheroidal graphite microstructure was not achieved for the SiMoAl specimens in article 6. Instead entirely degenerated graphite structure was formed, as shown in Figure 1(a). This can be a concern relative to the findings of article 6. When a nodularizer combining magnesium with rare earth metals is utilized for the melt treatment, spheroidal graphite microstructure exceeding 75 pct nodularity can be consistently produced for SiMoAl with 3 pct Al (weight pct used in chemical composition in this letter) iron castings, as presented in Figure 1(b) for the, as yet, unpublished work by the author of this letter. The second comment is about the discrepancy of high-temperature oxidation testing results. Aluminum-alloyed cast irons are attractive because of the resistance to scaling and growth at high temperatures. Figure 2 illustrates the results of static hot oxidation testing from article 6 and the unpublished work by the present author. The silicon equivalent (Si + 0.8Al) can be used as a good indicator for high-temperature oxidation resistance for cast iron in the range of chemistry studied. The silicon equivalent of SiMoAl (3 pct Si and 3 pct Al) iron is much larger than that of SiMo (<5 pct Si) iron. The SiMoAl (3 pct Si and 3 pct Al) iron is decidedly superior to SiMo iron in hot oxidation resistance including weight change, depth of oxide layer, and scale adherence, from the repeated testing by the present author. However, SiMoAl (3.1 pct Si and 3 pct Al) iron in article 6 displayed somewhat ‘‘catastrophic’’ oxidation behavior and even worse oxidation resistance than SiMo iron samples, as shown in Figure 2 (curve b), which appears to be contradictory to the data known to the present author. The third comment is on the relationship between molybdenum content and impact toughness of SiMo iron. There is a continuous increase in the strength at a given temperature with increasing Mo content. The most significant response to increased Mo content is realized over the range 0.4 to 1.0 wt pct Mo. High Mo content tends to generate more primary interdendritic carbides and an increasing volume fraction of Mo-rich phases, thereby reducing the impact toughness, as shown in Figure 3. For the 0.5 and 1.0 pct Mo SiMo samples, the toughness values agree between article 6 and Reference 8. However, the toughness of 0.7 pct Mo SiMo from article 6 appears to be an outlier among the data shown in Figure 3. Article 6 appropriately reported that the tensile elongation of SiMo iron was monotonically decreased with increasing the Mo content from 0.5 pct to 1.0 pct. Therefore, the toughness spike at 0.7 pct Mo in article 6 could have been caused by the variations in the testing and chemical compositions of iron samples.