Abstract
An age-hardening spheroidal graphite cast iron that contains Al, Ni and Cu is also expected to have an improved oxidation resistance. The following is a study on the effects of alloy elements and age-hardening of the cast iron on the oxidation resistance. The materials were oxidized at 525°C and 700°C in air.
At 525°C the cast iron was scarecely oxidized and thus it was considered to have practical applicability. In the test at 700°C, it was found that the addition of over 2%Al decreased the amount of oxidation markedly and proved that Al improved the oxidation resistance. However, Ni had an adverse effect because it produced a porous Wüstite layer by segregation in its inner layer. Cu was found to promote the effect of Al.
On a comparison of the caluculated and the observed weight gain by oxidation a difference was observed for cast iron with under 1%Al content, which was attributable to decarburization and the porous oxide film. The enrichment of Al and Si in the Wüstite layer improved the oxidation resistance. The concentration ratio was defined as follows.
FAlorSi=CAlorSi⁄(CFe) / C°AlorSi⁄(C°Fe)
Where C and Cº represent the concenration in the oxide film and the matrix respectively. FAl=3-4 and FSi=2 were almost constant in the Wüstite layer.
When the specimen was oxidized at 700°C after ageing the cast iron with 3.2% Al showed greater weight gain than the unaged materials. This was because a part of the undissolved precipitate, AlN, remained behind. On the other hand, the cast iron with Al, Ni and Cu had almost the same value in aged and unaged materials because nearly all the precipitates redissolved at 700°C.
The oxidation rate followed the parabolic law except in the early stage when therer are decarburization and preferential oxidation of Al and Si.
