| Abstrakt: |
Silicon and molybdenum (SiMo) ductile iron is commonly used for exhaust manifolds because these components experience thermal cycling in oxidizing environment, which requires resistance to fatigue during transient thermomechanical loads. Previous studies have demonstrated that alloying elements, such as Al, to SiMoductile iron reduces the amount of surface degradation during static high-temperature exposure. However, deterioration of sphericity of the graphite nodules and a decrease in ductility could affect the tendency of cracking during thermal cycling. In this article, the effect of Alalloying on static and transient thermomechanical behavior of SiMoductile iron was investigated to optimize the amount of Alalloying. A thermodynamic approach was used to confirm the effect of the Alalloying on the phase transformations in two SiMocast irons, alloyed by 1.8% Aland 3% Al. These two alloys were cast in a laboratory along with the baseline SiMoductile iron. Several experimental methods were used to evaluate the dimensional stability, physical properties, static oxidation, and failure resistance during constrained thermal cycling testing to compare their high-temperature capability. Experimental results verified that Alalloying increases the temperature range and decreases volume change during eutectoid transformation, which together with enhancement of oxidation protection improved the dimensional stability. Thermocycling tests showed that the number of cycles to failure depends on the amount of Alalloying and the applied high-temperature exposure during each cycle. SEM/EDX, high-resolution TEM and µCT analysis were used to verify the mechanism resulting from the Alalloying protection. It was shown that an optimal level of Alalloying for balancing oxidation and thermal cracking resistance depends on thermomechanical conditions of application. |
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