Effect of the Abnormal Skin Structure on Thermal Fatigue Property of Thin-Wall Ductile Cast Iron
Date Issued
2012
Date
2012
Author(s)
Lu, Shiang-An
Abstract
The primary purposes of this research are threefold: (1) to study the effects of metallurgical and processing parameters on the formation of the abnormal structure on the skin of the thin-wall ductile iron castings. The parameters examined include the hardener/resin ratio, type of coatings, the pouring temperature, and the addition amounts of nodularizer and inoculant, (2) to explore the mechanisms for the formation of the abnormal skin structure, and (3) to correlate the skin abnormal structure in terms of degraded area with the thermal fatigue life of the thin-wall ductile iron castings.
Taguchi’s analysis results indicate that for castings (2mm, 3mm and 6mm in section thickness) poured at around 1450oC, the optimal casting conditions regarding the prevention of the abnormal skin structure are: the hardener/resin ratio of 60-80%, graphite coating, and 1.2% nodularizer+0.6% inoculant. On the other hand, for castings poured at around 1400oC, the optimal casting conditions are: the hardener/resin ratio of 80%, SiO2 coating, and 1.2% nodularizer+0.6% inoculant. In addition, among the three types of coating (graphite, SiO2 and MgO) investigated, graphite coating exhibits the most effective when poured at a relatively high pouring temperature (1450oC), while SiO2 is the most favorite coating for a relatively low pouring temperature (1400oC). Nevertheless, no significant difference in effectiveness on the prevention of the abnormal skin structure was observed among these three coatings, but MgO coating shows the least variations in the degraded area of the abnormal skin structure. Furthermore, significant improvement was obtained when a coating was applied as compared to the case without coating.
Regarding the thermal fatigue property, the thermal fatigue life increases with decreasing the degraded area of the abnormal skin structure. For all the castings with three different section thicknesses, namely, 2mm (with fixed nodule count of 1380±100#/mm2), 3mm (with fixed nodule count of 920±100#/mm2) and 6mm (with fixed nodule count of 654±58#/mm2), specimen EGH-80% (1.2% nodularizer+0.6% inoculant, graphite coating, pouring temperature of 1450oC, and hardener/resin ratio of 80%) exhibited the best thermal fatigue property. In addition, increased thermal fatigue life was registered for castings poured at a relatively higher temperature, i.e., 1450oC, and with a thicker section, i.e., 6mm.
When the fatigue specimens were subjected to thermal cycles between room temperature and 800oC, phase transformations took place and the microstructures varied accordingly. In the first thermal cycle, most of pearlite and part of ferrite in the as-cast condition transformed to austenite during the heating and holding stage, and then the austenite formed transformed into martensite during the subsequent cooling cycle (water quench). As a result, martensite and un-transformed ferrite were present after the first thermal cycle. In addition, the transformed martensite can be observed to be distributed mainly in areas immediately adjacent to graphite nodules and also in the vicinity of grain boundaries where the original pearlite phase was present. In the second thermal cycle, part of ferrite and martensite again transformed to austenite during the heating and holding stage, while at the same time the un-transformed martensite were tempered. In the subsequent cooling cycle, austenite once again transformed into martensite, and therefore, martensite, tempered martensite and un-transformed ferrite coexist in the microstructure. The afore-mentioned pattern of phase transformation continued to operate in the following thermal cycles. However, at certain stage in the course of cyclic thermal fatigue test, minute graphite particles, the so-called secondary graphite, started to precipitate at grain boundaries, owing to the repeated tempering of both martensite and tempered martensite. When the martensite was heated to the pre-set 800oC, but failed to transform to austenite due to the temperature being not high enough for phase transformation to take place, the martensite was tempered at a relatively high temperature (800oC) instead, causing the precipitation of secondary graphite along the grain boundaries. With the progress of the thermal cycles, the precipitation of the secondary graphite also continued, which resulted in a gradual reduction in the dissolved carbon content in the matrix. Consequently, the volume fraction of the ferrite phase increases, while at the same time the amounts of martensite and tempered martensite decrease. Under the influence of residual tensile stress developed in each thermal cycle, cracks tended to be initiated at the graphite-matrix interface and/or in the vicinity of the precipitated secondary graphite particles, and once cracks occurred, they propagated along the secondary graphite particles at the grain boundaries and also the interface between the degenerate graphite phase and the matrix.
Subjects
Thin-wall ductile cast iron
Taguchi’s method
Abnormal skin structure
Coating
Thermal fatigue property
Type
thesis
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