Study on the High Temperature Thermal Fatigue Property of Thin-Section Sphere Graphite Cast Iron
Date Issued
2008
Date
2008
Author(s)
Chang, Hong-Yuan
Abstract
The primary purpose of this research is to establish the optimal casting conditions for producing thin-section (2~3 mm) spheroidal graphitic cast irons for high temperature applications (up to 800oC). Experimentally, the microstructures (include nodularity, nodule count, %ferrite, %pearlite, and %carbide) and thermal fatigue property will be evaluated and correlated with alloy design and casting parameters, such as molding material and pouring temperature. The results show that, for a fixed C content of some 3%, both the nodule count and nodularity increase first with increasing Si content, reach maxima at around 3.8~3.9%Si, and then decrease with further increase in Si content. On the other hand, the percent ferrite increases gradually with increasing Si content, reaches the maximum at around 4.6~4.8%Si, and then remains more or less constant or increases slightly. Regarding the effects of casting parameters on microstructure, the results show that higher nodule counts can be obtained in castings with a thinner section or with a lower pouring temperature. However, the effect of molding material (chemically-bonded sand and green sand) on nodule count is not significant. On the other hand, all the above three casting parameters exert little influence on graphite nodularity. Furthermore, the percent ferrite is higher in castings with a thicker section, a higher pouring temperature and molded with chemically-bonded sand. The optimal alloy design for attaining the best thermal fatigue property has been found to be a slightly hypereutectic composition, i.e., 4.5~4.6%CE, with a combination of relatively low C content and high Si content, e.g., 3.0%C + 4.8%Si. In addition, the best thermal fatigue property corresponds to a microstructure with the highest ferrite content and moderate nodule count. Furthermore, adding some 0.5%Mo to the iron significantly increases the thermal fatigue life. Based upon the results obtained herein, the optimal alloy design is: C: ~3%, Si: 4.7~4.8% (CE: 4.5~4.6%), Mo: 0.5%. During the thermal fatigue test, all the pearlite and part of the ferrite in the as-cast condition transform to austenite during the heating stage of the first cycle, and the austenite will transform to martensite after cooling (quench) to room temperature. As a result, un-transformed ferrite and martensite are present in the microstructure after one cycle. In the second cycle, part of un-transformed ferrite and martensite will again transform to austenite during the heating stage, while at the same time the un-transformed martensite will be tempered. Consequently, ferrite, martensite and temper martensite will be present in the microstructure after the specimen being cooled to room temperature. The afore-mentioned transformation mechanism continues to operate during the subsequent thermal fatigue cycles. However, at some point, the repeated tempering of both martensite and temper martensite causes the precipitation of secondary graphite particles at the grain boundaries, gradually reducing the dissolved carbon content in the matrix. As a result, the volume fractions of both martensite and temper martensite decrease gradually, while the ferrite and secondary graphite increase, with the progress of the thermal fatigue cycle. Under the influence of the tensile stress during each thermal fatigue cycle, cracks start to initiate at the graphite-matrix interface and/or at the vicinity of the precipitated secondary graphite particles. And then, cracks propagate along the grain boundary until the occurrence of complete fracture.
Subjects
thin-section spheroidal graphitic cast irons
nodule count, ferrite
thermal fatigue property
secondary graphite
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-97-R95522726-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):e08f5b05aeadbffccb4e2180d4369ebd