Eutectic cell wall morphology and tensile embrittlement in ferritic spheroidal graphite cast iron
- PDF / 1,868,072 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 69 Downloads / 210 Views
I. INTRODUCTION
OWING to its low production cost and fair mechanical properties, ferritic spheroidal graphite (SG) cast iron has been used in many engineering applications. These applications include exposures to various temperatures. To cite two examples: the temperatures are often sub zero and the climates severe when it is used as transport tubes[11] and storage containers[2] in the petroleum industry; and they are often in the range of about 400 8C to 600 8C or higher when it is used as the exhaust manifolds[3] in automobiles. However, ferritic SG cast iron may be subjected to embrittlement in certain temperature ranges. To ensure operational reliability, understanding of the embrittlement phenomena is essential. According to our recent tensile test results,[4] there exists a ductility drop at temperatures near the end of the upper elongation shelf and before the transition to the lower elongation shelf (which are around 230 8C for the chosen test materials and will be termed as the “intermediate low temperatures” in the following discussion). Associated with this tensile embrittlement, the fracture appearance reveals a zoned ductile area and a zoned brittle area (Figure 8). Whereas the zoned ductile area shows dimple voids and tearing ridges, the zoned brittle area is covered by quasicleavage cracks, which are initiated from the clusters of inclusion particles in the eutectic cell wall region.[4] Earlier in 1971, this zoned ductile and brittle fracture appearance was reported by Rickards.[5] But Rickards’ experimental data were too limited to depict whether the ductility deterioration would occur. Ductility deterioration can also take place at the temperatures around 400 8C.[6–10] To contrast from the high temperatures close to Ac1, these temperatures will be termed F.T. SHIAO, Graduate Student, and T.S. LUI and L.H. CHEN, Professors, are with the Department of Materials Science and Engineering, National Cheng Kung University, Taiwan 701, Republic of China. S.F. CHEN, formerly Graduate Student, Department of Materials Science and Engineering, National Cheng Kung University, is with Porite Taiwan Co., Ltd., Taiwan 722, Republic of China. Manuscript submitted May 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
“intermediate high temperatures” in the following discussion. (As a reminder, the Ac1 temperatures of those with 2.0 to 3.5 wt pct carbon increase from 755.5 8C 6 1.5 8C to 807 8C 6 1 8C when the silicon concentration increases from 2.4 to 4.8 wt pct.[11]) Instead of the zoned ductile and brittle fracture in the intermediate low-temperature range, the occurrence of this intermediate high-temperature embrittlement (which has also been called “intermediate temperature embrittlement,” “400 8C embrittlement,” or “673 K embrittlement” in the literature) leads to mixed regions of ductile fracture and brittle fracture (Figure 12). Dimple fracture occurs in the ductile regions, and intergranular fracture is the common feature in the brittle regions. Like the quasicleavage cracks in the brittle area of the z
Data Loading...
