Effect of Microstructural Refinement on Ductility Deterioration of High Silicon Ferritic Spheroidal Graphite Cast Iron Caused by Cyclic Heating

Abstract

This investigation applies cyclic heating and cooling to elucidate the effect of microstructural refinement on the tensile elongation deterioration of ferritic spheroidal graphite cast iron. In order to eliminate the oxidation factor, the cyclic heating/cooling test was performed in a 1:33 $ 0:133 Pa ambient vacuum atmosphere with cyclic heating at a maximum temperature of 1023 K. Severe embrittlement accompanied by intergranular fracture occurred after the ferritic spheroidal graphite cast iron was subjected to a certain number of heating and cooling cycles. A fair amount of inevitable inclusion particles were found to agglomerate in the eutectic cell boundary region, and so the cyclic heating induced embrittlement can be recognized to be strongly dependent on the solidification cooling rate of the materials. Based on experimental evidence, the cracking evolution can be divided into three steps: (1) crack initiation from the vicinity of the eutectic cell boundary at the surface, (2) crack linking and major crack formation, and (3) major crack inward extension. Cyclic heating cracks are mainly initiated at the eutectic cell boundary where a fair amount of MgO inclusions dispersed, and consequently propagated along the annealed eutectic cell boundary. While investigating the plastic deformation behaviors around the above mentioned MgO inclusions pertaining to the crack initiation and crack propagation, typical etch pit evidence was observed in the vicinity of the cell boundary area.

Coarsening and dendritic instability of spheroidal graphite in high silicon cast iron under thermal cycling in the ferritic domain

“…Thermal cycling in the ferritic domain has been shown to dramatically decrease mechanical properties of spheroidal high-silicon cast irons [3,4,6]. However, none of these previous works reported a microstructure evolution as observed in the present work.…”

“…However, none of these previous works reported a microstructure evolution as observed in the present work. In the case of the works by Lin et al [3,4] this was certainly due to the maximum temperature they considered, either 700°C or 750°C, which may have been too low for graphite dissolution to take place. It appears more surprising that Cheng et al [5] and Avery et al [6] did not observe microstructure changes similar to those detailed here as the highest temperature that they investigated was also 800°C.…”

“…These alloys are used in many applications including loading under thermal cycling. Studies on the effect of thermal cycling on tensile properties [3,4] and on the thermomechanical behaviour [5,6] of high silicon SGI have been carried out with maximum temperature up to 800°C which is within the ferritic domain for the investigated alloys. These studies involved a limited number of cycles and the only changes in microstructure that were reported concerned oxidation of inclusions associated with the last-to-solidify areas and partial recrystallization of ferrite for the highest maximum temperatures (750°C and 800°C).…”

Effect of Maximum Temperature on the Cyclic-Heating-Induced Embrittlement of High-Silicon Ferritic Spheroidal-Graphite Cast Iron

“…The inclusion particles were mainly composed of the elements of magnesium, phosphorous, oxygen and cerium. [4][5][6] It is well known that the presence of gaseous elements in the cast iron melt can cause casting defects such as blowholes and pinholes in the castings during the eutectic solidification process, and oxygen often concentrates in the eutectic liquid, increasing gas defects. Cast iron contains the two kinds of oxygen, which were oxides and soluble oxygen.…”

“…[1][2][3][4][5][6] High-silicon spheroidal graphite cast iron is considered to be one of the most promising candidate alloys for these components due to its castability, low oxidation, and good wear resistance. 7,8) In these cases, SG cast iron components periodically operate at elevated temperatures of up to 1123 K and, in particular, are subjected to cyclic thermal shocks from heating/cooling.…”