The purpose of this study is to carry out Charpy impact test of heavy section spheroidal graphite cast iron with varying Mn content and carbon equivalent (CE), and to investigate the influence of Mn content and carbon equivalent on impact characteristics. The Charpy impact specimen was cut out from a large ingot of spheroidal graphite cast iron equivalent to FCD400 and instrumented Charpy impact test was conducted. The hammer angle of fall was 140° and the test atmosphere was room temperature in air. The tests were conducted 14 times per material. The impact value was determined from the absorbed energy obtained from the measured voltage-time diagram. As a result, at any CE, the absorbed energy decreased with the increase in the Mn content. For materials with high Mn content, the absorbed energy decreased with decreasing CE, but for materials with low Mn content, the carbon equivalent did not affect the absorbed energy.
In order to investigate the cause of the decrease in the absorbed energy, all fracture surfaces were observed after the test by electron microscope, and the percent of brittle fracture, which indicates the percent of cleavage fracture, was calculated. As a result, the percent of brittle fracture rate increased with the increase in the Mn content. Moreover, the brittle fracture rate increased as the CE decreased at the same Mn content. Since most of the part showing brittle fracture on fracture surface was pearlite, the pearlite area on the fracture surface was calculated. Good correlation between the brittle fracture rate and pearlite area rate on the fracture surface was seen. These findings suggest that the reasons for the decrease in absorbed energy depends on the pearlite amount.
In sand mold additive manufacturing applying the two-liquid self-hardening process, changes in the strength of sand molds were investigated when the layer thickness was changed from 0.30mm to 0.70mm. The strength of sand molds excluding the influence of packing fraction was more or less constant when the layer thickness was 0.6mm or less. On the other hand, when the layer thickness was 0.7mm, a significant decrease in strength was observed and the sand mold was found to peel off in the stacking direction. To obtain robust sand molds, the printed resin should penetrate to the lower layer, and the layer thickness should be 0.6mm or less. When the layer thickness was 0.6mm or less, the sand mold strength monotonously increased with increasing density of the printing dots of the resin. These findings suggest that the strength of the sand mold is expressed as a function of the resin addition amount to the power of 0.57.
Amidst the growing needs to apply CV graphite cast iron to large castings, there have only been a few reports on the fatigue strength characteristics of CV graphite cast iron. Moreover, large castings are prone to producing chunky graphite and inhibiting mechanical properties.
In this study, FCV test castings capable of simulating the slow cooling rate of large castings with 150mm thickness were produced by adding rare earth element, and plane bending fatigue tests were conducted to investigate the influence of wall thickness on fatigue strength.
It was confirmed that the number of graphite grains decreased and the graphite particle size increased with increasing mass in each specimen. In addition, in the plane bending fatigue test, the fatigue strength of each specimen showed a similar value. These test results confirmed the weak correlation between the wall thickness and fatigue strength in the range of the experimental conditions. Moreover, in the observation of the crack growth behavior, an initial crack was generated at the end of the non-spheroidal graphite in the initial stage. With the increase of the number of repetitions, the crack propagated in the form of sewing the graphite while repeating the connection and coalescence with the leading crack generated from other graphite etc.