The purpose of this study is to investigate the influence of chunky graphite on the characteristics of fatigue crack propagation in heavy section spheroidal graphite cast iron. Samples containing and not containing chunky graphite were cut from a large ingot of spheroidal graphite cast iron. The fatigue crack propagation test conformed to ASTM. Stress ratio R was 0.1, and the specimen used was of the 1CT type with a thickness of 12.5mm, and humidity RH ＝ 0%.

  In the samples containing chunky graphite, the slope of the linear approximation m at the IIb stage of fatigue crack propagation becomes larger and fatigue crack growth rate becomes faster than the spheroidal graphite samples. The reason seems to be that crack propagates along the graphite connected to the chunky graphite. The decrease in the threshold stress intensity factor range ΔK_th of the chunky graphite samples was smaller than the decrease in tensile strength. In the chunky graphite samples, the fracture surface roughness became large and the crack closure induced by fracture roughness occurred conspicuously. The threshold stress intensity factor range decreased due to the influence of the chunky graphite. However, it was found that threshold stress intensity factor range does not decrease as much as the tensile strength due to the effect of the crack closure induced by fractured surface roughness.

  The microstructure and mechanical properties of ductile cast iron are affected by the chemical compositions and thickness of casting products. Thus, a nondestructive estimation method for casting is required. This research investigated effects of casting surface on the eddy current evaluation of ductile cast iron matrix. Ductile cast iron specimens having various microstructures were prepared by adding alloy elements and varying the thickness. One of the surfaces of the specimen was machined by face milling, and annealed after machining. Eddy current testing and microstructure observation were performed on both the machined surface and casting surface. Pearlite ratio showed good correlation with the eddy current signals measured on the machined surface. Similar correlation was obtained by measurement on the casting surface. However, the eddy current signal was affected by the casting surface. The influence of surface roughness and pearlite ratio on the eddy current signal differed according to the signal component. Selecting the signal components enables the effects of the surface roughness to be reduced. The effects of the testing frequency were also examined, and the results showed that the variation in the correlation between the eddy current and the pearlite ratio increased at lower and higher testing frequencies than 1 to 5kHz. Comparison of the correlations between the pearlite ratio and eddy current signals obtained by measurements on the machined surface and the casting surface showed that of the casting surface tends to have low pearlite ratio, due to the effects of the microstructure transient layer beneath the casting surface. Thus, it can be suggested that choosing a suitable testing frequency and an eddy current signal component enables the estimation of the matrix of ductile cast iron by eddy current testing regardless of the surface conditions.

  Several characteristics such as thermal expansion, strength, ductility, and toughness were examined in thermal expansion, static tensile, dynamic Charpy impact, and fracture toughness (J-integral) tests for two series of 1.8%C-1%Si low thermal expansive spheroidal graphite cast iron (LE-FCD) samples. Co addition series was 33%Ni- X%Co (X ＝ 0, 3, 6, 9 and 12) and Co substituent series was (36-Y) %Ni-Y%Co (Y ＝ 0, 4, 6, 8 and 12). The coefficient of thermal expansion at 373K of the LE-FCD samples in both series decreased with increasing Co content except the 24%Ni-12%Co sample which contained about 50% martensite in austenite matrix. The tensile and impact tests were carried out below room temperature using LE-FCD samples of the Co addition series. The tensile strength and elongation at 77K were found to be large owing to the transformation-induced plasticity (TRIP) of martensite. On the other hand, the impact values decreased with decreasing temperature. It is thought that the TRIP effect seen in the dynamic impact testing does not occur at all. From SEM observation, fracture surfaces tested in the temperature range from 123K to 573K appear not in the brittle but in ductile mode. The fracture toughness J_IC of the LE-FCD samples was more or less the same as or larger than those of fully ferritic spheroidal graphite ductile irons.

  High chromium-based multi component white cast iron is a promising erosion-resistant material because the crystallization of mainly chromium carbide and the various carbides around reduces the influence of solid particles. In this study, two kinds of high chromium-based multi component white cast irons with 3.0 % each of C, V, Mo and W plus varying Cr of 18% and 27 % were prepared to investigate their erosive wear properties. High chromium cast iron and multi white cast iron were used as the comparison materials. Solid particle erosion tests were conducted using silica sand of 290μm, 1030HV and irregularly shaped steel grits of 770μm, 810HV with impact angles of 30°, 60° and 90° and particle velocity of 100m/s.

  The erosive wear tests revealed that the erosion rates of high chromium white cast iron were half of the comparison materials and it showed excellent erosion resistance. In order to clarify the factors that improve erosive wear resistance, EDS and XRD analyses were performed to determine the carbide volume fraction and observe the cross-sections near the wear surface.

  The result suggested that since both the carbide ratio and macro matrix hardness are high, it is presumed that high chromium multi white cast iron exhibited excellent wear resistance. In addition, in this cast iron, the matrix is scraped preferentially over the progress and dropout of the protrusions as plastic deformation of the surface progresses.

  In most spheroidal graphite cast iron, tiny casting defects such as abnormal graphite, micro shrinkage, and inclusions, etc. occur near the casting surface. These defects deteriorate the fatigue strength depending on the size and distribution. Since the defects size is mostly within a range of 100 to 1000 μm, it is difficult to detect them in the production process. In this study, ferritic-pearlitic spheroidal graphite cast iron specimens were prepared with casting surface and machined surface. Then, a simple measurement of the defects size was carried out by microstructure observation or low temperature fracture testing. The estimated value of the maximum defect size could successfully be evaluated based on the statistics of extremes analysis for both micro shrinkage and dross defect near the casting surface. Moreover, four-point bending fatigue tests were performed at an R of 0.3 to investigate the influence of the defects near the casting surface on the fatigue strength. The S-N data showed a scatter because of the influences of the different defect sizes in respective specimens. The influence of the defects on the fatigue limit could be predicted by both statistics of extremes analysis of the defect size and the √area_max parameter model. Several examples will be also given on how to apply these methods for designing ductile cast iron products.

  High Mn cast iron improves the recycling rate of high tensile steel sheets and increases their strength, thereby expanding the range of use of cast iron materials. It is thought that erosion wear resistance can be improved by adding Cr to high-Mn cast iron. In this study, Cr was added to high Mn flake graphite cast iron, the microstructure and wear resistance were investigated, and the wear mechanism was elucidated. The erosion rate decreased with increasing Cr content. When the Cr content increased to 0.4%, the pearlite was refined and the matrix hardness increased. When the Cr content exceeded 0.6%, Cr carbide formed, and wear decreased as the matrix hardness increased. By changing the matrix microstructure from ferrite and pearlite to all pearlite, and further from pearlite and carbide, surface plastic deformation was suppressed and erosion wear was reduced. The changes in microstructure and hardness of the matrix were caused by the reduction of erosion wear.