• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.11
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• pp.2742-2748
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• 2000

In this study, the quantitative evaluation was performed by examination for gatigue crack origin in two prepared ductile irons specimens which have different microstructures using rotary bending fatigue tester, Series A has mixed microstructure. ferrite plus pearlite, and series B has bainitic microstructure. Obtained main results are as follows. The scatterings of fatigue strength were observed on σ(sub)α-Ν diagram of both specimen series, and it is observed that scatterings of series A were more serious. It is reasonable to evaluate the size of mesocrack range by means of √A(원문참조), where A means the area of mesocrack range including globular graphite nodule. As a result of reconsideration for the fatigue data by introduction of K(sub)α-Ν diagram, the scattrings of fatigue life were, remarkably. reduced. Therefore, it is more reasonable to evaluate of mesocrack range on fatigue life by parameter K(sub)α rather than σ(sub)α.

• Journal of Korea Foundry Society
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• v.12 no.2
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• pp.102-107
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• 1992

• Journal of Industrial Technology
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• v.18
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• pp.203-208
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• 1998

Fatigue tests were performed to examine the 4 parameter method for specimens prepared by various heat treatment which resulted in different mechanical properties. Obtained main results are as follows. (1) Samples treated by austempering did not show the expected improvement of fatigue limit although hardness and strength increased. This is attributed to the fact that defect sensitivities of materials increase as increasing of hardness and tensile strength, it is also shown that the graphites acting as stress concentration place become larger by austempering heat treatment than by normal annealing. (2) It is very reasonable to predict the fatigue limit of ductile irons with 4 parameter method based on ${\sigma}_{TS}$, $H_v$, ${\Delta}K_{th}$ and ${\sqrt{area}}_{max}$. (3) The half-austempering treatment appeared to be more useful than the full-austempering method to improve the fatigue limit in the spheroidal graphite cast iron with multi defective material.

• Journal of Industrial Technology
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• v.19
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• pp.75-79
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• 1999

Rotary bending fatigue tests were performed to investigate the effects of 2-phase matrix structure on fatigue limit with prepared specimens in high strength ductile irons. Two types of the specimens with different microstructures have been used. Series A has sorbite and series B has bainite. Fatigue limits of both specimens are improved comparing with as cast specimen. The fatigue limit is higher in series B than in series A. The reason why the fatigue limit of series A shows inferiority to that of series B is due to the transition of micro fatigue cracks to mesocrack occurs very rapidly, so increased stress intensity factor drives the fatigue crack growth. The higher fatigue limit of series B which has bainite is caused by the ${\gamma}$ layer contained in microstructure impede the rapid growth of micro fatigue crack to mesocrack and ${\alpha}$ layer around graphite has the higher capacity for the absorption of plastic deformation energy than sorbite.

• Journal of Korea Foundry Society
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• v.26 no.6
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• pp.258-266
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• 2006

The purpose of this study was to investigate the effects of silicon and nickel additions that influenced the impact and tensile properties of heavy section as-cast ductile irons for wind mill. Based on the results of the metallographic analysis and the mechanical testing on the 2.0 wt.%Si (LS group) and 2.4 wt.%Si (HS group) ductile irons, which contains 0.0, 0.3, 0.6 and 0.9 wt.%Ni, respectively, the following conclusions could be obtained. The nickel additions increased the tensile properties, the microhardness of pearlite, and the pearlite fraction of matrix for the specimen with the same silicon content. The mechanical properties of LS group specimen were in the range of the specification for the ductile iron wind mill castings. The LS group specimens showed higher absorbed impact energy at room temperature and $-20^{\circ}C$ than that of the HS group specimens. However, the absorbed impact energy at $-20^{\circ}C$ for the HS group specimens was observed to be sharply decreased under 10 J by addition of the nickel.

• Journal of the Korean Society for Heat Treatment
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• v.12 no.3
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• pp.215-220
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• 1999

Rotary bending fatigue tests were carried out to investigate the artificial defect sensitivity of fatigue limit in annealed and austempered ductile irons. Artificial defect(hole, diameter${\leq}0.4mm$) machined on specimen surface did not bring about an obvious reduction of fatigue limit in austempered ductile iron as compared with annealed. As a result of investigation on $\sqrt{area}$ c which is the critical artificial defect size. $\sqrt{area}$ c of austempered ductile iron is larger than that of annealed. This means that the crack initiation at artificial defect in austempered ductile iron is more difficult in comparison with annealed. In case that the $\sqrt{area}$ c of artificial defect and graphite nodule are same, the rate of crack initiation for graphite nodule is higher than that of artificial defect.

• Laser Solutions
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• v.2 no.2
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• pp.42-51
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• 1999

This study has been performed to investigate into some effects of the output power and traverse speed of laser beam on the microstructures, hardness and fatigue resistance of the ductile iron surface-hardened by $CO_2$ laser defocussed beam. Optical micrographs have shown that with increasing the output power and decreasing the traverse speed, the martensite was coarsened and some retained austenite were appeared in ductile iron. The microstructures of hardening zone were composed of bull's eye and some nodular graphite dissolved structures by the effect of self quenching. Fatigue fracture characteristics of ductile iron have appeared in the high stress and low stress ranges. The fracture initiated at nodular graphites in the surface hardened layer due to the stress concentration caused by a notch effect. The interior graphite nodules were broken away or popped out during crack propagation. Fatigue test has shown that values of fatigue strength considerably increased with increasing output power at a given traverse speed.

• Journal of Korea Foundry Society
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• v.12 no.3
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• pp.230-237
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• 1992

The purpose of present investigation is to obtain ductile cast iron with ferrite-bainite matrix by pearlite-bainite transformation treatment. Ductile cast irons having three kinds of Mn ampunt had been manufactured. Mn increased pearlite volume fraction iin as-cast ductile cast iron. Ductile cast irons of different pearlite fraction were austenitized at $875\;^{\circ}C$ for 230-350 sec or $925\;^{\circ}C$ for 130-170 sec followed by austempering at $300\;^{\circ}C$ or $400\;^{\circ}C$ for the various periods of time from 5 to 30 min. When specimen was austenitixed for 130 sec at $925\;^{\circ}C$ and for 230 sec at $875\;^{\circ}C$, pearlite was transformed into austenite. Bainite around graphite was found at $925^{\circ}C$ for 170 sec. Bainite in grain boundary of ferrite was happened at $875^{\circ}C$ for 350 sec. During the austempering process, acicular bainite was precipitated at $300^{\circ}C$ and lath bainite was precipitated at $400^{\circ}C$. Increment in manganese content restrained bainitic transformation. Retained austenie was of little quantity.

• Journal of the Korean Society for Heat Treatment
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• v.1 no.1
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• pp.13-23
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• 1988

The hardenability of alloyed ductile cast irons was studied for 54 different alloy compositions obtained from eight commercial and laboratory foundries. The alloying elements investigated for their effects on hardenability were Si(2.0 to 3.0%), Mn(0.0 to 0.8%), Mo(0.0 to 0.6%), Cu(0.0 to 1.5%), and Ni(0.0 to 1.5%). Two hardenability criteria, a first-pearlite hardenability criterion and a half-hard hardenability criterion, were used to determine hardenability of ductile irons. Prediction models for each hardenability criterion were developed by multiple regression analysis and were well agreed with previous experimental results. Molybdenum was the most potent hardenability promoting element followed by manganese, copper and nickel ; silicon had little effect on hardenability and reduced the hardenability as silicon content increased. When alloying elements were presented in combination, strong synergistic effects on the hardenability were observed especially between molybdenum, copper and nickel. The hardenability of ductile iron was strongly influenced by austenitizing temperature. Increasing austenitizing temperature up to $955^{\circ}C$, hardenability increased gradually but decreasing rate and then decreased as temperature increased above $955^{\circ}C$. Unless reducing segregation by very long-time annealing treatment, the hardenability of ductile iron was not significantly influenced by segregation of alloying elements.

• Journal of Power System Engineering
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• v.17 no.4
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• pp.109-114
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• 2013

In this study, the microstructure, mechanical properties and high temperature oxidation characteristics of HiSiMo and HiSiMoM ductile iron for exhaust manifold were investigated. The HiSiMoM ductile iron was developed by optimization of alloying element addition and casting design. The exhaust manifold prototype was fabricated using the HiSiMoM iron and this resulted in the weight saving of 0.73kg. The microstructures of the HiSiMo and HiSiMoM irons were similar each other and graphite nodularity was 89% and 93% respectively. Tensile strengths of them were 663.5 and 674.4 MPa and Brinell hardness were 235.3 and 243.9 respectively. Both irons showed parabolic weight gain behavior in high temperature oxidation atmosphere. Oxidation layer was divided into external and internal layers. The weight gain of the HiSiMoM iron was lower than that of the HiSiMo iron after isothermal oxidation test at $900^{\circ}C$. This should be rationalized by higher Si enrichment at the interface of the matrix and internal layer of the HiSiMoM iron.