• Journal of Korea Foundry Society
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• v.18 no.6
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• pp.563-569
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• 1998

A Study on microstructure and elevated temperature strength of 18Cr-2Mo ferritic stainless steel castings strengthened by alloying small amounts of titanium and carbon, has been conducted. The morphology of titanium carbides showed spherical in shape and their distribution depended on the amount of alloying elements. Maximum density ($7{\times}10^5/cm^2$) of titanium carbides has been formed in the alloy containing 2.0 wt.% titanium and 0.5 wt.% carbon as alloying elements and the size of carbide particles is in the range of 0.5 to $3.0\;{\mu}m$. High temperature tensile and fatigue strength of this alloy were the highest among the alloys tested in this research. The fracture mode of the alloys containing alloying elements less that 2.0 wt.% titanium and 0.5 wt.% carbon showed intercrystalline fracture at room temperature, while the alloys containing higher amounts of alloying elements showed transcrystalline fracture. All of the alloys showed creep or ductile rupture mode at elevated temperature.

• Proceedings of the KIEE Conference
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• 2000.11a
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• pp.265-268
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• 2000

In this study, a small punch creep (SP-Creep) test using miniaturized specimen has been described for the development of the new creep test method for high temperature structural components such as headers and tubes of boiler, turbine casing and rotor, and reactor vessel. The SP-Creep testing technique has been applied to 1Cr-0.5Mo steel used widely as boiler header material and the creep test temperature are varied at $550^{\circ}C{\sim}600^{\circ}C$. From the experimental results, e.g. SP-Creep curve behaviors, the creep rate in steady state and creep rupture life with test temperature and load, the load exponential value(n, m), the activation energy($Q_{spc}$), the Monkman-Grant relation and the creep life assessment equation etc., it can be summarized that the SP-Creep test may be a useful test method to evaluate the creep properties of the heat resisting material such as boiler header.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.7 s.166
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• pp.1120-1128
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• 1999

An elastic-viscoplastic finite element analysis is performed to investigate detailed growth behavior of creep cracks and the numerical results are compared with experimental results. In Cr-Mo steel stress fields obtained from the crack growth method by mesh translation were compared with both cases that the secondary creep rate is only used as creep material property and the primary creep rate is included. Analytical stress fields, Riedel-Rice(RR) field, Hart-Hui-Riedel(HR) field and Prime(named in here) field, and the results obtained by numerical method were evaluated in details. Time vs. stress at crack tip was showed and crack tip stress fields were plotted. These results were compared with analytical stress fields. There is no difference of stress distribution at remote region between the case of 1st creep rate+2nd creep rate and the case of 2nd creep rate only. In case of slow velocity of crack growth, the effect of 1st creep rate is larger than the one of fast crack growth rate. Stress fields at crack tip region we, in order, Prime field, HR field and RR field from crack tip.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.2
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• pp.135-139
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• 1985

A J$_{IC}$ test procedure by ultrasonic method performed to observe the crack opening behavior of fatigue precrack and detect the initiation of crack propagation of compact tension specimen in this paper. Pulse-echo method with 5 MHz transducer was used on the Cr-Mo steel quenched and tempered at 593.deg. C. We obtained the following results in the elastic-plastic fracture toughness test by ultrasonic method. Echo height is a little increased linearly and rapidly at the early stage of loading . Then it is decreased considerably, finally at the unstable crack growth stage, it is rapidly increased at an unpredictable rae. The initiation of crack propagation is supposed to be at the stage deviated from linearly decreased region and then blunted. J$_{IC}$ value(10.15-12.15 Kgf/mm) by ultrasonic method is lower than that(12.2 Kgf/mm) by R-curve method. But, it is required that the research for the more exact evaluation about correlation between echo height and the crack opening behavior of precrack tip will be continued. continued.ued.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.3
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• pp.159-167
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• 1998

In this study, friction-wear test was carried out on the carburized layer depth of a mechanical structure steel SNCM carburized with RX and LPG for 7hrs at $930^{\circ}C$ and also the wear properties of wear loss, wear rate, coefficient of friction, friction force and friction temperature were investigated. The wear properties for carburized layer of SNCM were tested on dry condition at the room temperature by the thrust load of 49~245N range at sliding speed of 0.2m/sec and the sliding speed of 0.2~1.0m/sec range at thrust load of 98N. Wear loss on the depth of carburizing layer was increased with increasing of thrust load and sliding speed, and with decreasing of hardness. The condition of worn surfaces were showed mild wear at less than the thrust load of 98N and sliding speed of 0.6m/sec but were showed severe wear at more than 98N and 0.6m/sec. The friction load and temperature were increased with increasing of thrust load but with increasing sliding speed was appeared minimum at 0.6m/sec. With increasing thrust load the wear rate was increased and the coefficient of friction was decreased, but with increasing sliding speed the wear rate and the coefficient of friction were decreased in 0.2~0.6m/sec and increased in 0.6~1.0m/sec, therefore 0.6m/sec in this testing is a transition velocity.

• Transactions of Materials Processing
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• v.26 no.1
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• pp.18-27
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• 2017

A comparative study was performed on strain-compensated Arrhenius-type constitutive models established with two regression methods: polynomial regression and regression Kriging. For measurements at high temperatures, experimental data of 70Cr3Mo steel were adopted from previous research. An Arrhenius-type constitutive model necessitates strain compensation for material constants to account for strain effect. To associate the material constants with strain, we first evaluated them at a set of discrete strains, then capitalized on surrogate modeling to represent the material constants as a function of strain. As a result, disparate flow stress models were formed via the two different regression methods. The constructed constitutive models were examined systematically against measured flow stresses by validation methods. The predicted material constants were found to be quite accurate compared to the actual material constants. However, notable mismatches between measured and predicted flow stresses were revealed by the proposed validation techniques, which carry out validation with not the entire, but a single tensile test case.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.37-42
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• 2000

The boiler tubes and steam Pipes operating both at high temperature and pressure for a long period of time in a power plant are degraded by creep because of internal pressure. So, the remaining life of a component is evaluated by the creep rupture strength. Although the conventional method to evaluate the creep damage is widely used, it has some disadvantages such as requires large size specimen and long employed to evaluate the correlation between fracture toughness and evaluation time. Recently, new method so called "small lunch test' is used to evaluate degradation of creep. In this study, a conventional creep test and a small punch test are conducted using 2.25Cr-1Mo steel which is mainly used for the boiler tubes and steam pipes in power plant. The creep life, approximately 1,500 hrs, is determined by conventional method under a severe condition then specimens for a small Punch test are obtained after certain time intervals such as 1/4, 1/2 and 3/4 of final rupture time, respectively.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.211-216
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• 2001

The components with the circular cross section have the symmetric combination parts for rotating balance and the crack emanates from the symmetric combination parts. The symmetric cracks from symmetric combination parts make a decrease in the component fatigue life more than single crack. In this study, to estimate the behavior of symmetric cracks, the fatigue test was performed using rotary bending tester on the specimen with a symmetric defects in circular cross section. The material used in this study is Ni-Cr-Mo steel alloy. Under the same stress, the result from the rotary bending fatigue test turned out that the symmetric cracks made a decrease in the fatigue life by 35% more than single crack and the relation between log a and cycle ratio $N/N_f$ obtained linearly.

• Journal of Welding and Joining
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• v.23 no.6
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• pp.61-66
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• 2005

A probabilistic approach for evaluating failure risk is suggested in this paper. Probabilistic fracture analyses were performed for a pressurized pipe of a Cr-Mo steel reflecting variation of material properties at high temperature. A crack was assumed to be located along the weld fusion line. Probability density functions of major variables were determined by statistical analyses of material creep and creep crack growth data measured by the previous experimental studies by authors. Distributions of these variables were implemented in Monte Carlo simulation of this study. As a fracture parameter for characterizing growth of a fusion line crack between two materials with different creep properties, $C_t$ normalized with $C^*$ was employed. And the elapsed time was also normalized with tT, Resultingly, failure probability as a function of operating time was evaluated fur various cases. Conventional deterministic life assessment result was turned out to be conservative compared with that of probabilistic result. Sensitivity analysis for each input variable was conducted to understand the most influencing variable to the analysis results. Internal pressure, creep crack growth coefficient and creep coefficient were more sensitive to failure probability than other variables.

• Korean Journal of Metals and Materials
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• v.49 no.9
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• pp.708-714
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• 2011

The premature failure of a servovalve used for six months in a thermal power plant has been analyzed. The servovalve was made of stainless steel, containing 16Cr-0.44Mo, along with other elements. An overload of oil-supply pumping and an abnormal increase in the oil flux were observed during operation. A study revealed that erosion and corrosion could be the main causes of the failure. The visual examination of the servovalve did not show any appreciable damage. However, corrosion and erosion of the servovalve were observed using scanning electron microscopy (SEM). Upon examination of the servovalve, the corrosion was found to have occurred throughout the bushing and spool; however, erosion occurred at only the edge-side. In addition, the condition of the electrohydraulic control system (EHC) oil was investigated with respect to its satisfaction of the management standard.