• Journal of Ocean Engineering and Technology
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• v.3 no.2
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• pp.638-638
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• 1989

It has been very important in various industry fields to improve the fatigue strength characteristics of bearings such as bearing life, fatigue limit, etc., because such poor properties could result in shortening the machinery life as well as in decreasing the accuracy. However, no successful heat treatment criterion seems to be available at present. In this study, the effect of the $170^{\circ}C\times120min$ tempering cycles repeated after $380^{\circ}C\times80min$ oil quenching for $800^{\circ}C$ spheroidizing-annealed bearing steel (STB2) as base metal on the $120^{\circ}C$ high temperature rotary bending fatigue strength characteristics were investigated, including the effects on hardness, Charpy impact value and micro-structure, in order to seek for the best heat treatment condition finally. The important results obtained are as follows : 1) The optimal cycle of tempering so that the fatigue strength .sigma. could become the highest was the 4th cycle. And it is confirmed that this $\sigma_{F}$ is about 6 times more increased than that of base metal, and about 1.3 times more increased than the case of the 1 cycle tempered. 2) As a result of the investigation for the effects of tempering cycles on hardness, the hardness at the tempering number of 2 thru 5 cyles was not decreased severely ; only about 10% decrease from those of the quenched and 1 cycle tempered case. Such hardness is equivalent to $H_{R}$/C61-62 with no bad effect on anti-abrasion of bearing steel. 3) In the case of 2 thru 5 cycle tempering as well as 1 cycle tempering, the impact value was not so improved comparing with the case of quenching, but an increase of 5 to 10% could be expected at least. 4) It was experimentally confirmed that the control of the mechanical properties improvement such as fatigue strength and fatigue life for bearing steels could be possible by the number of tempering cycles.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.65-69
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• 2003

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue and crack propagation, of thixoformed 357-T5 (Al-7%Si-0.6%Mg) alloy were examined. The resistance to impact and high-cycle fatigueof thixoformed 357-T5 tended to improve greatly with increasing solid volume fraction. An almost three-fold increase in impact energy value was, for example, observed with increasing solid volume fraction from 59 to 70%. The improvement in both impact and fatigue properties of thixoformed 357-75 with increasing solid volume fraction in the present study appeared to be related to the magnitude of stress concentration at the interface between primary and eutectic phase, by which the fracture process was largely influenced. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high solid volume fraction in thixoformed 357-T5 alloy was discussed.

• Proceedings of the Korean Nuclear Society Conference
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• 2002.05a
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• pp.265.2-265
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• 2002

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.17-22
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• 2007

Low-cycle environmental fatigue tests of cast austenitic stainless steel CF8M at the condition of fatigue strain rate 0.04%/sec were conducted at the pressure and temperature, 15MPa, $315^{\circ}C$ of a operating pressurized water reactor. The used test rig was limited to install an extensometer at the gauge length of the cylindrical fatigue specimen inside the small autoclave. So the magnet type LVDT's were used to measure the fatigue displacement at the specimen shoulders inside the high temperature and high pressure water autoclave. However, the displacement and strain measured at the specimen shoulders is different from the one at the gauge length for the geometry and the cyclic strain hardening effect. FEM calculated the displacement and the strain of the gauge length from the data measured at the shoulders. Tensile test properties in elastic and plastic behavior of CF8M material were used in the FEM analysis. A series of low cycle fatigue tests simulating the cyclic strain hardening effect verified that the FEM calculation was well agreed with the simulated tests. The process and method developed in this study would be so useful to produce reliable environmental fatigue curves of CF8M stainless steel in pressurized water reactors.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.44-48
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• 2001

The pipelines for natural gas transmission were buried in the ground of 1.5m depth. The pipelines were continuously subjected to vehicle load and internal pressure change by the quantity consumed of natural gas. In this paper, high cycle fatigue properties of natural gas transmission pipelines were studied. Fatigue specimens were obtained from the base and weld metal of circular pipe. Fatigue strength increased with increasing yield strength. Especially, the fatigue strength of base metal was higher than the yield strength of base metal and the fatigue strength of weld metal by manufactured process of TMCP.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.4
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• pp.477-486
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• 1986

본 연구에서는 SM20C를 모재로 하여 입경의 크기가 다른 3종의 복합조직강을 제작 동일한 분위기에서 저 및 고사이클 전영역에 걸쳐 피로특성을 검토하고져 한다. 제일보는 그 중 저사이클특성에 대한 보고이다. 일반적으로 저사이클 피로현상은 재 료가 탄소성 상태하에서 전위, 미소크랙, 보이드(void) 등의 인자가 복합적으로 작용 하여 발생함으로 변형률속도, 제어파형, 온도, 시험방법 및 분위기에 따라 많은 영향 을 받는다. 따라서 본 연구에서는 두가지 실험방법을 사용, 응력-변형율거동을 검토 복합조직강의 피로특성과 입경크기가 피로거동 및 강도에 미치는 영향을 비교 고찰하 였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.4
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• pp.427-434
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• 2004

Ferritic stainless steel is recently used in high temperature structures because of its good properties of thermal fatigue resistance, corrosion resistance, and low price. Tensile and low-cycle fatigue (LCF) tests on 429EM stainless steel used in exhaust manifold were performed at several temperatures from room temperature to 80$0^{\circ}C$. Elastic Modulus, yield strength, and ultimate tensile strength monotonically decreased when temperature increased. Cyclic hardening occurred considerably during the most part of the fatigue life. Dynamic strain aging was observed in 200~50$0^{\circ}C$, which affects the cyclic hardening behavior. Among the fatigue parameters such as plastic strain amplitude, stress amplitude, and plastic strain energy density (PSED), PSED was a proper fatigue parameter since it maintained at a constant value during LCF deformation even though cyclic hardening occurs considerably. A phenomenological life prediction model using PSED was proposed considering the influence of temperature on fatigue life.

• Journal of Mechanical Science and Technology
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• v.20 no.5
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• pp.675-680
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• 2006

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.

• Proceedings of the Materials Research Society of Korea Conference
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• 1995.11a
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• pp.108-108
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• 1995

• Proceedings of the Korean Nuclear Society Conference
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• 2005.05a
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• pp.59-60
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• 2005