• 한국압력기기공학회 논문집
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• 제20권1호
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• pp.25-31
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• 2024

The core shroud of rector vessel internals (RVI) of OPR1000 and ARP1400 is made of Type 304 stainless steel (SS) by bending and welding process that may induce high deformation and residual stress in manufacturing. This work aims to evaluate the susceptibility of stress corrosion crack (SCC) initiation of bent parts of RVI in high temperature primary water environment. For SCC initiation test, tensile specimens were fabricated from the 90 degree bent plate of Type 304 SS (DT specimen), that is an archived part of a Korean APR1400. After the SCC initiation test, the specimen surface was thoroughly examined by optical and scanning electron microscopy, and compared to the specimen fabricated from the as-received plate of Type 304 SS (AR specimen). The surface observation revealed that SCC initiated on the AR specimen surface in typical intergranular (IG) mode, while SCC on the DT specimen occurred in transgrannular mode as well as IG mode. It was also found that the size and number of SCC on the DT specimen were larger than that on the AR specimen. This was attributable to a strain-hardening during the bending process. To compare the susceptibility of SCC initiation, total crack density (TCD) was calculated from the total crack length divided by the measured area of AR and DT specimens. TCD of DT specimen was 4.6 times higher than AR specimen in average, indicating that higher possibility of degradation of bent parts of RVI for a long-term operation.

• 한국주조공학회지
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• 제9권1호
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• pp.58-66
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• 1989

The effect of heat treating temperature and ${\alpha}$ phase In the ${\beta}$ phase matrix were investigated for ${\beta}-CuZnAl$ shape memory alloys by tension test, fatigue test, and shape memory effect test. After heat treatment at $677^{\circ}C$, $750^{\circ}C$, $800^{\circ}C$ and $850^{\circ}C$ for 10 min. respectively, static fracture stress(${\sigma}_f$), fatigue fracture stress(${\tau}_{max}$) at $10^6$ cycle, and elongation(${\epsilon}_f$) was $24.2kg/mm^2$, $17.21kg/mm^2$ and 11.8%, respectively. As heat treating temperature decreased, fracture surfaces of the specimens were changed from the intergranular to the transgranular fracture mode. Especially, the a phase precipitated in the ${\beta}$ phase matrix had an effect on crack propagation and the fracture surface was characterized by dimple-like pattern when crack propagated in transgranular cracking mode. Precipitation of the ${\alpha}$ phase in the ${\beta}$ phase matrix lowered the transformation temperature by $10^{\circ}C$, and about 2.5 vol.% ${\alpha}$ phase did not affect the shape memory effect examined by the bending test.

• 한국산업융합학회 논문집
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• 제23권1호
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• pp.73-81
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• 2020

This study analyzed the causes of failure of SCM435 bolts that fix the springs of automobile air brakes that have been fractured during use. The cause of failure was analyzed using SEM, EDS, metallogical microscope and Vickers hardness tester. In the fracture, the ratchet mark began at the outer boundary of crack origin, and the grains at crack origin were found to have clear intergranular corrosion. One SCM435 bolt was subjected to a stress of 398 MPa, it's a stress of about 80% of the fatigue limit. As a result of such a large applied stress, cracks occurred at the corrosion origin and were fractured. In order to prevent the SCM435 bolt from fracture, it is necessary to use the correct composition, the accuracy of heat treatment, preventing damage by external impact, preventing corrosion of the damage part by moisture, and introduction a compressive residual stress by peening.

• Journal of Welding and Joining
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• 제3권2호
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• pp.16-26
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• 1985

Post weld heat treatment(PWHT), at more than 600.deg. C, is essential to remove residual stress and hydrogen in weld HAZ and improve fatigue characteristics. However, residual stress during PWHT is responsible for PWHT embitterment and it promotes precipitation of impurities to grain boundary. In this paper, the effect of stress simulated residual stress on fatigue failure was evaluated by fatigue test, microhardness test and fractograph. The obtained results are summarized as follows; (1) The fatigue crack growth rate(da/dN) of parent and heat treated parent was affected by microstructure due to heat treatment and it depended on stress intensity factor (.DELTA.k). (2) The fatigue strength of weld HAZ was dependent on applied stress during PWHT and da/dN after PWHT was slower than as-weld. (3) Softening amount of weld HAZ was bigger than any other due to PWHT. Hardness value of weld HAZ was affected by heat treatment under the applied stress of 10 $kgf/mm^2$, but beyond 20 $kgf/mm^2$ it was increased by the applied stress rather than heat treatment. (4) Beyond the applied stress of 20 $kgf/mm^2$ during PWHT, intergranular fracture surface was observed and its amount was increased with applied stress during PWHT. (5) Effect of applied stress during PWHT on aspect of fracture surface was larger rather than that on fatigue crack growth behavior.

• 대한금속재료학회지
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• 제49권1호
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• pp.1-8
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• 2011

To measure the sensitivity of the hydrogen embrittlement from plated high strength bolts (SWCH18A, F11T), the bolt was stressed by a corresponding to the amount of tensile strain of 5% and 20%, and then the bolt developed a crack on the surface. The bolt that didn't have concentrated hydrogen, even though it was stressed by a corresponding the amount of tensile strain of 5% and 20%, no crack developed. However, the bolt that had concentrated hydrogen, developed cracks from the thread crest to the root. It is impossible to measure the hydrogen amount from plated high strength bolts using Hydrogen-Determinator, because of the limitation of the minimum sample size (about 1g as a mass or $5{\times}5{\times}5 mm^3$ as a volume). Therefore, the sensitivity of hydrogen embrittlement can be measured by observing the presence of cracks on the surface of plated high strength bolts which are stressed by a corresponding to the amount of tensile strain of 5% and 20%.

• Corrosion Science and Technology
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• 제23권4호
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• pp.266-277
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• 2024

Due to its good corrosion and heat resistance with excellent mechanical properties, 304L stainless steel is commonly used in the fabrication of spent nuclear fuel dry storage canisters. However, welds are sensitive to stress corrosion cracking (SCC) due to residual stress generation. Although SCC resistance can be improved by stress relieving the weld and changing the chloride environment, it is difficult to change corrosion environment for certain applications. Stress control in the weld can improve SCC resistance. Ultrasonic shot peening (USP) needs further research as compressive residual stresses and microstructure changes due to plastic deformation may play a role in improving SCC resistance. In this study, 304L stainless steel was welded to generate residual stresses and exposed to a chloride environment after USP treatment to improve SCC properties. Effects of USP on SCC resistance and crack growth of specimens with compressive residual stresses generated more than 1 mm from the surface were studied. In addition, correlations of compressive residual stress, grain size, intergranular corrosion properties, and pitting potential with crack propagation rate were determined and the improvement of SCC properties by USP was analyzed.

• 한국재료학회지
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• 제14권1호
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• pp.35-40
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• 2004

In this study, dissolution characteristics of four types of commercial calcium phosphate ceramics were investigated in distilled water with respect to chemical composition and microstructure. For all samples, no significant damage was observed after 3 days of immersion. Following the 7 days of immersion, surface dissolution of the ceramics containing a crystalline phase susceptible to water such as TCP, even pure hydroxyapatite, was initiated at grain boundaries and the dissolution was extended interior to the material along the grain boundaries. In the considerably dissolved area, there was grain separation followed by the formation of 20 $\mu\textrm{m}$ of cavities. In at least one case, the residual pores on the surface appeared to initiate dissolution. In a dissolved area, a crack during the fracture propagates along the grain boundaries resulting in intergranular fracture, while transgranular fracture occurs in a dense area without significant dissolution.

• Corrosion Science and Technology
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• 제7권3호
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• pp.187-191
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• 2008

Transgranular stress corrosion cracking of nickel base alloys was reported by Copson and Dean in 1965. Study to establish this cracking mechanism needs to be carried out. Laboratory stress corrosion tests were performed for mill annealed(MA) or thermally treated(TT) steam generator tubing materials in a high temperature water containing lead. An electrochemical interaction of lead with the alloying elements of SG tubings was also investigated. Alloy 690 TT showed a transgranular stress corrosion cracking in a 40% NaOH solution with 5000 ppm of lead, while intergranular stress corrosion racking was observed in a 10% NaOH solution with 100 ppm lead. Lead seems to enhance the disruption of passive film and anodic dissolution of alloy 600 and alloy 690. Crack tip blunting at grain boundary carbides plays a role for the transgranular stress corrosion cracking.

• 한국기계연구소 소보
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• 통권19호
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• pp.43-52
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• 1989

Material damage of Unifiner Change Heater: Tube used for nearly 20 years was evaluated and Mechanical tests such as tensile tests and creep-rupture tests were conducted to predict the residual life. After the investigation, any major damage or degradation was not found except the welded zone. Microstructural observation showed that most of delta-ferrite was transformed. to sigma-phase and consequently, the ductility was very much reduced. A KLA(Knife-Line Attack) crack with 60mm in length and 2.8mm in depth was found just near the welded zone, which is believed to be caused by intergranular corrosion. Creep-rupture tests, which are very essential to predict the residual life, showed that both used base and weld metals have similar results with the reference data.

• 한국세라믹학회지
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• 제35권6호
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• pp.551-558
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• 1998

Detection of microcrack in {{{{ {Al }_{2 } {O }_{3 } }} ceramics were studided by AE(acoustic emission) technique with 4-point bending test in order to evaluate the fracture process and formation of microcrack. Fully-dense alu-mina ceramics having a different grain size were fabricated by varing the hot-pressing temperature. The grain size of alumina increased with increasing the hot-pressing temperature whereas the bending strength decreasd. The microcracks were observed by SEM and TEM. The generation of AE event increased with increasing the applied load and many AE event was generated at maximum applied load. Alumina with smaller grain size shows the generation of many AE event resulting in an increase of microcrack formation. An intergranular fracture is predominantly observed in fine-grained alumina whereas intragranular fracture occurs predominantly in coarse-grained alumina,. Analysis of micorstructure and AE prove that primary mi-crocracks occur within grain-boundaries of alumina. The larger microcracking were formed by the growth and/or coalesence of primary microcracks. Then the materials become to fracuture by main crack gen-eration at the maximum applied load.