• 한국정밀공학회지
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• 제20권5호
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• pp.140-147
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• 2003

As the huge energy transfer systems like as nuclear power plant and steam power plant are operated for a long time at a high temperature, mechanical properties are changed and ductile-brittle transition temperature is raised by degradation. So it is required to estimate degradation in order to assess the safety, remaining life and further operation parameters. The sub-sized specimen test method using surveillance specimen was developed for evaluating the integrity of metallic components. In this study, we would like to present the evaluation technique of the ductile-brittle transition temperature by the sub-sized specimen test. The four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method. The tensile test and fracture toughness test were performed. The results of the fracture toughness tests using the sub-sized specimens were compared with the evaluation technique of the ductile-brittle transition temperature.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.579-584
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• 2001

As the huge energy transfer systems like as nuclear power plant and steam power plant are operated for a long time, mechanical properties are changed and ductile-brittle transition temperature is raised by degradation. So it is required to estimate degradation in order to assess the safety, remaining life, and further operation parameters. The sub-sized specimen test method using surveillance specimen was developed for evaluating the integrity of metallic components. In this study, we would like to present the evaluation technique of the ductile-brittle transition temperature by the sub-sired specimen test. The four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method. The tensile test and fracture toughness test were performed. The results of the fracture toughness tests using the sub-sized specimens were compared with the evaluation technique of the ductile-brittle transition temperature.

• 한국재료학회지
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• 제24권1호
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• pp.13-18
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• 2014

In order to clarify the effect of Nb addition on the ductile-brittle transition property of sintered TiC, TiC-10 mol% Nb composites were researched using a three-point bending test at temperatures from room temperature to 2020 K, and the fracture surface was observed by scanning electron microscopy. It was found that the Nb addition decreases the ductile-brittle transition temperature of sintered TiC by 300 K and increases the ductility. The room temperature bending strength was maintained at up to 1800 K, but drastically dropped at higher temperatures in pure TiC. The strength increased moderately to a value of 320MPa at 1600 K in TiC-10 mol% Nb composites, which is 40% of the room temperature strength. Pores were observed in both the grains and the grain boundaries. It can be seen that, as Nb was added, the size of the grain decreased. The ductile-brittle transition temperature in TiC-10 mol% Nb composites was determined to be 1550 K. Above 1970 K, yieldpoint behavior was observed. When the grain boundary and cleavage strengths exceed the yield strength, plastic deformation is observed at about the same stress level in bending as in compression. The effect of Nb addition is discussed from the viewpoint of ability for plastic deformation.

• 한국재료학회지
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• 제25권10호
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• pp.559-565
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• 2015

The ductile-brittle transition behavior of two austenitic Fe-18Cr-10Mn-N-C alloys with different grain sizes was investigated in this study. The alloys exhibited a ductile-brittle transition behavior because of an unusual brittle fracture at low temperatures unlike conventional austenitic alloys. The alloy specimens with a smaller grain size had a higher yield and tensile strengths than those with a larger grain size due to grain refinement strengthening. However, a decrease in the grain size deteriorated the low-temperature toughness by increasing the ductile-brittle transition temperature because nitrogen or carbon could enhance the effectiveness of the grain boundaries to overcome the thermal energy. It could be explained by the temperature dependence of the yield stress based on low-temperature tensile tests. In order to improve both the strength and toughness of austenitic Fe-Cr-Mn-N-C alloys with different chemical compositions and grain sizes, more systematic studies are required to understand the effect of the grain size on the mechanical properties in relation to the temperature sensitivity of yield and fracture stresses.

• 한국재료학회지
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• 제25권8호
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• pp.417-422
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• 2015

In this study, low-carbon hypoeutectoid steels with different ferrite-pearlite microstructures were fabricated by varying transformation temperature. The microstructural factors such as pearlite fraction and interlamellar spacing, and cementite thickness were quantitatively measured and then Charpy impact tests conducted on the specimens in order to investigate the correlation of the microstructural factors with impact toughness and ductile-brittle transition temperature. The microstructural analysis results showed that the pearlite interlamellar spacing and cementite thickness decreases while the pearlite fraction increases as the transformation temperature decreases. Although the specimens with higher pearlite fractions have low absorbed energy, on the other hand, the absorbed energy is higher in room temperature than in low temperature. The upper-shelf energy slightly increases with decreasing the pearlite interlamellar spacing. However, the ductile-brittle transition temperature is hardly affected by the pearlite interlamellar spacing because there is an optimum interlamellar spacing dependent on lamellar ferrite and cementite thickness and because the increase in pearlite fraction and the decrease in interlamellar spacing with decreasing transformation temperature have a contradictory role on absorbed energy.

• 한국재료학회지
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• 제23권7호
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• pp.385-391
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• 2013

The influence of Cu and Ni on the ductile-brittle transition behavior of metastable austenitic Fe-18Cr-10Mn-N alloys with N contents below 0.5 wt.% was investigated in terms of austenite stability and microstructure. All the metastable austenitic Fe-18Cr-10Mn-N alloys exhibited a ductile-brittle transition behavior by unusual low-temperature brittle fracture, irrespective of Cu and/or Ni addition, and deformation-induced martensitic transformation occasionally occurred during Charpy impact testing at lower temperatures due to reduced austenite stability resulting from insufficient N content. The formation of deformation-induced martensite substantially increased the ductile-brittle transition temperature(DBTT) by deteriorating low-temperature toughness because the martensite was more brittle than the parent austenite phase beyond the energy absorbed during transformation, and its volume fraction was too small. On the other hand, the Cu addition to the metastable austenitic Fe-18Cr-10Mn-N alloy increased DBTT because the presence of ${\delta}$-ferrite had a negative effect on low-temperature toughness. However, the combined addition of Cu and Ni to the metastable austenitic Fe-18Cr-10Mn-N alloy decreased DBTT, compared to the sole addtion of Ni or Cu. This could be explained by the fact that the combined addition of Cu and Ni largely enhanced austenite stability, and suppressed the formation of deformation-induced martensite and ${\delta}$-ferrite in conjunction with the beneficial effect of Cu which may increase stacking fault energy, so that it allows cross-slip to occur and thus reduces the planarity of the deformation mechanism.

• 한국재료학회지
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• 제26권11호
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• pp.590-597
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• 2016

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

• 한국재료학회지
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• 제23권11호
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• pp.649-654
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• 2013

The effect of interstitial elements on the ductile-brittle transition behavior of austenitic Fe-18Cr-10Mn-2Ni alloys with different nitrogen and carbon contents was investigated in this study. All the alloys exhibited ductile-brittle transition behavior because of unusual low-temperature brittle fracture, even though they have a faced-centered cubic structure. With the same interstitial content, the combined addition of nitrogen and carbon, compared to the sole addition of nitrogen, improved the low-temperature toughness and thus decreased the ductile-brittle transition temperature (DBTT) because this combined addition effectively enhances the metallic component of the interatomic bonds and is accompanied by good plasticity and toughness due to the increased free electron concentration. The increase in carbon content or of the carbon-to-nitrogen ratio, however, could increase the DBTT since either of these causes the occurrence of intergranular fracture that lead to the deterioration of the toughness at low temperatures. The secondary ion mass spectroscopy analysis results for the observation of carbon and nitrogen distributions confirms that the carbon and nitrogen atoms were significantly segregated to the austenite grain boundaries and then caused grain boundary embrittlement. In order to successfully develop austenitic Fe-Cr-Mn alloys for low-temperature application, therefore, more systematic study is required to determine the optimum content and ratio of carbon and nitrogen in terms of free electron concentration and grain boundary embrittlement.

• 한국가스학회지
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• 제19권6호
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• pp.62-66
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• 2015

LNG선이나 해양플랜트 같은 설비나 구조물은 작동 조건을 고려할 때, 특히 부재마다 가지는 고유한 연성 취성천이온도(Ductile to Brittle Transition Temperature, DBTT)를 고려하여 설계되어야 한다. 본 연구에서는 해양플랜트 및 LNG선에 hull plate로 쓰이는 A-grade 연강(mild steel)에 대해 DBTT를 샤르피 V-노치(CVN) 실험을 통해 알아보았고 파괴형상을 통한 파괴거동을 살펴보았다. 그 결과 온도가 감소함에 따라 충격 흡수에너지는 감소함을 보였다. Upper shelf energy region과 lower shelf energy region이 나타나고 그 사이 구간의 천이점을 통해 DBTT가 결정되었다. 파괴형상에서는 upper shelf energy region에서 수많은 딤플이 연성파괴 형상으로 관찰되고 lower shelf energy region에서는 전형적인 취성파단형상이 관찰되었다. 이를 통해 BCC 구조를 가지는 A-grade 연강은 upper shelf energy 구역과 lower shelf energy구역을 보이고 그 사이 구간의 천이점에서 급격하게 온도가 떨어지는 DBTT구간을 뚜렷하게 보이는 특성을 알 수 있었다.

• 열처리공학회지
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• 제28권1호
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• pp.1-6
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• 2015

The ductile-to-brittle transition behavior of two austenitic Fe-18Cr-10Mn alloys with the combined addition of nitrogen and carbon was investigated in this study. The alloys exhibited a ductile-to-brittle transition behavior because of unusual brittle fracture at low temperatures unlike conventional austenitic alloys. The alloy with higher carbon content had higher yield and tensile strengths than that with lower carbon content due to the solid solution strengthening effect resulting from carbon addition. However, the increase in carbon content promoted the occurrence of intergranular fracture, and thus deteriorated the impact toughness. In order to develop successfully the austenitic Fe-18Cr-10Mn alloys with the excellent combination of strength and toughness in the future, therefore, more systematic studies are required to find the appropriate amount and ratio of nitrogen and carbon.