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발전기용 오스테나이트계 18Mn-18Cr 고질소강의 제조와 인장강도 예측
황병철,이태호,Hwang, Byoungchul,Lee, Tae-Ho 한국재료학회 2013 한국재료학회지 Vol.23 No.9
Over the past few decades, high-nitrogen austenitic steels have steadily received greater attention since they provide a unique combination of high strength and ductility, good corrosion resistance, and non-magnetic properties. Recently, highnitrogen 18Mn-18Cr austenitic steels with enhanced strength have been developed and widely used for generator retaining rings in order to prevent the copper wiring from being displaced by the centrifugal forces occurring during high-speed rotation. The high-nitrogen austenitic steels for generator retaining ring should be expanded at room temperature and then stress relief annealed at around $400^{\circ}C$ to achieve the required mechanical properties. In this study, four kinds of high-nitrogen 18Mn-18Cr austenitic steels with different nitrogen content were fabricated by using a pressurized vacuum induction melting furnace, and then the effects of nitrogen content, cold working, and stress relieving on tensile properties were investigated. The yield and tensile strengths increased proportionally with increasing nitrogen content and cold working, and they further increased after stress relieving treatment. Based on these results, a semi-empirical equation was proposed to predict the tensile strength of highnitrogen 18Mn-18Cr austenitic steels for generator retaining rings. It will be a useful for the effective fabrication of high-nitrogen 18Mn-18Cr austenitic steels for generator retaining rings with the required tensile properties.
황병철,Hwang, Byoungchul Materials Research Society of Korea 2013 한국재료학회지 Vol.23 No.11
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.
대기 플라즈마 용사법으로 제조된 철계 혼합 코팅층의 마모특성
黃昞哲,安志勳,李聖鶴,吳重錫 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.3
The objective of this study is to investigate the correlation between microstructure and the wear resistance of various ferrous blend coatings applicable to cylinder bores. Seven types of ferrous spray powders, two of which were stainless steel powders and the others were blend powders of ferrous powders mixed with Al_2O_3-ZrO_2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that iron oxides were formed in the austenitic (or martensitic) matrix by oxidation during spraying, while Al_2O_3-ZrO_2 oxides were mainly formed in the matrix for the blend coatings. The wear test results revealed that the blend coatings showed the better wear resistance than the ferrous coatings because they contained a number of hard Al_2O_3-ZrO_2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during wear. The STS 316 coating showed the slightly worse wear resistance than the blend coatings, but they showed the excellent wear resistance to a counterpart material resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides.
황병철,김양곤,이성학,김낙준,유장용 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.4
This study is concerned with the effects of microstructure on inverse fracture in the hammer-impacted region during drop-weight tear test(DWTT) of three high-toughness API X70 pipeline steels. Pressed notch or Chevron notch DWTT and Charpy V-notch impact test were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from tensile and Charpy impact tests of pre-strained specimens. In the hammer-impacted region of the DWTT specimens, cleavage-type inverse fracture appeared, and its fracture area was correlated well with upper shelf energy (USE) obtained from the Charpy impact test and with microstructure. The steel specimens having higher USE or having coarse polygonal ferrite had the larger inverse fracture area than those having lower USE or having fine acicular ferrite because of the larger compressive pre-strain induced by work hardening of the hammer-impacted region. These results were confirmed by relating to the Charpy impact test data of pre-strained steel specimens. (Received January 19, 2004)
황병철,이성학,김영민,김낙준,유장용 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.3
This study is concerned with the effects of microstructure on fracture properties of a high-toughness API X70 pipeline steel. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot-rolling, and their microstructures were varied by the rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below -100℃ when the finish cooling temperature range was 350~500℃, implying excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture area occurred at some finish cooling temperatures.