SA508 Gr.4N Ni-Cr-Mo계 저합금강의 이상영역열처리에 따른 미세조직 및 기계적 특성 평가

이초롱 과학기술연합대학원대학교 2017 국내석사

Reactor pressure vessel steel is a material that operates for a long time at high temperature and high pressure environment of about 300℃. The next-generation nuclear power plants are being developed with materials that have been operating for a long period of time with a large-sized and extended operating life for high efficiency. Excellent mechanical properties are required to apply existing commercial materials to next generation nuclear power plant. The content of Ni and Cr increases from the commercial reactor pressure vessel SA508 Gr.3 to improve the hardenability. At present, the research for the application of SA508 Gr.4N is progressing steadily. However, it was reported that temper embrittlement phenomena occurred in low alloy steels having chemical components similar to SA 508 Gr.4N when exposed to high temperatures for a long time. There is a difference between the temperature at which temper embrittlement occurs and the actual operating temperature of the reactor pressure vessel, but in order to improve the safety of nuclear power plant, it can occur during long-term operation verification of embrittlement properties is necessary. In this study, the temper embrittlement behavior of SA508 Gr.4N Ni-Cr-Mo low alloy steel was evaluated. The mechanical properties and the microstructural change due to intercritical heat treatment were compareed and evaluated and the influence on temper embrittlement was analyzed. A reference model alloy with the typical composition of SA508 Gr.4N steel. In order to maximize the temper embrittlement effect of the reference model alloy in the ASME specified composition and to compare it, the content of impurity P was increased from the standard and analyzed using two kinds of model alloys. It was confirmed that the grains of both model alloys which the intercritical heat treatment was applied were fine. There was little change in the impact characteristics applied to intercritical heat treatment. After aging heat treatment, the change of transition temperature was less than that before the application of the intercritical heat treatment. It was found that the temper embrittlement resistance was improved. These fracture mode changes are considered due to differences in impurity segregation behavior. As a result of the EBSD analysis, it was found that the impurities were segregated mainly at the random grain boundaries like the prior austenite grain boundaries and were not segregated at the CSL grain boundaries. So fine crystal grains and increased grain boundaries with the intercritical heat treatment disperse the segregation of impurities and suppress the temper embrittlement behavior. In fact, the impurity content in the intergranular fraction surface also decreased slightly when applying the intercritical heat treatment. The temper embrittlement resistance increased in the application of intercritical heat treatment of SA508 Gr.4N low alloy steel is attributed to dispersed segregation due to an increase in intergranular fraction surface where impurities can segregate by grain refinement. Therefore by applying intercritical heat treatment to SA508 Gr.4N low alloy steel, it is possible to suppress the temper embrittlement phenomenon that can occur during the long-term operation of the primary system on the nuclear power plant, it will contribute greatly to securing. 원자로 압력용기강은 약 300℃의 고온 및 고압의 환경에서 장시간 가동을 하는 소재이다. 차세대 신형원전은 고효율을 위해 대형화 및 가동수명이 연장되면서 장기간 가동을 대비한 소재로 개발되고 있다. 기존 상용소재를 차세대 신형원전에 적용하기 위해서는 우수한 기계적 특성이 요구된다. 현재 상용 원자로 압력용기강인 SA508 Gr.3보다 Ni과 Cr의 함량이 증가되어 경화능이 향상된 SA508 Gr.4N의 적용을 위한 연구가 꾸준히 진행되고 있다. 그러나 SA508 Gr.4N과 유사한 화학성분을 갖는 저합금강에서 고온에 장시간 노출될 경우 템퍼취화 현상이 발생하는 것으로 보고되었다. 템퍼취화가 발생하는 것으로 알려진 온도와 실제 원자로 압력용기의 가동 온도와는 차이가 있으나, 원전의 안전성을 높이기 위해서는 장기가동 시 발생할 수 있는 취화특성에 대한 검증이 필요하다. 본 연구에서는 SA508 Gr.4N Ni-Cr-Mo계 저합금강의 템퍼취화 거동을 평가하였다. 이상영역열처리에 따른 미세조직 변화와 기계적 특성을 비교 평가하여 템퍼취화 저항성에 미치는 효과에 대해 분석하였다. ASME 규격 내 기준 모델합금과 템퍼취화 효과를 극대화하여 비교하기 위해 기준보다 불순물 P 함량을 증가시킨 두 종류의 모델합금을 이용하여 분석을 수행하였다. 이상영역열처리를 적용한 두 모델합금 모두 결정립이 미세해지는 것을 확인하였다. 이상영역열처리 적용에 따른 충격천이특성의 변화는 거의 없었으나, 취화열처리 이후 천이온도의 변화량은 이상영역열처리 적용 전보다 감소하여 템퍼취화에 대한 저항성이 향상되는 것으로 나타났다. 충격시험 파단면에서도 입계파괴모드가 다소 감소하고 입내파괴모드가 증가하였다. 이러한 파단모드의 변화는 불순물의 편석거동의 차이에 기인한 것으로 판단된다. EBSD 분석 결과 불순물은 주로 구 오스테나이트 결정립계와 같은 random 입계에 편석되며, 결정립 내부의 CSL 입계에는 편석되지 않는 것으로 나타났다. 따라서 이상영역열처리로 미세해진 결정립과 증가한 결정립계는 불순물의 편석을 분산시켜 템퍼취화 거동을 억제하는 것으로 판단된다. 실제 입계파면에서의 불순물 함량도 이상영역열처리를 적용한 경우에서 약간 감소하였다. SA508 Gr.4N 저합금강의 이상영역열처리 적용으로 증가한 템퍼취화 저항성은 결정립 미세화에 의해 불순물이 편석할 수 있는 입계면이 증가하여 편석이 분산되는 것에 기인한다. 따라서 SA508 Gr.4N 저합금강에 이상영역열처리를 적용함으로써 원전 1차측 주요 기기의 장기간 가동시 발생 할 수 있는 템퍼취화 현상을 억제하여 원자로용기의 안전성 확보에 크게 기여할 수 있을 것이다.

二相領域處理한 中炭素 2Si-3Ni 複合組織鋼의 破壞擧動에 미치는 合金元素의 影響

최창기 國民大學校 1991 국내석사

二相領域 熱處理에 의한 Mn-Mo-Ni 低合金鋼의 相變態 및 材質改善 硏究

안연상 충남대학교 2001 국내박사

Mn-Mo-Ni low alloy steel has been applied to the nuclear reactor pressure vessel (RPV) for several decades. Since the RPV is a very important component which greatly affects the safety of nuclear power plant, the materials used for RPV must have excellent mechanical properties to maintain the integrity of the vessel during its life time. It has been well known that the fast neutron irradiation gives rise to the decrease in the toughness and to the shift of ductile-to-brittle transition temperature (DBTT) to a higher temperature. To compensate for the material degradation due to neutron irradiation, the material of high initial toughness and low initial DBTT should be adopted for RPV. Although the toughness and strength of recent RPV steel are sufficient for safe operation during the current design lifetime, efforts to improve the mechanical properties are necessary to extend the lifetime of power plants as well as to obtain sufficient operating margins. In many embrittlement models for RPV materials, the shift of DBTT is described by the function of fast neutron fluence and chemical composition. This means that the modification of microstructure without compositional change may not affect the embrittlement behavior significantly. Therefore, this work has been performed with the intention to improve the fracture toughness of RPV steel by the modification of heat treatment process to change the microstructure. In this research, the new heat treatment processes for manufacturing high toughness RPV steel have been developed by application of intercritical heat treatment (IHT). Application of IHT at 710 to 740 ℃ for 4 to 8 hours between quenching and tempering of the conventional heat treatment increased the room temperature impact energy significantly. IHT temperature of 725℃ and holding time of 6 hours was determined as an optimum IHT condition. The application of the optimum IHT resulted in the increase of the upper shelf energy and ductility and in the decrease of transition temperature and strength. The modifications of the tempering conditions, from 660℃/10 hours to 640℃/6 hours or to 620℃/6 hours, reduced the loss of strength resulting from the IHT and maximize the improvement of toughness. The large beneficial effects from the IHT were consistantly matained in spite of the change of heating and cooling rates. Additionally, the cause of the increase in toughness was investigated in relation to the microstructural change. The IHT produces a composite structure of hard tempered martensite and soft double-tempered bainite. More sub-grain boundaries are contained in the composite structure. Furthermore, most coarse and long carbides become spheroidized and the size of them also becomes smaller by the IHT. High toughness is obtained when the microstructure contains 20 to 60 % tempered martensite. These changes in the microstructural characteristics due to the IHT are the important causes for high toughness because they retard the void initiation and crack propagation.

이상영역 어닐링 처리한 미량합금첨가 Fe-C-1.5Si 강의 베이나이트 형성특성에 관한 연구

하성룡 서울大學校 大學院 1989 국내석사