소형펀치시험에 의한 연성-취성천이온도 평가 및 국부파괴강도 해석에 의한 표준시험법과 연계성 분석

오용준(Yong Jun Oh),최병찬(Byung Chan Choi),홍준화(Jun Hwa Hong) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.3

Ductile-brittle transition temperatures for the reactor pressure vessel(RPV) steels were determined using a small punch(SP) test and compared to the results from the Charpy impact test and fracture toughness test using a pre-cracked Charpy V-notched(PCVN) specimen in accordance with ASTM E 1921. The correlation between the transition temperatures from the SP test and standard specimen tests was investigated on the theoretical bases regarding the local criteria for cleavage fracture for different specimen geometries and loading conditions. SP test was performed for the seven conventional RPV steels in the temperature range of RT to -196℃. The samples were taken from 1/4T location of the vessels and machined into 10×10×0.5㎜ dimension. SP transition temperature (T<SUB>sp</SUB>) determined at the middle of upper and lower SP energies showed a good correlation with Charpy index temperature, T<SUB>41J</SUB>ㆍFurthermore, it was found that the T<SUB>sp</SUB> had a meaningful correlation with T<SUB>o</SUB> value from the PCVN test. The finite element(FE) calculation for the SP and PCVN test specimens gave a clue to the correlation. Fracture in SP specimen occurred at the highest stress and strain location and the local fracture stress at the cleavage initiation location for SP specimens showed a linear relationship with that for PCVN specimens in the transition region.

Evaluation of Fracture Toughness Degradation by Neutron Irradiation in RPV Steels Using Small Punch Test

오용준(Yong Jun Oh),이봉상(Bong Sang Lee),홍준화(Jun Hwa Hong) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8

A small punch (SP) test was performed to evaluate the ductile-brittle transition temperature shift by neutron irradiation in reactor pressure vessel (RPV) steels. The results were compared to the standard transition temperature shifts from Charpy test and Master Curve fracture toughness test in accordance with the ASTM standard E1921. The samples were taken from 1/4T location of the vessel thickness and machined into 10×10×0.5mm dimension. Irradiation of the steels was carried out in the research reactor at KAERI (HANARO) at about 290℃ to the different fluence levels respectively. SP tests were performed in the temperature range of RT to -196℃ using a 2.4mm diameter ball. For the materials before and after irradiation, SP transition temperatures (T<SUB>sp</SUB>), which are determined at the middle of upper and lower SP energies, showed a linear correlation with the Charpy index temperature, T<SUB>41J</SUB>. But, the transition temperature shift from the Charpy test (ΔT<SUB>41J</SUB>) shows better correlation with the transition temperature shift (ΔT<SUB>sp(E)</SUB>) at a specific SP energy level (E) than that (ΔT<SUB>sp</SUB>) determined at the middle energy of SP curve. The T<SUB>sp(E)</SUB> had a meaningful correlation with the reference temperature (T<SUB>o</SUB>) from the master curve method using a pre-cracked Charpy V-notched (PCVN) specimen. From the microscopic approach to the fracture mechanism in the SP and standard PCVN specimens, the authors found a linear correlation between local fracture stresses in the two different specimens. This provides the feasibility for applying the SP technique to predicting T<SUB>o</SUB> value from the standard test.

Type 347 스테인레스강 용접부의 미세조직 및 파괴기구

오용준,이봉상,류우석,국일현,홍준화 ( Yong Jun Oh,Bong Sang Lee,Woo Seog Ryu,Il Hiun Kuk,Jun Hwa Hong ) 대한금속재료학회 ( 구 대한금속학회 ) 1997 재료강도 심포지엄 Vol.- No.-

논문 : 부식환경 ; 질소첨가 Type 316L 스테인리스강의 입계석출 및 예민화특성에 미치는 냉간가공의 영향

서무홍 ( Moo Hong Seo ),오용준 ( Yong Jun Oh ),유우석 ( Woo Seog Ryu ),천병선 ( Byong Sun Chun ),홍준화 ( Jun Hwa Hong ) 대한금속재료학회 ( 구 대한금속학회 ) 1998 대한금속·재료학회지 Vol.36 No.6

The precipitaion and sensitization behavior of nitrogen added Type 316L stainless steels(SS) were investigated by using specimens cold worked for 0~40%. The alloys had a variation in nitrogen content from 0.04 to 0.15%. To quantify the degree of sensitization double-loop electrochemical potentiokinetic reactivation (DL-EPR test was performed in a 0.1M H2SO4 + 0.01M KSCN solution at 30℃. The addition of nitrogen increased sensitization resistance by retarding the onset of M23C6 precipitation and shifted Time-Temperature-Sensitization (TTS) curve to higher temperature and longer time range. Cold work accelerated the M23C6, precipitation and sensitization kinetic due to the increase in dislocation density. However, the acceleration of sensitization was found to depend on the added nitrogen content in the alloys. The alloys with high nitrogen(>0.1%N) content exhibited higher accelation of the sensitization as a function of the cold work than that with low nitrogen content. From the microstructural analysis, this was found to he attributed to the development of intensive slip bands during cold work and retardation of dislocation annihilation during subsequent aging in the alloys with high nitrogen content.

Type 316L 스테인리스가의 입계 석출 및 예민화에 미치는 질소의 영향

홍준화,류우석,오용준 대한금속재료학회(대한금속학회) 1997 대한금속·재료학회지 Vol.35 No.8

Nitrogen effect on sensitization behavior of type 316L stainless steel was investigated in view of thermal precipitation. The investigated alloys had a variation in carbon and nitrogen contents. The sensitization resistance was improved with decreasing of carbon content and increasing of nitrogen content. From a microstructural observation on the sensitized alloys, it was found that the addition of nitrogen up to 0.15% increased sensitization resistance through suppressing the initiation and growth of M_(23)C_6 type carbide. M_6C type carbide was only formed at the alloy with medium nitrogen content. Besides carbides, Mo-rich precipitates such as intermetallic η and χ phases, and five-fold symmetry phase were observed at the grain boundary. These phases were more popular in the medium nitrogen alloy than in low nitrogen alloy at the same aging condition. From a TEM-EDS(energy despersive spectroscope) analysis, it was found that these Mo-rich phases had not a significant deterimental effect on the sensitization resistance.

소형펀치시험에 의한 원자로압력용기용 저합금강의 강도 및 연성-취성 천이온도 평가

홍준화,김길무,오용준,최병찬 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.2

Small punch(SP) test was performed to evaluate tensile properties and ductile-brittle transition behavior of various reactor pressure vessel(RPV) steels with bainitic microstructure in the temperature range of R.T. to -196℃. The testing specimens were taken from 1/4t location of one weld and six forged RPV steels manufactured by the different processes and machined to a dimension of 10×10×0.5 ㎜. Yield strength and maximum strength from SP test were successfully correlated with yield strength in all temperature ranges and tensile strength in the upper shelf region of SP energy versus temperature curves. Weibull statistical analysis was a useful method to determine the mean value of SP energy in the transition region and the SP transition temperature even for the very limited number of SP specimens. Transition temperature determined at mid-point of upper and lower SP energy(T_(SP)) was successfully correlated to the index transition temperatures from standard Charpy impact test and T_o value from the precracked V-notched three-point bend fracture(PCVN) test. From the observation of the outer surface of the tested SP specimens, it was found that the SP energies were proportional to the lengths of the largest residual crack that did not lead to a fatal main crack. Small punch local fracture stress obtained by considering Griffith theory exhibited some meaningful agreement with local fracture stress obtained from the PCVN test.

SA516-Gr.70 탄소강의 파괴저항성에 미치는 동적변형시효의 영향

홍준화,이봉상,오용준,윤지현 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.6

The effects of dynamic strain aging (DSA) on the fracture resistance of ASME SA516-Gr.70 steels for nuclear piping were investigated through J-R tests conducted in the wide ranges of temperatures (RT-500℃) and loading rates (0.3-1000 ㎜/min). Tensile properties were also investigated in the temperature range of 100 to 316℃ over a range of strain rates (2.5×10^(-5)-1.33×10^0/s). The test results indicated that the SA516-Gr.70 steel was very susceptible to DSA. The minimum fracture resistance values were obtained in the temperature range between 200℃ and 400℃ for each given loading rate. The temperature for the minimum moved to higher temperature region as loading rates increase. The minimum was discussed in the viewpoint of DSA that is manifested by tensile tests. Furthermore. differences of DSA behaviors between two test materials of same specification were described and discussed with respect to the chemical composition.

2 상 영역 열처리 조건에 따른 Mn-Mo-Ni 저합금강의 미세조직과 기계적 성질의 변화

홍준화,김길무,오용준,안연상 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.10

The change of microstructures and mechanical properties in a Mn-Mo-Ni low alloy reactor pressure vessel steel with respect to intercritical heat treatment(IHT) conditions has been examined. The introduction of IHT after quenching step of commercial heat treatment process of the steel significantly improved the Charpy impact toughness and J-R fracture resistance. The IHT conditions showing improved toughness were the temperature range of 710-740℃ and the holding time of 4-8 hours. At the early stage of IHT, fine distribution of newly formed martensite (austenite in the intercritical temperature region) in tempered bainite matrix produced the fine microstructure with small rounded carbides after tempering, which resulted in the improved toughness. With the higher IHT temperature or longer time, however, the positive effect of IHT disappeared due to the coarsening of austenite. High toughness was obtained when the austenite volume fractions formed during heating to the intercritical temperature region were about 20-60%.

Mn-Mo-Ni 저합금강의 취성균열 전파에 미치는 입계 결정방위차의 영향

홍준화,오용준,김민철 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.5

The effect of orientation relationship between two adjacent grains on the brittle crack propagation was systematically investigated in the viewpoint of misorientation at the boundaries using Electron Back-Scatter Diffraction(EBSD) technique. Crystallographic orientation around the path of secondary crack under the fracture surface has been observed with fractured 3 point band specimen at -100℃. Use of EBSD technique allows the crystallographic cracking plane of the material to be inferred. For every crack observed, the trace of the plane was perpendicular to a $lt;001$gt; direction of the cracked packet. The concept of the geometric compatibility factor has been introduced in order to explain and predict the crack propagation between two adjacent grains. This factor can be calculated from the measuring the two angles from {001} pole figure. A possible cracking plane predicted from calculation of a geometric compatibility factor was coincident with those found in observation.

Mn-Mo-Ni 저합금강의 인성과 강도에 미치는 2 상영역 열처리와 템퍼링 조건의 영향

홍준화,김길무,오용준,안연상,김홍덕 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.3

The effect of intercritical heat treatment (IHT) and tempering conditions on impact toughness and strength in a Mn-Mo-Ni low alloy pressure vessel steel (SA508 Gr.3 steel) has been investigated. The optimum condition of the IHT for toughness improvement was developed. The application of IHT resulted in the increase of ductility and upper shelf energy and in the decrease of strength and ductile-to-brittle transition temperature (DBTT). The modification of tempering conditions reduced the loss of strength resulting from the IHT and maximized toughness improvement. The beneficial effects from the IHT were consistently maintained in spite of the changes 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 martensite and soft tempered bainite. Tempering following IHT results in spheroidization of carbides and finer effective grain size, which enhance resistance to the brittle fracture.