Fatigue life curves of alloy 617 in the temperature range of 800–950 °C

사인진,박재환,김응선 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.2

The cyclical behavior of Alloy 617 was examined at 25 C and high temperatures of 800, 850, 900, and 950 C in air to obtain its fatigue life curves. The specimens tested at 25, 800, and 850 C cyclically hardened, whereas those tested above 900 C cyclically softened from the first cycle, that is, their fatigue life was reduced at high temperatures owing to loss of strength. Parameters of the typical Coffin MansonBasquin relationship were determined for each test temperature. Interestingly, no significant difference in fatigue life was observed for the specimens tested in the range of 800e950 C. Owing to the similarity in fatigue life, we determined fatigue strength and fatigue ductility exponents that could be applied for this temperature range. The parameters obtained were close to the universal slopes, although the fatigue ductility exponent was slightly different. The proposed fatigue life curves were compared with those presented in ASME code

확산접합된 페라이트/마르텐사이트강의 미세조직 및 기계적 특성에 미치는 열처리 효과

사인진,김성환,홍성훈,장창희,Sah, Injin,Kim, Sunghwan,Hong, Sunghoon,Jang, Changheui 한국압력기기공학회 2015 한국압력기기공학회 논문집 Vol.11 No.1

As a measure of improving the mechanical properties of a diffusion bonded joint of a ferritic/martensitic steel (FMS), the post-bonding heat treatment (PBHT) is applied. In the temperature range of normalizing condition ($950-1,050^{\circ}C$), diffusion bonding is employed with compressive stress (6 MPa). Due to the martensite structure distributed in the matrix, Vicker's hardness values of the as-bonded are much higher than those of the as-received. Through the PBHT for 1 h at $720^{\circ}C$, hardness values are recovered to as low as those of the as-received condition. Also, tensile properties of PBHT are similar to those of the as-received at up to the test temperature of $550^{\circ}C$, when the diffusion bonding is carried out over $1,000^{\circ}C$. Based on the creep-rupture testing performed at $650^{\circ}C$ in air environment, the joint efficiency of the PBHTed specimens is about 80% in, which is higher than that of the as-bonded specimens.

Creep Behavior of Diffusion-Welded High-Temperature Alloy at 800℃

사인진,황종배,박재환,김응선 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5

A plate-type heat exchanger is considered as an intermediate heat exchanger in next-generation nuclear industry. Solid state diffusion welding is a key technology to achieve such high-temperature components with sufficient structural integrity. High-temperature mechanical properties (creep, fatigue, and creep-fatigue) on diffusion weldment have to be provided to regulatory body and/or designers. As a pioneering study, creep deformation and fracture behavior of a diffusion weldment were evaluated in this study. Stress-rupture test was conducted at 800 ℃ under various stress levels. Time-to-rupture, time-to-tertiary stage, and time-to-1% creep strain were measured using an extensometer physically attached to the gauge section of the specimens. Time-to-rupture of the diffusion weldment was comparable to that of the as-received alloy. It was revealed from a plot of log stress versus log time-to-rupture that the underlying creep deformation and fracture mechanism is dislocation glide and climb. All specimens were fractured away from the interface in the gauge section in a ductile manner. Fracture surface and cross section are analyzed with electron microscope.

Alloy 617 확산용접재의 고온 인장강도

사인진,황종배,김응선,Sah, Injin,Hwang, Jong-Bae,Kim, Eung-Seon 한국압력기기공학회 2018 한국압력기기공학회 논문집 Vol.14 No.1

A compact heat exchanger is one of critical components in a very high temperature gas-cooled reactor (VHTR). Alloy 617 (Ni-Cr-Co-Mo) is considered as one of leading candidates for this application due to its excellent thermal stability and strengths in anticipated operating conditions. On the basis of current ASME code requirements, sixty sheets of this alloy are prepared for diffusion welding, which is the key technology to have a reliable compact heat exchanger. Optical microscopic analysis show that there are no cracks, incomplete bond, and porosity at/near the interface of diffusion weldment, but Cr-rich carbides and Al-rich oxides are identified through high resolution electron microscopic analysis. In high-temperature tensile testing, superior yield strengths of the diffusion weldment compared to the code requirement are obtained up to 1223 K ($950^{\circ}C$). However, both tensile strength and ductility drop rapidly at higher temperature due to the insufficient grain boundary migration across the interface of diffusion weldment. Best fit curves for minimum yield strength and average tensile strength are drawn from the experimental tensile results of this study.

확산접합된 Ni기 합금 Hastelloy X에 미치는 후열처리 영향

사인진,황종배,홍순익,김응선,김민환 대한금속·재료학회 2017 대한금속·재료학회지 Vol.55 No.2

The effect of post-bond heat treatment (PBHT) on diffusion-bonded polycrystalline Ni-base alloy Hastelloy X (Ni-22Cr-18Fe-8Mo) was evaluated in this study. Diffusion bonding was conducted under a compressive pressure of 14 MPa in the temperature range of 1100-1200 ℃. While (Cr,Mo)-rich and (Mo,Si)-rich precipitates were developed along the interface at 1100 ℃, the precipitates were dissolved and grain boundary migration across the interface occurred above 1150 ℃. As the dissolution of precipitates is related to the temperature rather than compressive pressure, a modified diffusion bonding condition (1120 ℃/10 MPa) was applied. Brittle fracture with low tensile ductility was observed at the testing temperature of 700 ℃, but substantial recovery of ductility was achieved up to a testing temperature of 900 ℃ through PBHT (1180 ℃/1 h). The reduction of discontinuity at the interface was found to bring about the ductile behavior at high temperature.

Alloy 617 확산용접재의 저주기피로 특성

사인진(Injin Sah),박재환(Jaehwan Park),김응선(Eung-Seon Kim) 대한기계학회 2022 대한기계학회 춘추학술대회 Vol.2022 No.3

표면개질이 확산용접부 기계물성에 미치는 영향

사인진(Injin Sah),황종배(Jong-Bae Hwang),박재환(Jaehwan Park),김응선(Eung-Seon Kim) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

고온로 중간열교환기로 판형열교환기가 고려되고 있다.^(1,2) 판형열교환기는 확산용접 방법을 이용하여 얇은 시트를 접합하여 제작된다. 통상기술로 제작된 확산용접부의 접합계면은 접합계면을 따라 형성된 이차상으로 인해 외부하중에 취약하다.^(3-5) 본 연구에서는 확산용접부 접합계면 미세조직과 기계적 물성 향상을 위한 표면개질 방법을 제안하였다. 전자현미경 분석을 통해 접합계면에 크롬에 의한 탄화물과 알루미늄에 의한 산화물이 거의 형성되지 않는 것을 확인하였다. 이는 접합계면을 가로지르는 결정립계 이동을 촉진시켰고, 접합계면 영역의 미세조직이 접합계면에서 떨어진 영역의 미세조직과 유사하게 만들었다. 상온과 고온 인장실험에서 확산용접부는 모재 수준의 인장강도와 연신율을 보였다. 파단은 접합계면과 떨어진 인장시편 표점영역 안에서 발생하였다. 800 ℃에서 수행된 예비 응력-파단실험에서 확산용접부의 파단강도는 모재의 파단강도 범위 내에 포함되었다. Plate-type heat exchangers are expected to be applied in an intermediate heat exchanger (IHX) in high-temperature reactors (HTRs).^(1,2) Plate-type heat exchangers are manufactured by joining thin sheets through diffusion welding method. Following the conventional method in manufacturing the diffusion weldment, the interface is vulnerable to external loading condition due to secondary precipitates formed along the interface.^(3-5) A surface modification method is proposed in this study to improve the microstructural feature and mechanical properties of the interface. It was revealed in microscopic analysis that the interface is almost free from Cr-rich carbides and Al-rich oxides. This facilitates the grain boundary migration across the interface, and in turn the microstructure of the interface region becomes similar to that of regions remote from the interface. Fracture occurred at a location away from the interface in the gauge section of tension specimens. In room- and high-temperature tensile test, tensile strength and elongation of the diffusion weldment are similar to those of the as-received alloy. In preliminary stress-rupture test at 800 ℃, rupture strengths of the diffusion weldment fall within the range of the as-received alloy.

θ투영법을 이용한 초내열합금 크리프 거동 평가

사인진(Injin Sah),김상지(Sang Ji Kim) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

고온로 구조재료로 Ni-Cr-Co-Mo 초내열합금이 고려되고 있다.^(1,2) 크리프 변형 및 파단은 고온 운전 조건에서 구조재료 수명을 결정하는 주요 요인이다.^(3-7) 본 연구에서는 θ투영법을 이용하여 초내열합금의 크리프 거동을 평가하였다. 고온 응력-파단 실험 데이터에 비선형최소자승법을 적용하여 θ투영법 매개변수들을 도출하였다. 손상허용인자(λ) 값을 2.0 로 지정하여 3 차크리프 거동을 평가하였다. 비선형최소자승법으로 계산된 매개변수들은 응력 함수에서 선형성을 가졌다. 계산된 최소크리프율은 실험 데이터 범위 내에 포함되는 것으로 나타났다. 설계 관점에서 낮은 응력 수준에서 크리프 곡선들을 제안하였다. Ni-Cr-Co-Mo superalloy is expected to be used as a structural material in high-temperature reactors (HTRs).^(1,2) Creep deformation and rupture is a dominant factor in determining lifetime of structural material at high temperature operating condition.^(3-7) In this study, theta (θ) projection method was employed to characterize creep behavior of a superalloy. Parameters in the theta projection method were derived by applying nonlinear least square fitting (NLSF) method to experimental data of stress-rupture test at high temperatures. Damage tolerance factor (λ) of 2.0 was assigned to evaluate tertiary creep stage. Calculated parameters from NLSF had a good linearity as a function stress. It was revealed that calculated minimum creep rate falls within a range of experimental data. Creep curves at low stress levels were proposed for design aspect.

축방향 열전도와 유로 변형을 고려한 인쇄기판형 열교환기 열적 성능

박병하,사인진,김응선,Park, Byung Ha,Sah, Injin,Kim, Eung-seon 한국압력기기공학회 2018 한국압력기기공학회 논문집 Vol.14 No.1

Printed circuit heat exchangers (PCHEs) are widely used with an increasing demand for industrial applications. PCHEs are capable of operating at high temperatures and pressure. We consider a PCHE as a candidate intermediate heat exchanger type for a high temperature gas-cooled reactor (HTGR). For conventional application using stainless steels, design and manufacturing of PCHEs are well established. For applications to HTGR, knowledge of longitudinal conduction and deformation of channel is required to estimate design margin. This paper analyzes the effects of longitudinal conduction and deformation of channel on thermal performance using a code internally developed for design and analysis of PCHEs. The code has a capability of two dimensional simulations. Longitudinal conduction is estimated using the code. In HTGR operating condition, about ten percent of design margin is required to compensate thermal performance. The cross-sectional images of PCHE channels are obtained using an optical microscope. The images are processed with computer image process technique. We quantify the deformation of channel with dimensional parameters. It is found that the deformation has negative effect on structural integrity. The deformation enhances thermal performance when the shape of channel is straight in laminar flow regime. It reduces thermal performance in cases of a zigzag channel and turbulent flow regime.

Stress-Rupture Behavior of Diffusion-Welded Alloys for Application in PCSG System

황종배,사인진,김상지 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5

Diffusion welding is one of key technologies for building printed circuit steam generator (PCSG) in small modular reactors (SMRs). Evaluation of mechanical properties, such as tensile, stress-rupture, and fatigue, of the diffusion weldment are essential to employ the PCSG at operating conditions. In this study, microstructural evaluation and stress-rupture test were carried out using diffusion-welded SS 316L and Incoloy 800H(T). The microstructure of the diffusion-welded SS 316L was featured in the grain boundary migration across the welding interfaces. These features account for the comparable stress-to-rupture values of the diffusion-welded SS 316L to the expected minimum stress-to-rupture values presented in ASME Section III. In contrast, the grain boundary migration across the interfaces was limited owing to the presence of secondary precipitates for Incoloy 800H(T). The microstructure explained the degradation of the stress-to-rupture values for diffusion-welded Incoloy 800H(T). The time-temperature dependent reduction factor of the diffusion-welded SS 316L will be covered in detail.