Yong Huh

FATIGUE CHARACTERISTICS OF SM490A WELDED JOINTS

We compared the fatigue characteristics of weld metal with those of base metal heat-treated and non heat-treated. Also, we examined the influence of bead on fatigue life. From the experimental results of this study, it has been seen that the fatigue characteristics of welded specimens grinded the toe of bead are slightly better than those of welded specimens not grinded. Fatigue life is affected more by the stress concentration due to the profile change in the weld toe rather than by the residual stress, and heat-treatment had almost no influence on the fatigue characteristics.

Evaluation of the Plastic η-Factor Considering Strength Mismatch in a Narrow Gap Welding Part (II)

Abstract This study would like to evaluate the influences with the strength mismatch and the variation of the welding width of the narrow gap welding for the plastic parameter, the major constant determining the plastic -factor of J -Integral, using 3-D FEM. For this, we evaluate the plastic -factor according to the variation of the strength mismatch of weldment with same materials and welding width through FEM. Also, we proposed the equation of plastic -factor considering the variation of the strength mismatch of weldment with similar materials and welding width. 1. 서 론 최근 원자력 발전소 배관과 같은 대형 구조물 배관에 대한 협계용접(narrow-gap welding)이 증가되고 있다. 협계용접은 기존 용접법에 비해 용접부의 면적과 폭이 감소하며, 이에 따라 용접시간이 단축될 뿐만 아니라, 용접부의 수축 및 변형, 그리고 이에 따른 잔류응력이 감소되는 장점이 있다. 현재 이러한 협계용접부에 대한 대부분의 연구는 협계용접부의 용접 건전성 평가에 집중되어 있고, 원전 배관 설계 및 수명관리에 중요한 LBB(leak before brake) 해석을 협계용접부에 적용하는 연구가 일부 수행되고 있다. (1,2) †책임저자, 회원, 성균관대학교 기계공학부 E-mail : seok@skku.edu TEL : (031)290-7446 FAX : (031)290-7482 * 성균관대학교 기계공학부** 성균관대학교 대학원 기계공학과

THE MICROSTRUCTURAL ANALYSIS OF THE EFFECT OF FIC ON GAS TURBINE BLADES

Advancements in superalloys permit hot gas path components to operate for many thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine must withstand the most severe combination of temperature, stress, and environment. After a certain period of operation, the blade is damaged by the turbine operation mode. To recover the similar initial mechanical properties, the blade of nickel-based superalloy component undergoes a replacement repair process. A fluoride ion cleaning (FIC) process is used to assist in the successful welding repair of nickel-based superalloy components. This work is to study the FIC influence on a gas turbine blade that was serviced 25,748 equivalent operating hour (EOH) after first overhaul. The blades, including the fractured regions were obtained from a plant. The conditions of blade samples were observed by optical microscope and SEM, and the chemical composition of the flaws in the blade was investigated by EDS. γ′ phase morphology is also an important indicator of the mechanical properties of blades at the high temperatures. The blade condition was examined from morphology and size of the γ′ phase, because the γ′ phase coarsening rate depends largely on the exposure time and temperature.

A Study on the Degradation of the Superalloy Inconel 718 Using Ultrasonic Technique

The Ni-based superalloy Inconel 718 (IN-718) was developed in the 1950s and is currently used for several critical gas turbine components due to its good balance of mechanical properties. Because of its stable mechanical properties at high temperature, the alloy is used for turbine bolts. The mechanical properties of in-service facilities are required to maintain safety operation in power plants. Ultrasonic tests, tensile tests and hardness tests were performed to evaluate the degree of the material degradation of Inconel 718. The mechanical properties decreased as degraded, but the longitudinal velocity of the ultrasonic signal increased. Also, the microstructure of the degraded Inconel 718 was to study the relationship between the result from ultrasonic test and the results from destructive methods.

The Evaluation of Tensile Properties at High Temperatures Using a Ring Specimen

In the case of inner pressurized pipe, the hoop stress mainly causes the failure of pipe. For evaluating the hoop tensile properties of pipes, a ring test is used although it has an inevitable shortcoming of disturbance by the bending. Arsene suggested a central piece-inserted ring test for reducing the bending. At high temperatures, the friction between the specimen and the central piece exerts a serious influence on the results, so an effect on friction should be considered in the analysis. In this study, for the evaluation of the hoop properties using the central piece-inserted ring model of Arsene, we performed the ring tensile tests at several temperatures. From the ring tensile tests and numerical analysis the load-displacement conversion relationship of ring specimen (LCRR) was determined. We could obtain the hoop tensile properties by means of applying the LCRR to the results of the ring tensile test. From the result, it was observed that the strength decreased as the temperature increased, and the elongation dropped a little around 400°C.

Influence of the Strength Mismatch of a Narrow Gap Welded Joint of SA508 on the Plastic η Factor

In this article, the influence of the strength mismatch of a narrow gap welded joint of SA508 on the η factor was evaluated. The η factor is the principal parameter that determines the plastic portion of the J-integral. The specimens for tensile and hardness tests were collected from piping with narrow gap welding and the stress-strain curve and hardness were obtained from those. From these results, the Ramberg-Osgood (R-O) constant was obtained. Also, the finite element analysis was performed with variations in the strength mismatch and the weld width. The η factor equation considering the strength mismatch and the weld width of a narrow gap welded joint was suggested.

A Study on the Evaluation of the Fracture Toughness for the Narrow Gap Welding Part of Nuclear Piping

초록: 원전배관의LBB 해석을위한파괴인성(J-R곡선)은일반적으로ASTM E1820에따라시험을수행하여획득한다. 그러나ASTM E1820에제시되어있는파괴인성에관한식은균질재의물성평가를위한것이며, 협개선용접부에적용할경우실제와는다른파괴인성을나타낼수있다. 따라서본연구에서는ASTM에제시되어있는균질재의파괴인성식에의한결과와협개선용접재의특성을고려하여제안된소성η계수식에의한파괴인성의차이를비교검토하기위하여협개선용접이적용된배관에서CT 시험편을채취한후파괴인성시험을수행하였다.Abstract: The fracture toughness for the LBB analysis of piping is generally determined by the J-integralaccording to ASTM E1820. However, since this evaluates a base metal, the fracture toughness for narrow gapwelding can be differently than the real value. This study evaluated the plastic  factor of the narrow gapwelding part of a nuclear piping with SA508 Cl.1a and SA312 TP316. Also, it performed the fracturetoughness test for the narrow gap welding part and applied the new plastic  factor equation by Huh, et al.and then compared the results with those according with the ASTM standard.

Calculation of the Crack Length for a Pipe Specimen using the Modified Load Ratio Method

The objective of this paper is to apply the load ratio method to the measurement of the crack length of the real scale pipe specimen. The load ratio method was modified and finite element analyses were performed to derive the relationship between the normalized compliance and the normalized crack length for the pipe specimen. In order to measure the crack length, the direct current potential drop method and the modified load ratio method were applied to the pipe test. The applicability of the modified load ratio method was confirmed by comparing the calculated crack length with the measured crack length from the pipe experiment.

Plastic-Factor for the Fracture Toughness Test of the SA508Cl.1a Narrow-Gap Welding Part

In this study, the plastic η -factors of the SA508Cl.1a narrow-gap welding part, which is used for the primary piping system in a nuclear power plant were obtained by using finite element analysis and the modified fracture toughness testing method was suggested for the narrow-gap welding part. Also, we have performed the fracture toughness test for the SA508Cl.1a narrow-gap welding part by applying the new testing method and then we compared the results with those from the ASTM fracture toughness test.

Evaluation of the Ductile-Brittle Transition Behaviour of Aging Specimens by the Fracture Toughness Test

The safety and reliability of industrial machineries and structures used in various, and severe conditions has become an increasing concern. This study proposes ductile-brittle transition temperature (DBTT) evaluation technique by the sub-sized specimen test to prevent cleavage fracture. Four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared. The fracture toughness tests were performed on sub-sized specimens both at room and low temperatures. The results of the fracture toughness tests were analyzed to obtain the DBT behaviours of the specimen materials