Jae-Seong Kim

The Effect of Welding Residual Stress for Making Artificial Stress Corrosion Crack in the STS 304 Pipe

The stress corrosion crack is one of the fracture phenomena for the major structure components in nuclear power plant. During the operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loop components. In particular, stress corrosion crack usually occurs when the following three factors exist at the same time: susceptible material, corrosive environment, and tensile stress (residual stress included). Thus, residual stress becomes a critical factor for stress corrosion crack when it is difficult to improve the material corrosivity of the components and their environment under operating conditions. In this study, stress corrosion cracks were artificially produced on STS 304 pipe itself by control of welding residual stress. We used the instrumented indentation technique and 3D FEM analysis (using ANSYS 12) to evaluate the residual stress values in the GTAW area. We used the custom-made device for fabricating the stress corrosion crack in the inner STS 304 pipe wall. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced, and it could be controlled by welding residual stress.

A Study on the Optimal Welding Methods of Vertical Welding Positions in the Vehicle Body

서 보여주고 있는 시스템 장치를 구성하여 수직자세 용접을 실시하였다. 용접기는 오스트리아 Fronius사의 CMT 프로세서 용접기를 사용하였으며, 아크 모니터링 시스템을 이용하여 용접 중 전류와 전압 신호를 획득하였다. 동시에 수직자세 용접에서 나타나는 용융풀의 거동 및 금속이행현상을 확인하기 위해 고속카메라를 사용하여 촬영하였다. CMT 용접 장치는 토치 부분에 AC 서보모터를 장착하여 용접이 되는 동안 와이어를 뒤로 당겨줌으로써 단락이행모드가 주기적이고 안정적으로 발생하게 제작된 용접기이다. 또한 기존 용접기와 비교하여 동일 전류에서 저입열로 용접됨으로써 용락과 같은 문제를 해결할 수 있는 용접기이다.

A Study on the High Temperature Tensile Characteristics of Lap Weld of 15Cr Ferritic Stainless Steels

Ferritic stainless steels of the 400 series have been available for automotive exhaust system, heat exchanger, radiater etc. in various industrial because heat resistance, corrosion resistance and strength are excellent. Especially, automotive exhaust system is required good heat resistance because typical temperature of exhaust system exposed during operation of engine is reach up to . However, research for effect of high temperature in ferritic stainless steels is not enough. In this study, high temperature tensile properties of lap weld of ferritic stainless steels(STS 429) were investigated. In accordance with heat input, lap welds had been produced and were evaluated at high temperature() to compare high temperature tensile properties. In addition, room temperature tensile tests were carried out for non-aging and aging specimens. As a result of R.T tensile test, non-aging specimens were fractured in base metal except for low heat input specimen and aging specimens were fractured in weld metal. Also high temperature tensile test were carried out by aging specimen. After high temperature tensile test, fracture of aged specimen was occurred in base metal except for low heat input specimen. Fracture surface of low heat input specimen in weld metal was confirmed as brittle fracture with observation using scanning electron microscope(SEM). Significant decrease in ultimate tensile strength (between 82 and 85%) was observed for aged ferritic stainless steels(STS 429) when tested at high temperature.

Study on the Micro Crack Detection in Joints by Using Ultrasound Infrared Thermography

This study detected SCC defects of dissimilar metal welded(STS304 and SA106 Gr. b) pipes using the ultrasonic infrared thermography method and the lock-in image treatment method among infrared thermography method. The infrared excitement equipment has 250 Watt of output and 20 kHz of frequency. By using the ultrasound infrared thermography method, the internal defects of dissimilar metal weld joints of pipes used at nuclear power plants could get detected. By an actual PT test, it was observed that the cracks inside the pipe existed not as a single crack but rather as a multiple cracks within a certain area and generated a hot spot image of a broad area on the thermography image. In addition, UT technology could not easily defects detected by the width of fine hair cracks. but, ultrasound infrared thermography technique was defect detected.

A Study on the Mechanical Properties of MAG Weld on Ferritic Stainless Steel Sheets

Ferritic stainless steels have a good heat resistance and economic advantage. They are used for applications such as automotive exhaust systems where resistance to general corrosion is superior to carbon steels. However, there are not enough research for ferritic stainless steels on weldability mainly used as automotive exhaust manifolds. In this study, mechanical and microstructure properties of as-welded STS 429L and STS 444 ferrite stainless steels were confirmed by tensile, bending, hardness test, optical microscopy and scanning electron microscopy. Tensile strength of the STS 444 is higher than the STS 429L when it is a raw material. In contrast to this fact, STS 429L indicated higher tensile strength after butt welded. In addition, the hardness have a increasing tendency as getting down on the bead.

A Study on the Dissimilar Metal Joining of Aluminum to Steel Using the Arc Heat Source (IV) - Fatigue Behaviors Using Ultrasonic Elastic Resonance -

In order to commercialize the Fe-Al joining technology using arc heat source, it is necessary to guarantee about static and dynamic strength. However, no studies have been conducted to evaluate the fatigue strength of Al-Fe joints using arc heat sources. Therefore, ultrasonic fatigue test to the overlap joint structure of dissimilar materials was examined at first. For the ultrasonic elastic resonance experiment, the plate type specimen of the overlap joint was designed based on the design theory of the ultrasonic fatigue specimen. As a result, it was confirmed that the ultrasonic fatigue test for the overlap joint was sufficiently applicable through the thermal imaging camera measurement. Also the result of ultrasonic fatigue test according to the heat input and the presence of gap, the fatigue life depend on increasing heat input was reduced due to inverse proportion to the growth of the root pore. On the other hand, if the gap is present, fatigue life is improved up to 25% as much as the static strength by removing a considerable amount of pores. When compared to other studies on the fatigue of aluminum lap joints, this fatigue limit is sufficiently competitive if a method to control the pores is constructed.

Improvement of butt-welding characteristics of double wall polyethylene pipes

In this study, a new heat plate with a groove for butt-welding was developed to investigate the misalignment effects on the joining strengths of polyethylene double walled pipes. Thermal analyses of the material determined the temperature profile of the welding. After the welding, the joining characteristics of the pipes were compared with those welded by the conventional heat plate. The results of the stiffness and flattening tests showed that there were no big differences between the joints made from these two plates. In the leakage test, however, the pipes welded with the conventional heat plate leaked below the required test conditions, while the pipes welded with the grooved heat plate did not show any leakage, even in a range higher than the required conditions. The tensile and bending strength were also improved. The results demonstrated that the grooved heat plate accomplished more complete fusion at the pipe joints and induced high quality joints.

Characteristics of Mechanical Properties on 600MPa Grade TRIP Steel Using the Various Shielding Gases in the GMAW

1990년대 자동차 산업에 있어 최대 관심사는 엔진 개발을 통한 최대의 연료 효율을 확보하고 자동차의 무게를 증가시켜 안전성과 편안함을 개선하고 차량 사용에 있어 기능적인 부분을 부가하는 것이었다. 하지만, 최근 전 세계적인 에너지 및 환경규제 등의 문제가 대두되기 시작하면서 일본, 유럽 등지에서 법제화를 통해 자동차 중량을 감소시키는 것이 큰 이슈로 대두되고 있다. 자동차에 있어 무게를 줄이게 되면 차량의 연비를 줄일 수 있으며, 배기가스 배출을 감소시키게 되어 화석연료의 소비 감소 및 환경오염 방지에 큰 역할을 할 수가 있다. 반면 자동차의 안전성, 편의성 등을 바라는 다양한 소비자의 욕구를 충족시키기 위해 많은 장치들을 자동차에 추가 장착함으로써 자동차의 중량은 증가하게 되었다. 따라서 이러한 상반되는 문제를 해결하기 위해 철강 소재에서 마그네슘 합금, 알루미늄 합금, 플라스틱 등의 가벼운 비철합금 소재의 사용이 제안되고 있으나 가격, 안전성, 내구성 등의 문제로 그 사용은 제한되고 있는 실정이다. 이에 따라 자동차구조용 소재는 안전성 및 내구성을 위해 높은 강도와 연성을 동시에 확보하고 있는 고장력강재의 사용이 증가하고 있다. DP 강, TRIP 강, CP 강, Hot-stamping 강 등 600MPa부터 1,500MPa까지 다양한 강도의 강재들이 차체 제작에 있어 사용되고 있다

A Study on the Arc Position which Influence on Quality of Plug Welding in the Vehicle Body

Abstract Welding is an essential process in the automotive industry. Most welding processes that are used for auto body is spot welding. And CO 2 arc welding is used in a small part. In production field, CO 2 arc welding process is decreased and spot welding process is increased due to welding quality is poor and defects are occurred in CO 2 arc welding process frequently. But CO 2 arc welding process should be used at robot interference parts and closed parts where spot welding couldn’t. CO 2 welding is divided into lap welding and plug arc spot welding. In case of plug arc spot welding, burn through and under fill were caused in various welding environment such as different thickness combinations of base metal, teaching point, over the two steps welding and inconsistent voltage/current. It makes some problem like poor quality of welding area and decrease the productivity. In this study, we will evaluate the effect of teaching point through the weld pool behavior and bead geometry in the arc spot welding at the plut hole. Welding position is horizontal position. And galvanized steel sheet of 2.0mm thickness that has plug hole of 6mm diameter was used. Teaching point was changed by center, top, bottom, left and right of the plug hole. At each condition, the phenomenon of weld pool behavior was confirmed using a high-speed camera. As the result, we find the center of plug hole is the most optimal teaching point. In the other teaching point, under fill was occurred at the plug hole. This phenomenon is caused by gravity and surface tension. For performance of arc spot welding at the plug hole, the teaching condition should be controlled at a center of plug hole.Key Words : Arc spot welding, Plug hole, Teaching point, Gravity, Surface tension

A Study on Characteristic of Fracture in Lap Joint Welded STS429L

Recently, a demand of ferritic STS is increasing rapidly in automobile exhaust system. Exhaust manifolds are the part nearest to the engine so that the material is exposed to high temperature exhaust gas. Excellent heat resistant properties, especially high temperature strength, thermal fatigue resistance and high corrosion resistance are necessary for these parts. STS429L contains 15 weight percent of Cr and low Mo, so has good price competitive. And it has excellent high temperature strength and corrosion resistance, so receives attentions as material that applying to exhaust manifold. In tensile test of lap joint welded STS 429L, most of specimens are failed in base metal, but occurs brittle fracture in weld metals at some specimens in the face of good welding conditions. In the process of tensile test, lap joint welded STS429L specimens are transformed locally. The brittle fracture occurs that local transforming area exists in weld metals. But, butt welding specimens made by same materials showed ductile fracture in tensile test and bending test. In this study, suppose the reason of brittle fracture is in the combined local transform and tensile stress, through analysis of bead geometry, evaluate geometrical factor of brittle fracture in lap joint welded STS429L.