STS-304 용접부 피로수명에 관한 연구

Abstract

STS-304 is being used widely as piping systems in many industries and the socket welding is well known for very vulnerable to the fatigue although it is being used most because of simple work process. However, there are few cases of the fatigue characteristic evaluation about the socket welding part of small-sized pipes in Korea. Kori unit 3 was shut downed in Oct. 21, 1985 due to the leakage of the weld area in small bore piping. Since then, we have several experiences of weld area leaking. Similar situations are reported in foreign Nuclear Power Plants. In 1990's, 450 fatigue damages of piping were occurred in USA Nuclear Power Plants. An investigation shows that approximately 80% of total failures are related to socket weld connections in small bore piping. The related technology development is necessary as it is going operating time of domestic plants. It is carried out the fatigue limit test of welded STS-304 for high cycle fatigue lifetime evaluation. In this research, it is analyzed the fatigue lifetime and characteristics of the socket welding part of 3/4 inch STS-304 pipe through S-N curve drawing, broken side observation, EDS analysis and hardness test. (1) In the fatigue strength test, it is carried out that the load is imposed, lowering it by 4% from 70% level of tension strength. Through the test, it is obtained the time to failure according to each shear stress working on specimen. As a result of the test, S-N curve is created that the high cycle fatigue limit is 95MPa and as the load increases, the life to failure decreases rapidly. It is thought that if it is imposed the load over the permissible shear stress of some material, it gets fatigued rapidly. (2) In the fatigue failure characteristics analysis, the failure occurs toe part of the welding rather than root part of that for the most part. It is thought that it results from the stress concentration. In order to relieve this phenomena, it is important to maintain conclave formation or good surface condition. (3) When a steel is welded, δ-Ferrite comes into existence at about 1500 ℃ and it makes worse the fatigue characteristics. It is recommended to compare the fatigue characteristics according to the amount of δ-Ferrite through heat treatment. Before that, it should be guaranteed that all specimens welded have the same quality. The fatigue failure occurs frequently in bead, the welding material rather than original metal because the toe of bead causes the stress concentration by notch effect and surface defects such as undercut. The vibration in high temperature-pressure pipes is well known for the biggest reason for the high cycle fatigue. In general, the tension residual stress is piled up around the part of welding and the part gets embrittled that it makes crack grow rapidly to the failure. In order to prevent the fatigue failure of welding material, it is needed to keep the design criteria such as strength, temperature, thickness and corrosion.