수퍼 2상 스테인리스강의 미세조직에 따른 부식과 피로균열 진전특성

Abstract

A demand for steel which has longer life-time in severe environmental conditions is increasing due to the industrial development. For this purpose, the duplex stainless steel was developed to improve strength and corrosion resistance. Recently, a super duplex stainless steel which contains 25 % of chromium has been developed. Because the pitting resistance equivalent number(PREN) of super duplex stainless steel is more than 40, it has better corrosion resistance and mechanical properties than that of the conventional duplex stainless steel. But the increase of chromium, molybdenum and nitrogen cause σ embrittlement and 475 ℃ embrittlement of super duplex stainless steel. The deleterious Cr and Mo rich σ-phase which formed between 600 and 1000 ℃ is a very brittle precipitation, and it is generally known as the cause of reduction in both mechanical properties and corrosion resistance. And the σphase, that is formed in the relatively low temperature between 300 ℃ and 525 ℃, is the cause of 475 ℃ embrittlement in duplex stainless steel. On the other hand, although many researchers have studied the properties of super duplex stainless steel, the studies on the mechanical properties under cyclic loading using nondestructive evaluation are insufficient. Acoustic emission monitoring is a very sensitive method with a wide dynamic range and it can be used as diagnostics for continuous assessment of damage in materials and components. Various methods based on acoustic emission could be applied to aspect of materials strength as well as nondestructive evaluation to structural components subjected to static or fatigue loading. However, because cumulative counts and energy include many noises in acoustic emission experiment, quantitative evaluation has its limitation. Studies, which find out the fracture mechanism characteristics with classification the waveform and the spectra of signals, are being carried out actively. Also time-frequency analysis method, which can represent information of both time domain and frequency domain having signal at the same time, are being tried to study of nondestructive evaluation A stainless steel that contains aggressive negative ion was known to decrease the corrosion resistance. Stainless steel with super corrosion resistance was developed for improvement of corrosion resistance. Duplex stainless steel is widely used under sever environment because of good mechanical properties and corrosion resistance. But duplex stainless steel is not stabilized compared to austenite stainless steel in corrosion resistance. Newly developed duplex stainless steel is the super duplex stainless steel which contain 25 % of chromium. Because PREN of super duplex stainless steel is more than 40, it has better corrosion resistance and mechanical properties than the normal duplex stainless steel. Now marine strainer filters that are used in a ship are made of 304 stainless steel. It causes a economic problem because these marine strainer filters made of 304 stainless steel are corroded terribly for a shot period. Therefore materials that can substitute for 304 stainless steel need to be developed. The object of this study is the development of the material for marine strainer filter that have better corrosion resistance. Super duplex stainless steel that has good corrosion resistance and high PREN is applied as a material for marine strainer filter. On this study, we evaluated the corrosion resistance of super duplex stainless steel that contains 25 % Cr and can maintain long life under sever circumstances. The corrosion resistance of super duplex stainless steel that has two structure(fibrous structure, dispersed structure) was evaluated through immersion test and impingement test in 35% HCl and sea water. And we compared super duplex stainless steel with 316L stainless steel and 304 stainless steel. The corrosion resistance of super duplex stainless steel was superior than 304 stainless steel. The dispersed structure of super duplex stainless steel was stabilized than fibrous structure in corrosion, The corrosion rate was fast in order of 304 stainless steel, 316L stainless steel, fibrous structure of super duplex stainless steel and dispersed structure of super duplex stainless steel. Also, fatigue crack propagation behavior and corrosion characteristic in 3.5 % NaCl solution were investigated using polarization test and immersion test for super duplex stainless steel with various volume fractions of the austenite phase. The various volume fraction and distribution of austenite structure of specimen were obtained by changing the heat treatment temperature and cycle. From the result, fatigue crack propagation rate trends to the kinds of two fatigue crack propagation rate of the first half was distinguished from that of the second half. Corrosion resistance of super duplex stainless steel in 3.5 % NaCl solution shows that the σ-phase precipitation corrode more easily. Based on the analysis of time-frequency method, it should be possible to monitor real-time crack propagation and final fracture process resulted from various damages and defects in structural members.