SA533B壓力槽鋼材水媒環境效應低週疲勞行為研究

The Low-Cycle Fatigue Behavior of SA533B Reactor Pressure Vessel Steel under High Temperature and High Pressure Water Environments

SA533B低合金鋼係核能電反應爐壓力槽之構造鋼才。本報告主要探討於10 MPa高壓循環流動水媒中壓力槽鋼材低週疲勞行為受水媒溫度、應變振幅及水中溶氧量等不同因素之影響情形。測試結果顯示，於飽和溶氧水媒中，水媒溫度（150℃-300℃）對疲勞壽命影響之效應相當顯著，疲勞壽命差異大；然於除氧水媒中，疲勞壽命相近，溫度效應不明顯。另隨著應變振幅之增加，鋼材低週疲勞壽命週期數明顯降低。水媒中溶氧量之多寡對鋼材低週疲勞壽命具有決定性的影響，降低溶氧量可抑低水媒環境中鋼材之疲勞劣化。比較空氣與水媒中之低週疲勞測試結果，明顯可見水媒環境效應對鋼材疲勞劣化之影響程度。腐蝕與疲勞協同作用的結果顯著降低水媒中鋼材疲勞壽命，鋼材在飽和溶氧水媒中之疲勞壽命最短；然隨著溶氧降低至1 ppb 左右，鋼材疲勞壽命漸趨近於空氣中測試者。X-光繞射（XRD）分析板狀試片表面腐蝕生成物結果發現，水媒中溶氧量多寡與水媒溫度兩者顯著影響腐蝕產物組成，外觀顏色變異情形亦深受該兩因素影響。腐蝕生成物之成分與顏色變化是環境效應的一種指標，有助於了解SA533B壓力槽鋼材在高溫高壓水媒中之低週疲勞行為。

SA533B low-alloy steel is the material mainly used for the reactor pressure vessels (RPVS) of nuclear power plants. This report is focused on the effects of temperature, strain amplitude and dissolved oxygen content in feed water on the low-cycle fatigue (LCF) behavior of SA533B RPV steel in circulating water environments at a pressure of 10 MPa. A significant effect of temperature on fatigue life was observed with the specimens tested in a water environment saturated with oxygen. The fatigue life was varied with temperatures between 150℃ and 300℃. But little or no dependence of fatigue life on temperature was noted with the water environment with an oxygen content of 213 ppb. The fatigue lives of the specimens tested at different temperatures are not much different. The fatigue life significantly decreased with increasing strain amplitude. The low-cycle fatigue life of the RPV steel is also deeply affected by the dissolved oxygen content in feed water. The degradation of SA533B steel in water environment could be effectively mitigated by lowering the dissolved oxygen content. The steel specimens have the longest fatigue life when tested in air, but, in contrast, they were observed to have the shortest one when tested in water environment with saturated oxygen. The fatigue life in water environment increased to approach that in air when the dissolved oxygen content was down to about 1 ppb. It implies the fatigue-corrosion interaction plays a significant role in determining the fatigue life of SA533B steel. The results of x-ray diffraction (XRD) analysis show the change of constituents of corrosion products is strongly affected by both temperature and dissolved oxygen content of water environment. Furthermore, the same factors also affect the variations of appearance colors of corrosion products. The change of constituents and colors of corrosion products could act as an indication of the environmental effects, which is instrumental in understanding the low-cycle fatigue behavior of SA533B steel in high temperature and high pressure water environments.