SS316L不銹鋼在高溫高壓水媒中之低週疲勞行為

Low-Cycle Fatigue Behavior of Type 316L Stainless Steel in a High Temperature and High Pressure Water Environment

SS316L不銹鋼為進步型沸水式反應器（ABWR）爐內組件與壓力邊界的重要材料。本研究探討應變振幅（Strain Amplitude）及疲勞頻率兩者對SS316L不銹鋼於300℃、10 MPa循環流動水媒中低週疲勞行為之影響。測試結果顯示，隨著應變振幅之下降，低週疲勞壽命明顯增長；應變振幅降至0.12%時，試片能通過5 × 105週次之測試而不斷。鋼材於測試初期階段呈現顯著之週期硬化（Cyclic Hardening）現象，最大應力值急速上升。低週疲勞測試頻率效應測試結果顯示，頻率越低鋼材疲勞壽命越短。另進行室溫及300℃空氣中之低週疲勞測試，比較鋼材於水媒中與空氣中疲勞行為之差異；因腐蝕與疲勞協同作用，鋼材在水媒中之壽命最短，300℃空氣中者次之，室溫空氣中者最長。掃描電子顯微鏡（SEM）觀察發現，水媒中測試鋼材樣品之破斷面上散布著大量的顆粒狀腐蝕生成物，而空氣中測試者則無。能量分布能譜儀（EDS）分析結果顯示，此等腐蝕產物主要成分為Fe, Cr, Ni元素，另含多量的氧，不同於富含S, Mn兩元素的二次相顆粒。

SS316L stainless steel is one of the important structural materials for the in-core components and pressure boundaries of the advanced boiling water reactors (ABWR). This work was mainly focused on the effects of strain amplitude and fatigue frequency on the low-cycle fatigue (LCF) behavior of SS316L stainless steel in a high temperature, high pressure water environment. The test results show the fatigue life is significantly increased with decreasing the strain amplitude. The specimens remain intact after being fatigue-tested up to 5×105 cycles, when the applied strain amplitude is no greater than 0.12%. During the early stage of the low-cycle fatigue test, the cyclic hardening phenomenon was observed. The results from the tests of frequency effect show the lower the fatigue frequency was applied, the shorter the specimen fatigue life was observed. The steel specimens have the longest fatigue life when tested in air at room temperature, but, in contrast, they were observed to have the shortest one when tested in the high temperature, high pressure water environment. From the above observations, it can be inferred that the fatigue-corrosion interaction plays a significant role in determining the fatigue life of SS316L stainless steel. SEM fractographic examinations reveal that a large quantity of granular corrosion products were desposited on the fracture surface of the specimen tested in an aqueous environment, but that none was observed with the specimen tested in air. The EDS analysis shows the corrosion products are mainly composed of Fe, Cr, Ni and oxygen. They appear different from the second-phase particles which are rich with S, Mn elements