本文選題：奧氏體不銹鋼 + 敏化�。� 參考：《大連理工大學》2016年碩士論文

【摘要】：奧氏體不銹鋼因其優(yōu)良的綜合性能被廣泛用于各行各業(yè)中,但敏化后的不銹鋼極易發(fā)生晶間腐蝕進而引起構件失效事故,因此實現(xiàn)對奧氏體不銹鋼晶間腐蝕敏感性的快速無損檢測具有非常重要的意義。本文研究不同敏化程度的304奧氏體不銹鋼對超聲非線性特性的影響,為實現(xiàn)奧氏體不銹鋼晶間腐蝕敏感性的無損表征與評價提供理論和試驗依據(jù)。以CSK-I A標準試塊為研究對象,本文探討了信號調(diào)制模式、增益、衰減等系統(tǒng)參數(shù)對非線性超聲傳播特性的影響,據(jù)此提出合理的參數(shù)選擇方案。實驗結果表明：在試樣厚度允許的情況下,設置較多的脈沖串周期個數(shù)并采用漢寧窗進行信號調(diào)制可以有效提高實驗精度；衰減和輸出電壓級別不會影響非線性系數(shù),增益在一定范圍內(nèi)以N-2N原則進行變化也不會影響非線性系數(shù),但是增益調(diào)節(jié)過高會使信號產(chǎn)生諧波失真進而嚴重影響實驗結果；脈沖串周期個數(shù)較少時,中頻帶寬設置過小會使信號失真進而嚴重影響實驗結果。本文對不同敏化狀態(tài)的304不銹鋼進行非線性超聲檢測,輔以電化學動電位再活化測試,討論了非線性超聲表征不銹鋼晶間腐蝕敏感性的可行性,并通過金相顯微組織觀察、X射線衍射等方法對其進行了分析,研究結果如下：1.不同敏化狀態(tài)試樣的聲速和衰減系數(shù)并無明顯區(qū)別,傳統(tǒng)超聲檢測并不能有效地實現(xiàn)304不銹鋼晶間腐蝕敏感性表征。2.不同敏化狀態(tài)試樣的非線性參量發(fā)生顯著變化：非線性參量β/β0隨敏化溫度升高、敏化時間延長呈現(xiàn)增大的趨勢。3.皮爾森相關系數(shù)計算結果表明非線性參量與敏化度之間存在很好的相關性,兩者關系曲線亦表明非線性參量隨敏化度升高而增大,因此分析認為非線性超聲表征304不銹鋼晶間腐蝕敏感性是可行的。4.金相顯微組織觀察與X射線衍射結果表明：經(jīng)敏化處理,試樣晶界析出了碳化鉻(Cr23C6),并且碳化物析出隨敏化溫度提高、敏化時間延長而增多。因此分析認為：304不銹鋼敏化過程中析出的碳化鉻與奧氏體基體產(chǎn)生錯配引發(fā)局部應變場,內(nèi)應力的增加使超聲波發(fā)生畸變滋生高次諧波,而碳化物析出越多超聲波畸變越嚴重,因此非線性參量隨敏化度升高而增大。
[Abstract]:Austenitic stainless steel is widely used in various industries because of its excellent comprehensive properties. However, the sensitized stainless steel is prone to intergranular corrosion, which leads to the failure of components.Therefore, it is of great significance to realize fast nondestructive testing for the sensitivity of austenitic stainless steel to intergranular corrosion.In this paper, the influence of 304 austenitic stainless steel with different sensitized degree on ultrasonic nonlinearity is studied, which provides theoretical and experimental basis for the nondestructive characterization and evaluation of intergranular corrosion sensitivity of austenitic stainless steel.In this paper, the effects of signal modulation mode, gain and attenuation on the characteristics of nonlinear ultrasonic propagation are discussed, and a reasonable parameter selection scheme is proposed.The experimental results show that when the thickness of the sample is allowed, the experimental accuracy can be improved effectively by setting a large number of pulse series periods and using the Hanning window to modulate the signal, and the attenuation and output voltage level will not affect the nonlinear coefficient.The change of gain in a certain range by N-2N principle will not affect the nonlinear coefficient, but too high gain regulation will make the signal produce harmonic distortion and seriously affect the experimental results.If the if bandwidth is too small, the signal will be distorted and the experimental results will be seriously affected.In this paper, the nonlinear ultrasonic detection of 304 stainless steel in different sensitized states and electrochemical potentiodynamic reactivation test are carried out. The feasibility of nonlinear ultrasonic in characterizing the sensitivity of stainless steel to intergranular corrosion is discussed.The X-ray diffraction method was used to analyze the microstructures. The results are as follows: 1: 1.There is no significant difference in sound velocity and attenuation coefficient between different sensitized samples. Traditional ultrasonic measurement can not effectively characterize the sensitivity of 304 stainless steel to intergranular corrosion.The nonlinear parameters of different sensitized samples changed significantly: the nonlinear parameter 尾 / 尾 0 increased with the sensitizing temperature, and the sensitization time increased with the increase of the sensitizing temperature.The calculation results of Pearson correlation coefficient show that there is a good correlation between nonlinear parameters and sensitivities, and the curve of the relationship also shows that the nonlinear parameters increase with the increase of sensitivities.Therefore, it is considered that it is feasible to characterize the intergranular corrosion sensitivity of 304 stainless steel by nonlinear ultrasound.The results of metallographic microstructure observation and X-ray diffraction show that Cr23C6C _ (6) carbide precipitates from the grain boundaries of the samples after sensitizing treatment, and the precipitation of carbides increases with the increase of sensitizing temperature and sensitization time.Therefore, it is considered that the mismatch between chromium carbide and austenite matrix in the sensitized process of stainless steel: 304 leads to the local strain field. The higher the ultrasonic distortion, the higher the ultrasonic distortion, and the more carbides precipitate, the more the ultrasonic distortion is.Therefore, the nonlinear parameter increases with the increase of sensitization.
【學位授予單位】：大連理工大學
【學位級別】：碩士
【學位授予年份】：2016
【分類號】：TG142.71

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