本文選題：鐵素體球墨鑄鐵 + 沖擊吸收功��； 參考：《沈陽工業(yè)大學(xué)》2015年博士論文

【摘要】：鐵素體球墨鑄鐵具有良好的韌性和不容易發(fā)生脆性斷裂等優(yōu)點,滿足大型風(fēng)力發(fā)電機組輪轂、變速箱、底座等部件在常溫和較低溫度(-20℃左右)工作環(huán)境下的使用要求,但在更低溫條件下(-40℃以下),其力學(xué)性能發(fā)生顯著變化容易發(fā)生脆性破壞。目前關(guān)于鐵素體球墨鑄鐵低溫沖擊斷裂機理的研究很少,對沖擊過程中材料斷裂特征、斷裂行為、石墨球和晶界在斷裂過程中的作用及溫度變化對沖擊斷裂的影響機理均不掌握�；谏鲜鲈�,本課題開展了鐵素體球墨鑄鐵低溫沖擊性能影響因素及斷裂行為的研究。對鐵素體球墨鑄鐵進行系列示波沖擊試驗結(jié)果表明：溫度對沖擊過程中裂紋形成和裂紋擴展能力均有明顯影響,在韌脆轉(zhuǎn)變溫度以上時,球墨鑄鐵沖擊斷裂所需的裂紋形成功變化不大,裂紋擴展功有所下降,而在韌脆轉(zhuǎn)變溫度以下時,裂紋形成功與裂紋擴展功均存在劇烈的下降。采用激光共聚焦顯微分析系統(tǒng)對QT400-18L球墨鑄鐵沖擊斷口進行定量研究發(fā)現(xiàn),隨著溫度的降低球墨鑄鐵的斷面粗糙度指數(shù)減小且斷面粗糙度指數(shù)隨溫度的變化與沖擊吸收功的變化趨勢一致�？昭〝U張比和分形維數(shù)的變化反映出材料沖擊斷裂行為隨溫度的變化,空穴擴張比隨溫度的下降而減小,沖擊吸收功的對數(shù)隨分形維數(shù)的增加呈單調(diào)下降的直線關(guān)系。通過掃描電鏡及透射電鏡等手段觀察分析了鐵素體晶粒及石墨球?qū)Φ蜏貨_擊斷裂的影響,QT400-18L球墨鑄鐵的有效解理斷裂單元尺寸比QT350-22L球墨鑄鐵的有效解理斷裂單元尺寸大,低溫沖擊作用下解理裂紋啟動和擴展遇到粗大晶粒的概率也相應(yīng)較高,低溫下位錯在晶界處的塞積導(dǎo)致微裂紋更容易在晶界處萌生。晶界處夾雜物的存在易引起顯微孔洞的連接導(dǎo)致材料最終斷裂。石墨對球墨鑄鐵沖擊斷裂行為的影響首先是通過石墨與基體界面的斷裂行為引起的,石墨與基體的彈性模量之差越大越容易在石墨與基體界面處萌生裂紋,二者之差越小則越容易在石墨內(nèi)部開裂。采用沖擊試樣斷口金相原位觀察方法分析了不同溫度下QT400-18L鐵素體球墨鑄鐵裂紋萌生與擴展路徑以及斷口附近組織的演變規(guī)律,韌脆轉(zhuǎn)變溫度以上,沖擊后斷口附近大量石墨-基體界面發(fā)生開裂；韌脆轉(zhuǎn)變溫度區(qū)間,鐵素體球墨鑄鐵沖擊試樣表現(xiàn)為韌窩和解理混合斷口形貌,兩種斷裂模式裂紋形核均與石墨球有關(guān)；韌脆轉(zhuǎn)變溫度以下,垂直交叉孿晶成核進而導(dǎo)致微裂紋擴展,解理斷裂主要是孿晶起裂。這種形變孿晶引起的裂紋形核、擴展方式造成了裂紋形成功與裂紋擴展功的劇烈下降。利用高分辨透射電子顯微鏡,分析了不同溫度下沖擊斷口附近位錯組態(tài)由自由態(tài)到胞狀結(jié)構(gòu)再到位錯纏結(jié)的變化規(guī)律,采用XRD衍射方法計算了不同溫度下沖擊斷口附近位錯密度,隨著溫度的降低位錯密度有所降低可動位錯數(shù)目減少。根據(jù)位錯理論及微觀斷口轉(zhuǎn)變機制,對鐵素體球鐵韌脆轉(zhuǎn)變溫度表達式進行理論推導(dǎo),表明鐵素體晶粒越大、石墨球尺寸越大、石墨球間距越小、韌脆轉(zhuǎn)變溫度越高�；谖诲e動力學(xué)理論及Johnson-Cook本構(gòu)模型建立了描述QT400-18L球墨鑄鐵塑性變形行為的數(shù)學(xué)模型,模型計算值與實驗結(jié)果有著相同的變化規(guī)律,發(fā)現(xiàn)在230K出現(xiàn)一臨界溫度即韌脆轉(zhuǎn)變溫度。在韌脆轉(zhuǎn)變溫度以上相對于非熱效應(yīng)而言,熱激活應(yīng)力的作用可以忽略不計,而在韌脆轉(zhuǎn)變溫度以下表現(xiàn)出明顯的熱激活效應(yīng),需要借助熱激活作用來克服位錯滑移過程中的短程障礙。
[Abstract]:Ferrite spheroidal graphite cast iron has the advantages of good toughness and brittle fracture, which can meet the requirements of large wind turbine hub, gearbox, base and other components in working environment at normal temperature and lower temperature (about -20 C), but the mechanical properties of the large wind turbine are easily changed at a lower temperature (below -40 C). Brittle failure. At present, there are few studies on the mechanism of low temperature impact fracture of ferrite spheroidal graphite cast iron. The fracture characteristics, fracture behavior, the role of graphite ball and grain boundary during the fracture process and the influence mechanism of temperature on the impact fracture are not grasped. Based on the above reasons, this subject has carried out the ferrite spheroidal graphite cast iron. A series of oscilloscope impact tests on ferrite spheroidal graphite cast iron show that temperature has an obvious effect on the formation of cracks and the ability of crack propagation in the process of impact, and the crack shape needed for the impact of ductile iron at the transition temperature above the ductile brittle transition is not very successful. The expansion work decreased, while the crack growth and the crack propagation work were greatly reduced at the toughened and brittle transition temperature. The quantitative study on the impact fracture of QT400-18L ductile iron by laser confocal microscopy found that the roughness index of the ductile iron decreased with the decrease of the temperature and the roughness of the cross section. The variation of the number with the temperature is consistent with the change trend of the impact absorption work. The variation of the hole dilation ratio and the fractal dimension reflects the change of the impact fracture behavior of the material with the temperature, and the void expansion ratio decreases with the decrease of the temperature. The logarithm of the impact absorption work is linear with the increase of the fractal dimension. The effect of ferrite grain and graphite ball on low temperature impact fracture is observed by means of electron microscope. The effective cleavage unit size of QT400-18L ductile iron is larger than that of QT350-22L ductile iron. The probability of the crack initiation and expansion under the impact of low temperature is higher than that of the coarse grain. The deposition of microcracks at the grain boundary causes the microcracks to occur more easily at the grain boundary. The existence of inclusions at the grain boundary causes the connection of the micropores to lead to the final fracture of the material. The effect of graphite on the impact fracture behavior of ductile iron is first caused by the fracture behavior of the graphite and the matrix interface, and the elastic model of the graphite and the matrix. The bigger the difference is, the more easily the cracks occur at the interface between the graphite and the matrix. The smaller the difference between the two is the easier to crack in the graphite. The crack initiation and propagation path of the QT400-18L ferrite spheroidal graphite cast iron at different temperatures and the evolution of the microstructure near the fracture surface, and the ductile and brittle transition at different temperatures are analyzed. A large number of graphite matrix interfaces are cracked near the fracture surface after the impact, and the toughness and brittleness transition temperature range, the impact specimen of ferrite spheroidal graphite cast iron is characterized by dimple and cleavage fracture surface, and the two fracture modes are related to the graphite ball. Below the toughening and brittle transition temperature, the vertical twinning twins are nucleated and then lead to micro cracks. The expansion, cleavage fracture is mainly the twin fracture. The crack nucleation caused by this deformation twin causes the crack shape success and the sharp decrease of crack growth work. By using high resolution transmission electron microscope, the change rules of dislocation configuration near the impact fracture in different temperatures are analyzed from self from state to cellular structure and then wrong tangles in place. The XRD diffraction method is used to calculate the dislocation density near the fracture surface at different temperatures. With the decrease of the dislocation density, the number of movable dislocation decreases with the decrease of the dislocation density. According to the dislocation theory and the microscopic fracture transition mechanism, the expression of the ductile brittle transition temperature of Ferritic Ductile iron is theoretically derived, which indicates that the larger the ferrite grain is, Shi Moqiu The larger the size, the smaller the graphite ball space, the higher the toughening and brittle transition temperature. Based on the dislocation dynamics theory and the Johnson-Cook constitutive model, a mathematical model is established to describe the plastic deformation behavior of QT400-18L nodular cast iron. The calculated values of the model have the same changing rules as the experimental results, and the critical temperature, that is, the toughened and brittle transition temperature appears at present of 230K. At the toughening and brittle transition temperature above the non thermal effect, the effect of the thermal activation stress can be ignored, while the thermal activation effect is obvious below the toughened and brittle transition temperature, and the short-range obstacle in the dislocation slip process needs to be overcome by the use of thermal activation.
【學(xué)位授予單位】：沈陽工業(yè)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG143.5

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