發(fā)布時(shí)間：2018-08-13 14:37

【摘要】：近年來,在極低溫條件下使用的工業(yè)裝備越來越多,例如大型超低溫BOG壓縮機(jī)的工作溫度一般在-160℃甚至更低,因而對超低溫鑄造材料有著較大的需求。通常情況下,以鐵素體為基體的球墨鑄鐵材料能夠承受的溫度最低在-60℃左右,無法滿足更低溫度的應(yīng)用需求。高鎳奧氏體球墨鑄鐵隨著溫度的降低沒有韌-脆轉(zhuǎn)變現(xiàn)象,擁有著良好的低溫力學(xué)性能,因而在超低溫(-100℃以下)工業(yè)制造領(lǐng)域有著廣泛的應(yīng)用前景。目前,針對高鎳奧氏體球墨鑄鐵的相關(guān)研究主要集中在高溫性能方面,對于超低溫奧氏體球墨鑄鐵的研究極少,對其微觀組織、沖擊斷裂特征、示波沖擊斷裂過程以及沖擊裂紋的萌生和亞穩(wěn)擴(kuò)展規(guī)律并未見報(bào)道,因此本課題開展了超低溫奧氏體球墨鑄鐵微觀組織與低溫沖擊斷裂行為的研究。對超低溫奧氏體球墨鑄鐵微觀組織及其對摩擦磨損行為的研究表明:超低溫奧氏體球墨鑄鐵的微觀組織主要由奧氏體、石墨球以及分布在晶界處的碳化物構(gòu)成,材料中的錳元素和鉻元素會偏析分布至材料基體中的奧氏體晶界處形成M23C6(M=Fe、Mn、Cr)型碳化物,其微觀硬度可達(dá)到1200HV以上,遠(yuǎn)高于奧氏體基體硬度值,因而使得材料的宏觀硬度得以提升。鉻元素有著比錳元素更強(qiáng)的碳化物形成能力,對材料的摩擦磨損性能影響更大。通過對不同鉻元素含量下的材料摩擦磨損后的形貌進(jìn)行分析發(fā)現(xiàn),該材料表現(xiàn)為磨粒磨損機(jī)制,其中鉻元素促進(jìn)形成的晶界碳化物作為硬質(zhì)顆粒使得材料的摩擦磨損性能顯著提高。采用低溫示波沖擊手段,針對不同合金(鎳、錳和鉻)元素下的超低溫奧氏體球墨鑄鐵低溫沖擊性能進(jìn)行研究,結(jié)果表明:隨著溫度的降低,不同合金元素下的超低溫奧氏體球墨鑄鐵沖擊性能均存在著相似的特征,即呈現(xiàn)先上升后下降的變化趨勢,且鎳元素含量的變化對低溫沖擊性能存在著正相關(guān)的影響,而過多的錳元素和鉻元素加入會導(dǎo)致低溫沖擊性能惡化。采用掃描電子顯微鏡對沖擊斷口形貌進(jìn)行分析發(fā)現(xiàn):材料在室溫至-193℃的溫度區(qū)間內(nèi)均呈現(xiàn)了以石墨球或石墨球凹坑作為韌窩中心的韌性斷裂形貌特征,并且其沖擊斷口中石墨球數(shù)量與沖擊性能有著直接的因果關(guān)系,即石墨球越多則沖擊性能越好;碳化物數(shù)量的改變在室溫下對材料的沖擊性能影響并不明顯,而隨著溫度的降低其影響呈現(xiàn)增大的趨勢,在-193℃的超低溫條件下會導(dǎo)致沖擊斷口中出現(xiàn)縱向微裂紋,嚴(yán)重破壞材料沖擊性能。在對超低溫奧氏體球墨鑄鐵低溫沖擊性能規(guī)律的研究基礎(chǔ)上,對不同溫度下的示波沖擊曲線進(jìn)行了深入分析,進(jìn)一步地揭示了材料的沖擊斷裂過程,結(jié)果表明:以斜率法與柔度變化率法相結(jié)合的方式對示波沖擊曲線進(jìn)行分段分析的方法,可以有效的定量表述材料的低溫沖擊斷裂過程;其中,沖擊裂紋的高載荷亞穩(wěn)擴(kuò)展能量的比重可以達(dá)到?jīng)_擊總能量的60%以上,且兩者的變化趨勢相一致,即隨溫度的降低呈現(xiàn)先上升后下降的趨勢,因而高載荷亞穩(wěn)擴(kuò)展能量是決定低溫沖擊性能的主要因素;而該材料的低溫沖擊性能之所以呈現(xiàn)先上升后下降的趨勢(在-80℃時(shí)為極大值),是因?yàn)樵谑覝刂?80℃時(shí),高載荷亞穩(wěn)擴(kuò)展段的平均載荷對低溫沖擊性能起到了主導(dǎo)作用,而當(dāng)溫度繼續(xù)降低時(shí),高載荷亞穩(wěn)擴(kuò)展段的位移則成為了主導(dǎo)因素,研究還發(fā)現(xiàn),即便鎳元素含量變化這一規(guī)律仍然存在。同時(shí),采用三維激光共聚焦顯微鏡對沖擊斷口的幾何形貌進(jìn)行定量分析,對不同溫度下高載荷亞穩(wěn)擴(kuò)展段的表面粗糙度指數(shù)進(jìn)行統(tǒng)計(jì),驗(yàn)證了上述分析結(jié)論的正確性。由于超低溫奧氏體球墨鑄鐵實(shí)際示波沖擊曲線中高載荷亞穩(wěn)擴(kuò)展段的能量正好對應(yīng)著沖擊裂紋萌生與亞穩(wěn)擴(kuò)展過程中所吸收的能量,因此進(jìn)一步研究了沖擊裂紋萌生與亞穩(wěn)擴(kuò)展過程,結(jié)果表明:隨著溫度的降低,超低溫奧氏體球墨鑄鐵有著更好的抵抗沖擊裂紋萌生的能力;而前期抵抗沖擊裂紋亞穩(wěn)擴(kuò)展的能力受到溫度的影響,后期則受到溫度和材料中鎳元素含量變化的共同影響;材料基體中的石墨球(特別是相鄰的石墨球)和位于晶界處的碳化物是影響沖擊裂紋亞穩(wěn)擴(kuò)展路徑的最主要因素,而溫度的降低和材料中碳化物數(shù)量的增加都會加劇材料的脆性斷裂傾向;同時(shí),采用Schindler方法對不同鎳元素含量的超低溫奧氏體球墨鑄鐵在動態(tài)載荷下的延性斷裂韌度JBl0.2進(jìn)行了計(jì)算后發(fā)現(xiàn),隨著溫度的降低,材料在動態(tài)載荷下的延性斷裂韌度呈現(xiàn)持續(xù)升高的趨勢,而當(dāng)溫度達(dá)到-80℃以下時(shí),這一趨勢會發(fā)生明顯的放緩。
[Abstract]:In recent years, more and more industrial equipments have been used under extremely low temperature conditions. For example, the working temperature of large-scale ultra-low temperature BOG compressor is generally - 160 C or even lower, so there is a great demand for ultra-low temperature casting materials. The high-nickel Austenitic Ductile iron has good mechanical properties at low temperature, so it has a broad application prospect in the field of ultra-low temperature (-100) industrial manufacturing. At present, the research on high-nickel Austenitic Ductile iron is mainly focused on. In the aspect of high temperature properties, there is little research on the microstructure, impact fracture characteristics, oscillographic impact fracture process and the law of initiation and metastable propagation of impact cracks of ultra-low temperature Austenitic Ductile iron. The microstructure and friction and wear behavior of ultra-low temperature Austenitic Ductile iron are studied. The results show that the microstructure of ultra-low temperature Austenitic Ductile iron is mainly composed of austenite, graphite nodules and carbides distributed at grain boundaries. Manganese and chromium elements in the material will segregate and distribute to austenite grain boundaries to form M. The micro-hardness of 23C6 (M=Fe, Mn, Cr) carbide can reach 1200HV, which is much higher than that of austenite matrix, so the macro-hardness of the material can be improved. The carbide forming ability of chromium element is stronger than that of manganese element, which has greater influence on the friction and wear properties of the material. The wear morphology analysis showed that the material exhibited abrasive wear mechanism, in which the grain boundary carbide promoted by chromium element was used as hard particles to improve the friction and wear properties of the material. The results show that the impact properties of ultra-low temperature Austenitic Ductile Iron with different alloying elements have similar characteristics as the temperature decreases, that is, the impact properties of ultra-low temperature Austenitic Ductile Iron increase first and then decrease, and the change of nickel content has a positive correlation with the impact properties at low temperature, while excessive manganese and chromium elements have a positive correlation. Scanning electron microscopy (SEM) was used to analyze the impact fracture morphology. It was found that the ductile fracture morphology with graphite sphere or graphite sphere pit as the dimple center was observed in the temperature range from room temperature to - 193 C. The number of graphite spheres and impact properties of the impact fracture surface were also observed. There is a direct causal relationship, that is, the more graphite spheres, the better the impact performance; the change of carbide number at room temperature has no obvious impact on the impact performance of the material, but with the decrease of temperature its impact shows an increasing trend, in the ultra-low temperature of - 193 C conditions will lead to the occurrence of longitudinal microcracks in the impact fracture, seriously destroying the impact of materials. On the basis of the study on the low temperature impact property of ultra-low temperature Austenitic Ductile iron, the oscillographic impact curves at different temperatures are analyzed in depth, and the impact fracture process of the material is further revealed. The results show that the oscillographic impact curves are segmented by slope method and flexibility change rate method. The analytical method can be used to quantitatively describe the low temperature impact fracture process of materials, in which the proportion of metastable propagation energy under high load can reach more than 60% of the total impact energy, and the change trend of the two is consistent, that is, the metastable propagation energy under high load increases first and then decreases with the decrease of temperature. The main factor determining the low temperature impact property is that the low temperature impact property of the material first increases and then decreases (the maximum value is at - 80 C), because the average load of the high load metastable extension plays a leading role in the low temperature impact property from room temperature to - 80 C, and the high load metastable property when the temperature continues to decrease. At the same time, the geometric morphology of impact fracture was quantitatively analyzed by using three-dimensional laser confocal microscopy, and the surface roughness index of metastable extended section with high load at different temperatures was calculated to verify the above results. The results show that the energy absorbed in the metastable growth stage of high load in the oscillographic shock curve of ultra-low temperature Austenitic Ductile Iron corresponds to the energy absorbed in the process of crack initiation and metastable growth. Therefore, the process of impact crack initiation and metastable growth is further studied. The results show that with the decrease of temperature, the energy absorbed in the metastable growth stage corresponds to the energy absorbed in the process of impact crack initiation and meta Low-temperature Austenitic Ductile iron has better resistance to impact crack initiation; the resistance to metastable propagation of impact crack in early stage is affected by temperature, while in later stage is affected by both temperature and nickel content in the material; graphite spheres in the matrix (especially adjacent graphite spheres) and carbonization at grain boundary Material is the most important factor affecting the metastable propagation path of impact crack, and the brittle fracture tendency is aggravated by the decrease of temperature and the increase of carbide content in the material. It is found that the ductile fracture toughness of the material increases continuously with the decrease of temperature under dynamic loading, and the trend slows down obviously when the temperature is below - 80%.
【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】：2017
【分類號】：TG143.5

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