USC電站鍋爐用T92鋼高溫下微觀組織演化及壽命預測
發(fā)布時間：2018-08-14 09:05
【摘要】：T92鋼(NF616)是上世紀90年代日本新日鐵公司在T/P91鋼的基礎上,將材料進行了進一步的合金化,增加W含量到1.8%,減少Mo含量到0.5%,并增加了適量的B元素。與T/P91鋼相比,T92鋼的持久強度有較大幅度的提高,可用于625℃以下的高溫蒸汽管道。目前,T92鋼憑借其優(yōu)異的綜合性能已成為新一代超(超)臨界火力機組的理想用鋼。然而,國產T92鋼投入使用的時間較短導致T92鋼長期運行后的的數(shù)據(jù)資料匱乏,同時國內外超(超)臨界機組的運行調峰方式的差異,造成材料在長期高溫服役過程中微觀組織結構與性能退化不一樣,對機理的研究也不夠深入,尤其是目前還沒有有效的T92鋼蠕變壽命預測方法,更談不上剩余壽命的預測及壽命預測的驗證�；诖�,本文以某國產T92鋼的模擬加速老化樣品為研究對象,研究材料在蠕變過程中的組織結構演變與力學性能變化的規(guī)律,并基于Larson-Miller參數(shù)法的蠕變壽命預測方程對其壽命進行了預測。本文運用掃描電鏡、透射電鏡和納米壓痕儀等現(xiàn)代材料分析方法對模擬不同工況參數(shù)T92鋼的顯微組織結構、斷裂機理及力學性能進行了分析。研究表明,供貨態(tài)T92鋼的組織為典型的板條狀回火馬氏體+碳化物,在長期的高溫及應力的作用下,材料的微觀組織無法保持其原始組織的穩(wěn)定性,逐漸開始退化。板條馬氏體發(fā)生回復,材料中呈彌散分布的原始第二相向原奧氏體晶界、亞晶界及板條馬氏體邊界偏聚,并發(fā)生聚集和粗化,并伴隨著新相Laves相的析出。T92鋼中的M23C6碳化物顆粒呈棒狀和球狀兩種形態(tài),在蠕變過程中,具有球狀形態(tài)的顆粒更為穩(wěn)定,Laves相則一般依附在大尺寸的M23C6顆粒上析出,析出位置主要在原奧氏體晶界、亞晶界等界面上。與M23C6相相比,Laves相的粗化速度較快,當Laves相長大到一定的尺寸時,將會誘發(fā)材料中蠕變孔洞的產生,孔洞的聚集和聯(lián)結造成材料的斷裂及過早失效。對比蠕變樣品中受應力區(qū)域與不受應力區(qū)域的析出相平均尺寸發(fā)現(xiàn),應力明顯加速Laves相的形核和粗化,而M23C6的平均尺寸則略有增加。此外,在整個蠕變過程中,基體的主要合金元素(Cr、W、Mo等)的百分含量均逐漸減少,合金元素逐漸由固溶態(tài)向化合態(tài)進行轉移,這些微觀組織結構的軟化共同造成材料宏觀顯微硬度的下降。其次,本文結合微納米壓痕硬度測試技術獲取了700℃下不同時間斷裂后的T92鋼蠕變樣品中板條馬氏體基體的硬度,排除了高角邊界對結果的影響,再結合微觀組織分析方法,探討了不同模擬工況條件下馬氏體基體的退化機制。結果表明T92鋼在長時高溫及應力的作用下,板條馬氏體中基體的主要強化因素都將發(fā)生不同程度的削弱,蠕變過程中Laves相和M23C6相等第二相的析出和粗化造成第二相間距增大;板條馬氏體發(fā)生回復,晶寬增加;基體中重要的固溶強化元素W、Mo等不斷流失,這些因素的綜合作用,導致了T92鋼在長時蠕變過程中的基體性能的下降,可以通過檢查馬氏體基體的硬度來判斷,結果表明基體硬度隨蠕變時間的增加而不斷下降。最后,本文對600℃,649℃和700℃下的T92鋼的加速老化數(shù)據(jù)進行擬合,建立了T92鋼高溫過程中的對數(shù)曲線,提出了一種基于Larson-Miller參數(shù)法的壽命預測方法,通過該方法嘗試了T92鋼外推10萬小時的持久強度,與ECCC的外推結果相當。
[Abstract]:T92 steel (NF616) was further alloyed on the basis of T/P91 steel by Nippon Steel Co. in the 1990s. The content of W was increased to 1.8%, the content of Mo was reduced to 0.5%, and the content of B was increased. Compared with T/P91 steel, the durable strength of T92 steel was greatly improved, which can be used for high temperature steam pipes under 625 C. At present, T92 steel has become an ideal steel for the new generation of ultra-supercritical (ultra-supercritical) thermal power units because of its excellent comprehensive properties. However, the short time of using domestic T92 steel leads to the lack of data after long-term operation of T92 steel. Meanwhile, the differences of peak-shaving modes in operation of ultra-supercritical (ultra-supercritical) thermal power units at home and abroad result in the long-term high temperature wear of materials. The microstructure and properties of T92 steel are different during service, and the research on the mechanism is not enough. Especially, there is no effective creep life prediction method for T92 steel, let alone the validation of residual life prediction and life prediction. The evolution of microstructure and the change of mechanical properties during creep were studied. The creep life prediction equation based on Larson-Miller parameter method was used to predict the creep life of T92 steel. The cracking mechanism and mechanical properties of T92 steel are analyzed. The results show that the microstructure of T92 steel is a typical lath tempered martensite + carbide. Under the action of long-term high temperature and stress, the microstructure of the material can not maintain the stability of its original structure and gradually degenerates. The primary secondary phase segregates to the original austenite grain boundaries, subgrain boundaries and lath martensite boundaries, and aggregates and coarsens with the precipitation of the new phase Laves. The M23C6 carbide particles in T92 steel are rod-like and spherical in shape. During creep, the spherical particles are more stable, while the Laves phase generally adheres to the large-sized M. Compared with M23C6 phase, the coarsening rate of Laves phase is faster. When the Laves phase grows to a certain size, it will induce the formation of creep voids in the material. The voids aggregation and bonding will cause the fracture and premature failure of the material. Mean sizes of precipitates in stress and non-stress regions show that stress accelerates the nucleation and coarsening of Laves phase, while the mean sizes of M23C6 increase slightly. In addition, the percentage of main alloying elements (Cr, W, Mo, etc.) in the matrix decreases gradually during the whole creep process, and alloying elements gradually change from solid solution state to chemical conformity state. Secondly, the hardness of lath martensite matrix in creep specimen of T92 steel was obtained by micro-nano indentation hardness testing technique, which excluded the effect of high angle boundary on the results. Then the microstructure analysis was combined with micro-nano indentation hardness testing technique. The results show that the main strengthening factors of lath martensite matrix will be weakened in varying degrees under the action of long-term high temperature and stress, and the precipitation and coarsening of Laves phase and M23C6 phase will result in the increase of the second phase spacing during creep. The results show that the matrix hardness of T92 steel decreases with the increase of creep time. Finally, the accelerated aging data of T92 steel at 600, 649 and 700 degrees Celsius are fitted, and the logarithmic curves of T92 steel at high temperature are established. A life prediction method based on Larson-Miller parameter method is proposed. By this method, the rupture strength of T92 steel is extrapolated for 100,000 hours, which is equivalent to that of ECCC.
【學位授予單位】：華南理工大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：TG142.1
本文編號：2182362
[Abstract]:T92 steel (NF616) was further alloyed on the basis of T/P91 steel by Nippon Steel Co. in the 1990s. The content of W was increased to 1.8%, the content of Mo was reduced to 0.5%, and the content of B was increased. Compared with T/P91 steel, the durable strength of T92 steel was greatly improved, which can be used for high temperature steam pipes under 625 C. At present, T92 steel has become an ideal steel for the new generation of ultra-supercritical (ultra-supercritical) thermal power units because of its excellent comprehensive properties. However, the short time of using domestic T92 steel leads to the lack of data after long-term operation of T92 steel. Meanwhile, the differences of peak-shaving modes in operation of ultra-supercritical (ultra-supercritical) thermal power units at home and abroad result in the long-term high temperature wear of materials. The microstructure and properties of T92 steel are different during service, and the research on the mechanism is not enough. Especially, there is no effective creep life prediction method for T92 steel, let alone the validation of residual life prediction and life prediction. The evolution of microstructure and the change of mechanical properties during creep were studied. The creep life prediction equation based on Larson-Miller parameter method was used to predict the creep life of T92 steel. The cracking mechanism and mechanical properties of T92 steel are analyzed. The results show that the microstructure of T92 steel is a typical lath tempered martensite + carbide. Under the action of long-term high temperature and stress, the microstructure of the material can not maintain the stability of its original structure and gradually degenerates. The primary secondary phase segregates to the original austenite grain boundaries, subgrain boundaries and lath martensite boundaries, and aggregates and coarsens with the precipitation of the new phase Laves. The M23C6 carbide particles in T92 steel are rod-like and spherical in shape. During creep, the spherical particles are more stable, while the Laves phase generally adheres to the large-sized M. Compared with M23C6 phase, the coarsening rate of Laves phase is faster. When the Laves phase grows to a certain size, it will induce the formation of creep voids in the material. The voids aggregation and bonding will cause the fracture and premature failure of the material. Mean sizes of precipitates in stress and non-stress regions show that stress accelerates the nucleation and coarsening of Laves phase, while the mean sizes of M23C6 increase slightly. In addition, the percentage of main alloying elements (Cr, W, Mo, etc.) in the matrix decreases gradually during the whole creep process, and alloying elements gradually change from solid solution state to chemical conformity state. Secondly, the hardness of lath martensite matrix in creep specimen of T92 steel was obtained by micro-nano indentation hardness testing technique, which excluded the effect of high angle boundary on the results. Then the microstructure analysis was combined with micro-nano indentation hardness testing technique. The results show that the main strengthening factors of lath martensite matrix will be weakened in varying degrees under the action of long-term high temperature and stress, and the precipitation and coarsening of Laves phase and M23C6 phase will result in the increase of the second phase spacing during creep. The results show that the matrix hardness of T92 steel decreases with the increase of creep time. Finally, the accelerated aging data of T92 steel at 600, 649 and 700 degrees Celsius are fitted, and the logarithmic curves of T92 steel at high temperature are established. A life prediction method based on Larson-Miller parameter method is proposed. By this method, the rupture strength of T92 steel is extrapolated for 100,000 hours, which is equivalent to that of ECCC.
【學位授予單位】：華南理工大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：TG142.1
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