本文選題：T23鋼　切入點：再熱裂紋　出處：《上海交通大學》2015年博士論文

【摘要】：T23鋼是一種新型鐵素體耐熱鋼,由于其具有良好的焊接性和較高的蠕變強度,被廣泛應用于超(超)臨界鍋爐的水冷壁、過熱器等組件。然而,在制造廠和電廠的T23鋼膜式水冷壁焊接接頭中均出現(xiàn)了裂紋,導致了水冷壁管的爆管及泄漏,嚴重影響了機組投運后的安全。因此,對T23鋼焊接接頭再熱裂紋(RC/SRC)的敏感性進行更深入的研究具有重要意義。采用實際焊接接頭的斜Y型坡口試驗及基于Gleeble-3500熱力耦合試驗機的熱模擬CGHAZ(coarse-grained heat-affected zone)的STF(strain-to-fracture)試驗,對國產T23鋼CGHAZ再熱裂紋敏感性進行評估。采用透射電鏡(TEM),電子背散射衍射(EBSD)及Jmat Pro軟件對晶界析出相,裂紋所在晶界的取向差分布,以及晶界附近應變的協(xié)調過程進行表征分析。結果表明,再熱裂紋在不同溫度下的敏感性受晶界及晶內的相對強度影響。晶界強度由晶界取向差,晶界析出相共同作用,晶內強度由晶內二次析出相及晶內位錯密度共同決定。此外,建立了再熱裂紋位置與晶界取向差的關系,為從晶界工程角度降低裂紋敏感性提供了理論依據(jù)。為闡明影響晶內和晶界強度因素的作用,進行了試驗設計。通過750℃焊后熱處理來調控晶內強度,采用STF試驗對再熱裂紋敏感性進行評估,結合SEM,EBSD,TEM及小角度衍射(SAXS)對析出相種類,位錯密度,空洞及微裂紋位置的變化進行了研究。結果表明,一定時間范圍內(1min-1.5h),位錯密度的快速下降有效軟化晶內強度,塑性變形主要在晶內而非在晶界上發(fā)生,再熱裂紋敏感性下降。超過該時間范圍后,晶內位錯密度下降速度放緩,同時晶內析出了大量的MC相,使得晶內強度有所增加,塑性變形可能會通過晶界的開裂進行協(xié)調;但是,由于在晶界上新析出了尺寸較小且與基體有較好共格/半共格關系的M23C6,它與其他碳化物相比增加了晶界的相對強度,因而抵消了晶內強度增加帶來的不利影響。由于晶內及晶界強度的同時增加,塑性變形只能通過強度較低的block的開裂或者剪切的方式進行協(xié)調,因此,再熱裂紋敏感性依然較小。上述結果為焊后熱處理工藝的設計提供了指導,以確保焊接接頭投入運行后能夠安全服役。本文還設計了一種臨界熱循環(huán)工藝,該工藝改變了晶界特征。采用TEM和EBSD對臨界熱循環(huán)工藝得到的再結晶晶粒的晶體學特征,晶界取向差分布,應變集中程度進行了表征和分析,研究表明,再結晶晶粒降低了原奧氏體晶界上大角度晶界的比例,并增加晶界的曲折度,阻礙裂紋沿著原奧氏體晶界進行直線擴展。另外,再結晶中的M/A(martensite-austenite)組元消耗了一定的碳元素,造成析出相在原奧氏體晶界上難以粗化長大。同時M/A組元形成時帶來的應變集中避開了原奧氏體晶界。該工藝實現(xiàn)了晶界的有效強化,降低了再熱裂紋敏感性。最后基于再熱裂紋的開裂機理及影響晶內和晶界強度的因素,構建了T23鋼CGHAZ再熱裂紋的開裂模型。此模型的建立為T23鋼焊接接頭的焊后熱處理提供了依據(jù),并為其他材料再熱裂紋的研究提供了新的方法和思路。
[Abstract]:T23 steel is a new type of heat-resistant ferritic steel, because of its high creep strength good weldability and high, is widely used in ultra (ultra) supercritical boiler water wall, superheater and other components. However, welding on T23 steel factory and power plant membrane type water wall joints are the crack, LED tube explosion and leakage of the water wall tube, seriously affecting the unit after operation safety. Therefore, the T23 steel welding joint of reheat cracking (RC/SRC) susceptibility to further research has important significance. The actual welding cable Y groove joint test and thermal simulation of CGHAZ Gleeble-3500 thermal coupling based on the testing machine (coarse-grained heat-affected zone) STF (strain-to-fracture) test, evaluation of domestic T23 steel CGHAZ reheat cracking. By transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and Jmat Pro software on grain boundary precipitation The grain boundary phase, crack orientation difference distribution, and grain boundary strain coordination process were analyzed. The results show that the reheat cracking susceptibility at different temperatures by the relative strength of effect of crystal boundary and in the grain boundary strength by grain boundary misorientation, grain boundary precipitates interaction, intragranular strength by intergranular phase and two a crystallization of dislocation density in the joint decision. In addition, to establish the relationship between the reheat crack position and boundary misorientation, provides a theoretical basis for reducing the crack sensitivity from grain boundary engineering perspective. To elucidate the influence factors in the grain and grain boundary strength, the experimental design. Intragranular strength controlled by 750 DEG C after welding heat treatment with STF test of reheat cracking susceptibility was assessed with SEM, EBSD, TEM and small angle diffraction (SAXS) on the precipitate, dislocation density, void variation and micro crack location are studied . the results show that within a certain time (1min-1.5h), the rapid decline of the dislocation density effectively soften intragranular strength, plastic deformation is mainly in the grain rather than in grain boundaries, decrease reheat cracking. Over the time range, intragranular dislocation density and the rate of decline slowed, intragranular precipitation of a large number of MC the crystal phase, strength increase, plastic deformation may be coordinated through the grain boundary cracking; however, due to the new grain boundary precipitation is smaller in size and with the matrix has good coherent / semi coherent M23C6, compared with other carbide increases the relative strength of the grain boundary, and thus offset the increase intragranular strength adverse effects. Due to the increase of grain and grain boundary strength and plastic deformation only through coordination, lower strength block cracking or shear mode. Therefore, reheat crack sensitivity is still small. Provides guidance for the design of the above results for heat treatment after welding, to ensure welding joint operation safe service. The paper also designs a critical heat recycling process, the process changes the grain boundary characteristics. The recrystallization crystal characteristics of TEM and EBSD on the critical thermal cycling process, grain boundary misorientation the distribution, degree of strain concentration were characterized and analyzed, research shows that the recrystallization grain reduces the original austenite grain boundaries the proportion of high angle grain boundaries, and increased grain boundary tortuosity, block the crack along the original austenite grain boundary line expansion. In addition, the recrystallization of M/A (MARTENSITE-AUSTENITE) component of carbon consumption of certain that caused the precipitates at the grain boundaries to coarsening. At the same time bring strain M/A component is formed when the concentration from the original austenite grain boundary. The process of implementation of the grain boundary Effect of strengthening, reduce the reheat cracking. Finally, based on the cracking mechanism of reheat crack and the influence factors in the grain and grain boundary strength, construct the model of CGHAZ cracking reheat crack of T23 steel. Provide the basis for the establishment of this model for T23 steel welded joints after welding heat treatment, and provides the new research methods and ideas for other materials reheat crack.

【學位授予單位】：上海交通大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG142.73

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