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采用乙炔真空渗碳方式对服役6 a的乙烯裂解炉管合金Cr35Ni45Nb进行了加速渗碳处理,并利用SEM,XRD及定量电子探针等手段对渗碳前后炉管内壁的渗碳行为及相演化机理进行系统研究.结果表明,炉管内表面形成了较厚的Cr2O3/Si O2复合氧化层.复合氧化层表现出良好的抗腐蚀能力,是阻止C渗入炉管内部的有效障碍.材料的抗渗碳能力主要取决于表层Cr2O3的连续性、致密性和亚表层Si O2的稳定性.在低氧分压且具有还原性的气氛中,表层的Cr2O3层逐渐碳化为Cr3C2,并且Cr3C2逐渐剥离和脱落,使得保护性Cr2O3膜的抗渗碳能力逐渐减弱甚至消失.亚表层的Si O2虽具有优良的热力学稳定性,但Si较低的活度使得Si O2层不够连续,仍有部分C从氧化层空隙间渗入.当移除该复合氧化层或者渗碳时间足够长使得复合氧化层抗渗碳能力急剧减弱时,炉管材料内部由于发生严重的内部渗碳使得组织结构发生了显著变化:枝晶间碳化物严重合并和粗化,并原位发生由M23C6到M7C3的转变,同时在碳化物内部析出类似于离异共析状的蠕虫状g相.距表面越近,C活度越高,导致在约0.5 mm深的范围内发生以大量石墨析出为特征的金属尘化现象.碳化物的严重合并粗化以及金属尘化现象会造成炉管组织的严重弱化、宏观裂纹的产生以及炉管服役寿命的降低.
The acetylene vacuum carburizing method was used to accelerate the carburizing treatment of ethylene cracking furnace tube alloy Cr35Ni45Nb, which was used for 6 years. SEM, XRD and quantitative electron probe were used to analyze the carburizing behavior and phase evolution Mechanism.The results show that a thick Cr2O3 / Si O2 composite oxide layer is formed on the inner surface of the tube, and the composite oxide layer shows good corrosion resistance and is an effective barrier to prevent C from penetrating into the interior of the tube. The carbon ability mainly depends on the continuity and compactness of surface Cr2O3 and the stability of subsurface Si O2. In the atmosphere of low oxygen partial pressure and reducing, the surface Cr2O3 layer is gradually carbonized into Cr3C2, and Cr3C2 gradually peels off , So that the protective carburizing ability of Cr2O3 film gradually weakened or even disappear.Although subsurface Si O2 has excellent thermodynamic stability, but the lower activity of Si makes the Si O2 layer is not continuous enough, there are still some C from the oxide layer Interstitial infiltration.When the composite oxide layer is removed or the carburizing time is long enough so that the composite oxide layer carburizing ability abruptly decreased, the internal tube material due to the occurrence of a serious internal carburization of the knot A significant change occurred in the interdendritic carbide carbides serious coarsening and coarsening, and in situ M23C6 to M7C3 transition occurred, while precipitated inside the carbide isolates ecliptic wormlike g phase.From the closer to the surface, The higher the C activity, resulting in the occurrence of a large amount of graphite precipitates within about 0.5 mm of the deep metal precipitation phenomenon characterized by severe coarsening of carbide and metal dusting phenomenon will cause serious weakening of the furnace tube structure, macroscopic Crack generation and furnace service life reduction.