服役态Cr35Ni45Nb合金高温真空渗碳行为及相演化机理研究  

HIGH TEMPERATURE VACUUM CARBURIZATION BEHAVIORS AND PHASE EVOLUTION MECHANISMS OF Cr35Ni45Nb ALLOY UNDER SERVICE CONDITION

作　　者：彭以超 张麦仓[1] 杜晨阳[2] 董建新[1] 

机构地区：[1]北京科技大学材料科学与工程学院,北京100083 [2]中国特种设备检测研究院,北京100013

出　　处：《金属学报》2015年第1期11-20,共10页Acta Metallurgica Sinica

基　　金：国家高技术研究发展计划资助项目2012AA03A513~~

摘　　要：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.Carburization in Ni-Cr-Fe-based alloys is an important phenomenon, especially in ethylene cracking tubes which serve at high temperatures under highly carburizing environment. In this work, the Cr35Ni45 Nb tube subjected to service condition for 6 a was carburized by low-pressure vacuum carburizing（LPVC） at 1080 ℃.The carburization behaviors and corresponding mechanisms of phase evolution in the inner wall were comprehensively analyzed through SEM, XRD and EPMA. The results showed that oxidation behaviors of the tube at high temperature were consisted of external oxidation of Cr and internal oxidation of Si, resulting in formation of com-posite oxide scales. Depletion of Cr in the subsurface caused by surface Cr2O3 leaded to carbide dissolution and formation of carbide free zone and carburized zone. The critical concentration of Cr for carbide dissolution is about19.0%（mass fraction）. By comparing carburization behaviors of specimens whose oxide scales were retained or removed, the carburization resistance of the composite oxide scales in carburizing environment was systematically investigated. The results showed that the composite oxide scales formed previously acted as an effective barrier to carbon infiltration. However, the outermost Cr2O3 scale tended to be carbonized to form carbide scale to spall from the surface in the strongly reducing environment with low oxygen partial pressure, while the Si O2 kept stable all along due to its excellent thermodynamic stability. However, a certain amount of carbon was still capable to penetrate the alloy interior through gaps of the Si O2 scale due to its discontinuity. Therefore, continuity, density and high-temperature stability of the oxide scales were crucial for the alloy to achieve excellent anti-carburizing performance. Once the oxide layers were removed or carbonized adequately, inconceivable internal carburization occured widely. Large amounts of secondary carbides precipitated again in the previous carbide free zone due to high carbon activity. Wides

关 键 词：Cr35Ni45Nb合金 真空低压渗碳 抗渗碳性 氧化膜 金属尘化 

分 类 号：TG113[金属学及工艺—物理冶金] TG132[金属学及工艺—金属学]