Cr35Ni45钢高温长期服役过程中的组织演化规律分析

Microstructure Evolution of Cr35Ni45 Steel after High Temperature and Long Time Service

采用光学显微镜、XRD、扫描电镜和电子探针等手段对不同服役时间(原始态、服役1.5年及6年)Cr35Ni45型裂解炉管的组织形态及析出相演化规律进行系统分析。结果表明,原始铸态炉管的组织为奥氏体基体、长条状M7C3碳化物及鱼骨状NbC与基体的共晶体;高温长期服役过程中,M7C3发生向M23C6碳化物的转变,共晶碳化物NbC发生向铌镍硅化物或η相转变,同时,晶内析出大量弥散分布的颗粒状二次M23C6碳化物;随着高温服役时间的进一步延长,碳化物形态从原始的条状、鱼骨状向块状转变并逐渐连接成网,晶内析出的二次碳化物随服役时间的延长逐渐溶解合并,数量减少。造成Cr35Ni45钢高温长时服役过程中析出相结构及形貌变化的主要原因在于该钢1000℃左右的服役条件及高温下析出相的稳定性。

Microstructure evolution characteristics of Cr35Ni45 type pyrolysis furnace tubes serving for different time, including the as-cast, serving for one and a half year and for 6 years were investigated by OM, SEM, electron probe and XRD analysis. The results indicate that the microstructure of the as-cast tube is consisted of austenitic matrix, M7C3 carbide and eutectics of NbC and austenitic matrix. With the service time increasing, different phenomena occur, such as the transformation of primary M7C3 to M23C6 carbides, the transformation of NbC to Ni-Nb silicide or r/phase and precipitation of secondary M23C6 carbides. After serving for 6 years, the shape of carbides changes from strip and fishbone type to the network type, and the secondary carbides gradually gather to grain boundary, then dissolve and merger in the grains. The high service temperature up to 1000 ℃ and the stability of precipitates under that condition is the main reason of the microstructure evolution of Cr35Ni45 steel.