HP40裂解炉管组织及裂纹产生原因分析

本文用SEM、EDS、XRD对开裂的HP40裂解炉管进行分析,发现多数裂纹均位于焊缝附近,且均沿枝晶间和晶界开裂的碳化物扩展.经EDS成分分析和XRD结构分析,这些开裂的碳化物为M23C6和富Nb、Si的碳化物.新炉管组织为奥氏体和枝晶间分布的M7C3及高Nb、Si的共晶碳化物.新旧炉管比较可知,开裂炉管的结构和形态在长时间高温下均发生了明显变化,即M7C3转变为M23C6且明显粗化,在晶界呈连续分布.这种脆而硬的碳化物在组织应力和热应力作用下碎裂成为潜在的裂纹源,它们加速了裂纹扩展进程,缩短了炉管的使用寿命.沿裂纹打开断口发现,整个断面被这两种碳化物所覆盖.这进一步证实了造成裂解炉管早期开裂的原因是这种沿晶界连续分布的粗大碳化物的存在.

Analysis on Microstructure and Crack of HP40 Cracking Furnace Tube

HP40 cracking furnace tube was analyzed by means of SEM,EDS,XRD. The result displays that most of cracks near the welding region and expanding along carbides which in the dendrite and grain boundaries. Carbides were the summation of M23C6 and one kind of carbide that had rich Nb and Si. For the new cracking furnace tube, its microstructure included austenite and the eutectic of M7 C3 and the Silicon - rich and Niobium - rich carbides. The structure and form of the cracking furnace tube changed obviously at high temperature for a long time. M7 C3 changed into M23 C6 which distributing in the grain boundaries continuously and coarsening obviously. These hard and brittle carbides breaking under the action of structure stress and thermal stress become the potential crack initiation. They speeded up the course of cracks expanding and shorten service life of cracking furnace tube assuredly. Opened fracture along the crack, cross - section which covered with two kinds of carbides was discovered. This result approved further that the reason on splitting of the tube early was the presence of coarse carbides which distributing in the grain boundaries continuously.