Investigation on microstructure and coking mechanism for Cr35-Ni45 steel tube during high temperature service

Regarding the failure of tube caused by coking, microstructure characteristics, coking mechanism and mechanical properties of Cr35-Ni45 steel tube serviced for 2.5 years were investigated by means of XRD, SEM, electron probe microanalysis and tensile tests. Experimental results show that the coke consists of filamentous coke and tar droplets, which impairs Cr2O3 oxide layer and causes serious carburization. No Cr2O3 layer and carbide depleted zone are found in the coked tube, but a thicker carburizing zone under the subsurface forms. Three types of carbides are observed in the coked tube, which are a small amount of chromium-rich M3C2 on inner surface, chromium-rich M7C3 in carburizing zone and M23C6 in the centre matrix. Moreover, mechanical properties of tube are degraded by coarsening of carbides resulting from coking. At the service temperature, the stability of precipitates in the steel is the main factor for changes of microstructure and mechanical properties in the coked tube.