Effect of cracking feedstock on carburization mechanism of cracking furnace tubes

In the present research, one of the primary problems which often arises in cracking furnaces of ethylene plants was pioneered. This problem is associated with high failure rate of heat resistant cast stainless steel tubes used in pyrolysis furnaces. The interaction between feed types and tubes which is accompanied by carburization is considered as the main origin of such a high failure rate. Hence, in this study three samples of tube (a heat resistant alloy of HP40 + Nb) involving as-cast sample (without exposure to feed), the sample exposed to liquid feed and the sample exposed to gaseous feed were studied. In this respect, two types of feed including liquid phase (i.e., naphtha), and gas phase (ethane) were selected to be compered in terms of their influence on the tube structure. In order to evaluate the interaction between the feed type and the tube destruction, microstructural changes were studied using SEM, EDS and microhardness analyses. The results revealed that higher values of hardness were obtained for the gas feedstock tubes compared to the liquid feedstock ones and as-cast sample. This can be attributed to easier carbon diffusion and more sever growth of carbide in the tube structure that makes it more brittle. Moreover, in inner surfaces of tubes, spalling of oxide layers in addition to reformation of chromium oxide on the surface cause chromium to diffuse into the metal surface, leading to reduction of the chromium content to 5 wt%. The results showed that generally the gas feedstock caused more microstructural changes and damages on the tube structure compared with the liquid feedstock with identical life services.