ASTM A 297 grade HP steels are widely employed for radiant tubes in reforming furnaces: this class of heat resistant alloys shows high creep and corrosion resistance, ensuring good performances in extreme pressure and temperature conditions. The typical microstructure of such materials is an austenitic matrix surrounded by a network of interdendritic carbides, which contain chromium and other carbide forming elements, namely Nb, Ti, W, Zr and Y.
During long service life, these high strength materials may suffer aging or even severe damage, especially when process conditions allow coke deposition, or maintenance procedures are not carried properly.
Service aging can be summarized, for HP steels, in terms of microstructure degradation: coalescence and coarsening of interdendritic precipitates, precipitation of secondary carbides in the austenite matrix and transformation of niobium-rich carbides in the G-phase silicide are the typical phenomena occurring on the microstructure of these alloys during service. Carburization can also occur in radiant tubes, since their inner wall side is exposed to hydrocarbon-rich process fluids: carbon diffuses into the metal matrix, causing massive precipitation of chromium-rich carbides. The alloy corrosion resistance is then reduced, resulting in surface attack, cracks development and a general wastage of the material. Furthermore, the high temperatures, which tubes are exposed to, can also induce creep, especially if a local tube overheating occurs: cavities and microcracks, mainly localized at precipitates, are the typical evidences of creep damage on HP steels.
The present work is aimed on the damage characterization of several radiant tubes in HP alloys, after long term service aging in reforming plants. We employed optical and electron microscopy, EDX elements mapping and mechanical tests, in order to characterize and evaluate the various damages affecting the alloys.
Microstructure evolution has been detected in all the analyzed tubes, but we found that such a phenomenon was strictly influenced by the chemical composition of each alloy, so that in presence of small amounts of titanium and tungsten, the chemical evolution of the secondary phases was appreciably contained. Creep also was observed in all the investigated tubes and its extent was found to be related to both alloy composition and process conditions. These latter have assumed to be the main driving factor for carburization, since we observed that slight differences in temperature, pressure, chemical composition of the process fluid and tube maintenance dramatically conditioned the performances of each tube. Massive precipitation and material degradation, in fact, were found in some cases, but, on the other side, no appreciable evidence of carburization damage was observed on other cases.