Effects of the Localized Overheat On the Microstructure … · Effects of the Localized Overheat On...
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Effects of the Localized Overheat On the Microstructure of Centrifugally Cast HP-Steels Used In Steam Reformer Furnaces L. H. de Almeida 1 , M. C. Mendes 2 , J. Dille 3 1 PEMM-COPPE/UFRJ. P.O.Box 68505. 21941-972 Rio de Janeiro RJ Brazil, [email protected] 2 Tenaris S.A. - Centro de Pesquisas e Desenvolvimento Brasil 3 ULB – 4MAT [email protected] Heat-resistant centrifugally cast grade HP stainless steels, ASTM A297, are used in the petrochemical industry as radiant tubes in steam reformer furnaces. The mechanical design of the catalytic tubes usually take 100.000 h as a reference life. Nowadays, the requirement of higher productivity have raised the demand for improved performance, in this context Nb and Ti were added resulting in the modified HP steels with a finer and more distributed secondary precipitation, a less-continuous dendritic and more stable primary dendritic carbide network, and also a smaller volume fraction of transformed G-Phase (Ni 16 Nb 6 Si 7 )[1]. These microstructural characteristics make this material more appropriate to the harsh operational conditions inherent to reformer tubes, presenting high strength and high creep resistance at elevated temperatures [1,2]. During service life, the gradual deterioration of the catalyst can result in a restriction of the feedstock flow, that cools the catalytic tubes located in the radiation chamber without reducing the fuel burn rate. This generated thermal instability can result in a temperature surge, exceeding 1000°C, large enough to cause the failure of the catalyst tubes through formation of large longitudinal cracks [2]. Mainly owing to overheating, some uncertainties remain concerning the integrity of tubes that do not collapse during an overheating event and the possibility of reusing them. In this context, the aim of this work is evaluate the effects of the localized overheating on these modified HP steels, aged in different conditions during long term exposure in service by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural characterization was performed using seven samples removed from different regions, including close to the longitudinal cracks, of 10 mm wall thickness operational tubes. Metallographic specimens were prepared by conventional grinding and polishing techniques without chemical etching for scanning electron microscopy examination. Those for transmission electron microscopy examination were prepared by electropolishing and Focused Ion Beam (FIB) techniques. SEM sample observations were carried out in three regions along the wall thickness using backscattered electrons and energy-dispersive X-ray spectroscopy (EDS) for the primary identification of precipitates. TEM observations, to identify precipitates, were performed using EDS and electron diffraction. The results showed that, although the SEM analysis has indicated a total dissolution of the secondary precipitation after the temperature surge, TEM analysis allowed a better comprehension of this effect suggesting, in fact, a dissolution followed by a reprecipitation of these precipitates. Related to the NbC → G-Phase transformation, the results have shown a different behavior of the overheated samples, suggesting a possible reprecipitation of the NbC carbides over the G-Phase after the temperature surge.
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Effects of the Localized Overheat On the Microstructure of Centrifugally Cast HP-Steels Used In Steam Reformer Furnaces
L. H. de Almeida1, M. C. Mendes2, J. Dille3
1 PEMM-COPPE/UFRJ. P.O.Box 68505. 21941-972 Rio de Janeiro RJ Brazil, [email protected] Tenaris S.A. - Centro de Pesquisas e Desenvolvimento Brasil3 ULB – 4MAT [email protected]
Heat-resistant centrifugally cast grade HP stainless steels, ASTM A297, are used in the petrochemical industry as radiant tubes in steam reformer furnaces. The mechanical design of the catalytic tubes usually take 100.000 h as a reference life. Nowadays, the requirement of higher productivity have raised the demand for improved performance, in this context Nb and Ti were added resulting in the modified HP steels with a finer and more distributed secondary precipitation, a less-continuous dendritic and more stable primary dendritic carbide network, and also a smaller volume fraction of transformed G-Phase (Ni16Nb6Si7)[1]. These microstructural characteristics make this material more appropriate to the harsh operational conditions inherent to reformer tubes, presenting high strength and high creep resistance at elevated temperatures [1,2].During service life, the gradual deterioration of the catalyst can result in a restriction of the feedstock flow, that cools the catalytic tubes located in the radiation chamber without reducing the fuel burn rate. This generated thermal instability can result in a temperature surge, exceeding 1000°C, large enough to cause the failure of the catalyst tubes through formation of large longitudinal cracks [2]. Mainly owing to overheating, some uncertainties remain concerning the integrity of tubes that do not collapse during an overheating event and the possibility of reusing them. In this context, the aim of this work is evaluate the effects of the localized overheating on these modified HP steels, aged in different conditions during long term exposure in service by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microstructural characterization was performed using seven samples removed from different regions, including close to the longitudinal cracks, of 10 mm wall thickness operational tubes. Metallographic specimens were prepared by conventional grinding and polishing techniques without chemical etching for scanning electron microscopy examination. Those for transmission electron microscopy examination were prepared by electropolishing and Focused Ion Beam (FIB) techniques. SEM sample observations were carried out in three regions along the wall thickness using backscattered electrons and energy-dispersive X-ray spectroscopy (EDS) for the primary identification of precipitates. TEM observations, to identify precipitates, were performed using EDS and electron diffraction. The results showed that, although the SEM analysis has indicated a total dissolution of the secondary precipitation after the temperature surge, TEM analysis allowed a better comprehension of this effect suggesting, in fact, a dissolution followed by a reprecipitation of these precipitates. Related to the NbC → G-Phase transformation, the results have shown a different behavior of the overheated samples, suggesting a possible reprecipitation of the NbC carbides over the G-Phase after the temperature surge.
[1] de Almeida, L.H., Ribeiro, A.F., Le May, I., “Microstructural characterization of modified 25Cr-35Ni centrifugally cast steel furnace tubes”, Materials Characterization, vol 49, pp. 219-229, 2003.
[2] Perez, I.U., Júnior, L.N., Bueno, L.O., de Almeida, L.H., Silveira, T.F. “Short Duration Overheating in a Steam Reformer: Consequences to the Catalyst Tubes, Journal of Failure Analysis and Prevention, vol 13, 2013.
Figure 1. SEM images of the G-Phase in different conditions, a) aged condition and b) after the temperature surge. 2000X.
Figure 2. TEM images of secondary precipitation, in A aged condition and in B overheating condition. 53000X.
