Correlation of Internal Carburization and Creep damage in Furnace Tubes
Introduction
Petrochemical plant produces ethylene through thermal cracking of hydrocarbons. High-temperature creep, due to the inner pressure and piping stress, and carburization which occurs in the inner wall, make life prediction of the cracking furnace tube difficult. The. HP40Nb (25Cr35Ni1Nb) heat-resistant alloy is widely used for manufacturing the cracking furnace tube, owing to the good balance of cost and high-temperature performance.
The as-cast alloy is characterized by the supersaturated austenite matrix with the continuous skeletal-shaped primary carbides (M7C3, M23C6 and NbC) at the inter-dendritic regions.). In the initial service stage, i.e. before entering the feedstock, the primary Cr-rich M7C3 carbide is prone to transformation into M23C6 carbide, while the NbC carbide transforms into the G phase (Ni16Nb6Si7) at temperatures of 700 °C to 1000 °C. Furthermore, the austenite matrix is subjected to the secondary Cr-rich M23C6 carbide precipitation[1].
After the hydrocarbons entering, the microstructural evolution in the inner wall (carburized layer) is distinctively different from the outer wall (non-carburized layer).In the non-carburized layer, the inter-dendritic M7C3 carbide is replaced by the M23C6 carbide, together with the progressive phase transformation from NbC to G phase. Number density of secondary M23C6 reduces with the growth, and the shape of the inter-dendritic M7C3 becomes blocky. Additionally, creep cavities as indicated by black in Fig. 1(e) appear at grain boundaries (GB) and the triple points, where the stress concentration builds up due to the obstruction of GB sliding.
At the final stage, the microstructure is characterized by the coarsened M7C3 and NbC precipitates, Cavity nucleation and growth occur adjacent to the M7C3 at the inter-dendritic region with the further exposure, followed by the linkage of cavities and the development of micro- and macro-cracks. To summarize, after the thermal and environmental exposure under stress, the microstructure of the HP40Nb cracking furnace tube encounters a series of evolutions [1]
Experimental Investigation
Following the above coarse of model, metallographic examination was done by surface replication and also across the thickness of failed furnace tubes Moreover, EDS analysis was performed in a Line scan to examine the gradient of carbon content across tube thickness.
The microstructural examination by surface replicas displayed dis-integrated structure of primary carbides and significant coarsening and formation of blocky carbides was evident. However, no signs of creep cavitation was evident in surface replication.
The SEM image revealed presence of internal carburized microstructure and presence of creep cavities. EDS analysis was performed across the thickness to examine variation of wt.% of carbon content. The internal surface showed maximum carbon content and gradually decreased across the thickness and almost became uniform after 2.9 mm from ID. This was marked as the depth affected by carburization. The samples contained multiple creep cavities across the thickness while the maximum concentration of damage was observed in the carburized zone.
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The micrographs from the surface replication revealed coarse network of primary carbides in the austenite matrix. Multiple isolated and aligned creep cavities and micro-fissures were present along the primary carbide network. The presence of creep damage revealed significant microstructural ageing and degradation and marks the tertiary stage of creep damage in form of micro-fissures. EDS analysis was performed across the thickness to examine variation of wt.% of Carbon content. The internal surface showed maximum carbon content and gradually decreased across the thickness and almost became uniform after (4.2+ 4.2mm) from ID. So, 8.4mm was marked as the depth affected by carburization. The samples contained multiple creep cavities across the thickness with multiple creep micro-fissures originating from the ID side of the carburized zone.
Conclusion!!
Based on the testing and examination results it was concluded that surface replication can only detect creep damage in cast furnace tubes until the damage reaches the external surface. The creep damage in furnace tubes follow a specific growth pattern as displayed in figure 9 and 10.
Secondly, the internal carburization significantly reduce the creep strength and preferential nucleation of creep cavities takes place in the vicinity of the carburized layer. Therefore future works in modelling creep cavitation should consider this important factor and keep this in consideration.
References.