**1. Introduction**

Creep embrittlement is the predominant failure mechanism in cast reformer tubes in the fertilizer, petrochemical, and petroleum refining industries [1–7]. Reformer tubes are used in the process industry for the production of hydrogen by reaction between natural gas and steam in the presence of a catalyst. The reforming reaction is highly endothermic. Reformer tubes are very critical components being exposed to severe conditions of temperature and process for a long time. The tubes typically operate at temperatures over 900°C for prolonged time durations. Failure of reformer tubes in the process industry mostly results in plant shutdowns and thus results in economic loss due to lost production, in addition to replacement costs. Considerable efforts have been made in alloy development to mitigate creep embrittlement and to extend the service life of reformer tubes [8–11]. The reformer alloys are not controlled by any international specifications and are mostly proprietary. The alloys contain high contents of nickel and chromium along with minor alloying additions of elements such as niobium, titanium, silicon, etc. The alloying elements provide the required oxidation and carburization resistance along with excellent creep resistance. Nickel additionally provides a stable austenitic matrix and microstructural stability. Nickel also retards the precipitation of intermetallic phases in high-performance alloys [3, 12, 13]. The microstructure of as-cast alloys is the austenitic dendritic type with inter-dendritic carbides. In service, when exposed to high temperatures, microstructural changes occur in reformer alloys with precipitation of secondary carbides along with intermetallic secondary phases [9, 10, 14–25]. The precipitation of secondary phases that occurs during high-temperature service drastically affects the mechanical properties of the reformer tubes [7, 8, 18, 21]. The high-temperature exposure for prolonged durations can also lead to the initiation of creep voids in the material. Eventually, the creep voids will link together to form cracks and tube rupture.

The new generation reformer alloys of type 25Cr-35Ni modified with minor alloying additions typically perform well under reforming conditions at temperatures well beyond 900°C [3, 21]. The minor alloying elements added to the reformer alloys such as titanium, niobium, zirconium, tungsten, etc., provide a fine dispersion of carbides that are stable at temperatures well in excess of 900°C [8–10, 16, 21]. The elements also promote the fragmentation of the as-cast microstructure and partial replacement of chromium-rich carbides by more stable alloy carbides [9]. The distribution and geometry of carbides play an important role in imparting creep resistance to the reformer alloys [6]. Optimally distributed fine carbides act to restrict the movement of dislocations, thus enhancing the creep resistance [3]. The primary carbide network typically consists of chromium-rich Cr23C6 and niobium-rich carbides, which get enriched with other alloying elements during high-temperature service [3, 22]. Continued exposure of the tubes to high temperatures can lead to coarsening and coalescence of the precipitated secondary carbides [3, 7, 26]. The reformer tubes were designed for a life of 100,000 hours at the operating temperature. Although reformer tubes have a service life of 12–15 years, premature failure of tubes is often encountered mainly because of microstructural degradations due to overheating during service. Overheating of tubes even for short durations can lead to precipitation and coalescence of carbides in 25Cr-35Ni reformer tubes, in turn leading to premature failure predominantly by creep embrittlement [27]. There are reports of premature failure of reformer tubes due to overheating and the resultant creep [1, 3–7, 26, 27]. HP40Nb microalloy grade reformer tube which was operating at 880°C suffered creep damage and failure in 2 years of service when the tubes experienced temperature excursion up to 1150°C for a short period of 5–10 minutes [1]. Similarly, the HP40Nb reformer tube failed in 7 years of service when the tubes operating at 870°C were exposed to temperatures higher than the design for short durations. The tensile strength and elongation of the tube specimens exposed to 1000°C were significantly lower due to the microstructural degradation [7]. The overheating of the tubes accelerates the dissolution of secondary carbides and coarsening of primary carbides at the inter-dendritic boundaries [1, 5, 7], leading to the initiation of creep cavities. The failure initiates with the nucleation of cavities and their evolution into fissures and microcracks and final rupture [1, 5].

The 25Cr-35Ni reformer alloys are termed HP alloys [28, 29]. The alloys contain about 0.40% carbon. The high carbon content in the alloy provides the required high-temperature strength and resistance to creep by forming carbides with the alloying elements. The carbide precipitation along the grain boundaries restricts grain

#### *Creep Failure of 25Cr-35Ni Centrifugally Cast Reformer Tube DOI: http://dx.doi.org/10.5772/intechopen.108766*

boundary sliding, while finely dispersed carbides within the grains provide strength, both together impart high-temperature strength and creep resistance [3].

Reformer tubes are manufactured through the centrifugal casting process [28]. The centrifugal force during the casting process produces a hollow cylinder product with minimal wall thickness variations. Directional solidification of the tube starting from the outer surface in contact with the mold leads to a high-quality sound cast metal, free of inclusions and cavities. The reformer tubes are made by pouring the molten alloy with controlled chemistry into a refractory-lined rotating die. The charge is pre-melted in an electric induction furnace. The die rotates at high speeds, which facilitates uniform metal thickness on the inside surface of the die. The molten metal quickly solidifies starting from the die-metal interface. Due to the cooling effect of the die, metal solidifies quickly forming columnar grains, extending from the outside of the tube toward the inner surface. As the solidification of molten alloy progresses, the rate of solidification slows down resulting in the formation of equiaxed grains near the inner tube surface. The Proper control of parameters is critical to ensure the manufacture of reformer tubes with good quality. The large rotational forces drive the lighter suspended particles, such as non-metallic phases and gas bubbles to the inner liquid phase before the complete solidification of the tube. The tubes are thus supplied with the inner surface machined to remove the layer containing defects. The tubes are typically manufactured with a rough external surface. The rough external surface gives a better radiation heat transfer. After the casting process, the tubes are subjected to inspection and testing to ensure that they are supplied defect free. The inspection includes visual inspection, liquid penetrant, and eddy current inspection, while the tests conducted are to ensure adherence to chemical composition and mechanical properties.

Despite continuous efforts in alloy development and creep life predictions, there are incidents of premature failure of reformer tubes in the fertilizer, petrochemical, and petroleum refining industries [1, 3–7, 26, 30, 31]. The paper discusses an investigation conducted on the premature failure of a 25Cr-35Ni reformer tube. The failed reformer tube was in hydrogen production service for 8 years at a temperature of 880°C. Since the tubes were designed considering the service life of 100,000 hours, the failure of the tube in 8 years is considered a premature failure, and conducted an investigation. The investigation involved microstructural studies and the assessment of mechanical properties. The properties of the service-exposed tubes were compared with that of a new alloy.
