Creep Failure of 25Cr-35Ni Centrifugally Cast Reformer Tube

Abstract

Cast 25Cr-35Ni alloys are extensively being used in the petrochemical and petroleum refining industries for high-temperature applications. A typical application of such alloys in the industry is in the manufacture of cast catalyst reformer tubes for the production of hydrogen. The cast 25Cr-35Ni catalyst reformer tubes possess the required mechanical properties, creep resistance, oxidation resistance, and high-temperature stability. Though reformer tubes are designed to give a service life of over 100,000 hours at temperatures beyond 900°C, there are incidents of failure due to creep damage, which is the predominant failure mechanism in reformer tubes. The paper discusses an investigation conducted on the premature failure of a 25Cr-35Ni reformer tube. The investigation involved microstructural assessments and the evaluation of mechanical properties. The microstructure and mechanical properties of the service-exposed reformer tube were also compared with a new tube. The investigation revealed that the failure of the tube was due to creep embrittlement. The creep embrittlement was due to the microstructural degradations that occurred as a result of overheating. Adherence to the design and operational parameters is critical in mitigating creep embrittlement failures.

1. Introduction

Creep embrittlement is the predominant failure mechanism in cast reformer tubes in the fertilizer, petrochemical, and petroleum refining industries [1, 2, 3, 4, 5, 6, 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, 9, 10, 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, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 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, 9, 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, 4, 5, 6, 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 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, 4, 5, 6, 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.

2. Experimental

The chemical analyses of the service-exposed tube and the new tube were conducted by optical emission spectrometry (OES). The service-exposed reformer tube section was subjected to visual examination to understand the macro features of failure. The features of the tube section were recorded using a digital camera. After the visual examination, specimens were cut from a location near the location of rupture for microscopic assessments and hardness measurements. The microscopic assessments were aimed at identifying the presence of cavities and cracks characteristics of creep damage [1, 2, 3], while the hardness measurements were made to understand the effect of secondary phase precipitation on the hardness [1, 2, 3] of the alloy. The specimens were mounted and metallographically prepared for the examination of the cross-sections. The mounting of the specimens was done in phenolic powder using a Buehler Simplimet 3000 automatic mounting press. A Buehler Automet 250 with Ecomet 250 power head was used for the grinding and polishing of the mounted specimens. The metallographic examinations were made in as-polished and etched conditions. The etching of the metallographically prepared specimens was made using 10% oxalic acid electrolytically at an applied voltage of 6 V, as reported elsewhere [3]. The optical microscopic examination was carried out using a Zeiss Axio Observer Z1m microscope to study the microstructure and to observe the cavities. The specimens prepared from the service-exposed tube section were subjected to scanning electron microscopic (SEM) examinations using TESCAN TS 5135 Vega to further examine the cavities and microstructure. Chemical characterization of the precipitated phases was carried out using Oxford Xmax 20 energy dispersive spectrometer (EDS) attached to the SEM. Microhardness measurements of the samples were carried out on the reformer tubes at a 200 g load with a dwell time of 5 s, while hardness measurements were made on the tube samples at a load of 5 kg and a dwell time of 5 s. Tensile tests were performed on the service-exposed and new tubes at room temperature as per ASTM A370 [32].

The secondary phases precipitated in the service-exposed reformer tube were extracted using electrochemical extraction as reported elsewhere for the extraction of secondary phases from the service-exposed stainless steel grade 347 heater tube [33] for the chemical characterization. The composition of the electrolyte was 5 g oxalic acid and 200 ml hydrochloric acid made up to 1000 ml in distilled water. A sample measuring about 10 cm × 1 cm × 1 cm was cut from the reformer tube and prepared to 220 grit finish by grinding using a water-cooled abrasive paper and grinding machine. Electrochemical extraction was carried out in a 250-ml beaker. The reformer tube sample served as the anode, and a stainless steel piece served as the cathode. The cell voltage was controlled at 1.5 V. The extraction process continued till a large section of the test specimen was dissolved. The secondary phase particles settled at the bottom of the beaker were collected and washed with distilled water. After repeated washing, the extracted secondary phase particles were separated by a centrifuge and dried in an oven. The collected secondary phase particles were characterized using a Panalytical X’pert X-ray diffractometer (XRD).

3. Results and discussion

The chemical composition of the service-exposed and new reformer tube samples are given in Table 1. The analyses meet the requirements of HP40-Nb reformer heater tubes for chemical composition.

Tube material	C	Cr	Ni	Fe	Si	Mn	Nb
Service-exposed	0.41	24.1	35.2	36.8	1.14	1.12	0.85
New tube	0.46	23.8	33.8	38.7	1.22	0.89	0.74

Table 1.

The chemical composition of the reformer tube samples.

A photograph of the service-exposed tube sample with a closer view of the cracks is shown in Figure 1. The sample showed cracks opened up in the longitudinal direction. The crack edges were thick-lipped and appeared brittle in nature. The tube section exhibited a limited degree of expansion but did not show any localized bulging. The macroscopic features of the failed tube section were similar to the reformer tubes that failed due to overheating and creep damage [1, 26]. The general appearance of the tube surface was black in color.

Figure 1.

Photograph of the service-exposed reformer tube sample.

Optical micrographs of the service-exposed tube and the new tube are shown in Figures 2–5. The micrographs of the service-exposed tube in the as-polished condition showed a dendritic network of primary carbides with numerous voids (Figure 2a). The voids were predominantly within the primary carbide network. The aligned voids were also linked at some locations and formed fissures (Figure 2b). The micrograph of the new tube also showed a dendritic network of primary carbides, but less continuous in comparison to that observed in the service-exposed tube (Figure 3). Voids or other imperfections were absent in the new tube sample. The micrographs of the service-exposed tube in the etched condition are shown in Figure 4. The formation of cracks by the linkage of aligned voids is evident in the micrographs. The primary carbides precipitated at the interdendritic boundaries appeared to have been coarsened to form a continuous network of carbides. In addition to the primary carbide network, the micrographs also showed numerous finely distributed secondary carbides within the grains. The secondary carbides were also found agglomerated at some locations. Similar observations on the presence of voids, linkage of aligned voids, precipitation of secondary carbides, and coalescence of carbides have been reported in the literature [1, 2, 3, 9, 14, 17, 34, 35]. The micrographs of the new tube sample in the etched condition showed the primary carbide network, while the secondary carbides were absent (Figure 5).

Figure 2.

Optical micrographs of the service-exposed reformer tube in the as-polished condition.

Figure 3.

Optical micrographs of the new reformer tube in the as-polished condition.

Figure 4.

Optical micrographs of the service-exposed reformer tube in the etched condition.

Figure 5.

Optical micrographs of the new reformer tube in the etched condition.

Figures 6 and 7 show the SEM micrographs of the service-exposed tube sample. The presence of voids and the formation of fissures by the linkage of aligned voids are evident in the micrographs. The voids are predominantly initiated within the dendritic primary carbide network, at the interface between the carbide precipitate and the matrix. The micrographs further showed that the dendritic network was comprised of different precipitates as indicated by different contrasts in the backscattered image (Figure 7a). The initiation of voids and formation of cracks by the linkage of voids within the carbide network has been reported in reformer tubes that suffered creep failure [1, 2, 3, 4]. The creep voids typically initiate at the primary carbide-matrix interface.

Figure 6.

SEM micrographs of the service-exposed reformer tube sample.

Figure 7.

SEM micrographs of the service-exposed reformer tube showing carbide precipitates and voids within the carbide precipitate. The EDS spectra obtained for the precipitates marked in Figure 7a are shown in Figure 8.

The EDS spectra obtained for the precipitates with different contrasts as seen in Figure 7a are shown in Figure 8. The spectra indicate the enrichment of different elements in the precipitate. The brightest precipitates are enriched in niobium, with relatively minor amounts of other alloying elements such as chromium, nickel, and iron (Figure 8a). The precipitate is most likely niobium-rich carbide. The light gray precipitate is enriched in chromium with some amounts of nickel, niobium, silicon, and iron (Figure 8b). G-phase is rich in niobium and silicon along with chromium. The darker precipitate is chromium-rich (Figure 8c) along with iron and nickel. These precipitates are likely to be chromium-rich carbides. The spectrum obtained for the matrix shows peaks corresponding to iron, chromium, and nickel, the alloying elements present in the tube material (Figure 8d). Chemical characterization of phases precipitated in service-exposed reformer tubes has been the subject of several studies and the studies have characterized the precipitated phases [7, 9, 17, 22, 23, 24, 25]. Niobium-rich carbide, niobium-silicon- rich G-phase, and chromium carbide and typically identified in the service-exposed reformer tube samples. The present results are also well in agreement with the reported studies. The precipitation of such phases is a precursor to the initiation of creep voids and creep embrittlement of reformer alloys that experienced overheating [7, 10, 14, 15].

Figure 8.

EDS spectra obtained for the precipitates observed in the service-exposed reformer tube sample at locations marked in Figure 7a.

Figure 9 shows the XRD spectrum obtained for the phases extracted from the service-exposed reformer tube sample. The phases that precipitated in the service-exposed tube sample are chromium carbides (M₂₃C₆ and M₇C₃), niobium carbide, and austenite. The presence of the phases is evident in the EDS spectra of the carbides also. The presence of these carbides in the service-exposed tube sample is well in agreement with the XRD data reported in the literature [36, 37, 38]. The precipitation and coarsening of carbides occur in reformer alloys during long-term exposure at elevated temperatures. Such precipitated phases significantly affect the properties of reformer alloys, due to the chemical nature of the precipitated phases and the crystallographic mismatch [6, 10].

Figure 9.

XRD spectrum of the secondary phases extracted from the service-exposed reformer tube sample.

The measured hardness values of the service-exposed and new reformer tube samples are given in Table 2. The hardness values indicate the higher hardness of the respective phases in the service-exposed tube sample in comparison to the new reformer tube sample. The higher hardness observed for the respective phases in the service-exposed reformer tube sample is due to the precipitation and subsequent coarsening of the secondary carbides that occurred due to exposure to high temperatures. The phases that precipitate in the alloy due to high-temperature exposure are hard and thus leading to an increase in the hardness.

Tube material	Microhardness (HV)		Hardness (HV)
	Matrix	Dendritic carbide
Service-exposed	197	219	213
New tube	164	206	198

Table 2.

The hardness of the reformer tube samples.

The tensile properties of the service-exposed and the new reformer tube samples are given in Table 3. The service-exposed reformer tube sample showed a relatively higher yield strength and tensile strength in comparison to the new tube sample. The service-exposed tube possessed an elongation of only 3% compared to 12% observed for the new reformer tube sample. The microstructural changes that occurred in the alloy during exposure to high temperatures i.e., precipitation of secondary carbides and coalescence of carbides, induced brittleness in the alloy, thus resulting in a significant drop in the ductility of the alloy. The secondary phases that precipitated during exposure to the high temperature are brittle and hard. The hardness measurements also indicated an increase in the hardness of the service-exposed tube sample.

Tube material	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
Service-exposed	398	481	3
New tube	368	456	12

Table 3.

The tensile properties of the reformer tube samples.

The microstructure of the service-exposed tube contained finely distributed secondary carbides, in addition to the primary dendritic carbide network. The primary carbide network was more continuous in the service-exposed tube. The precipitated secondary carbides have coalesced at some locations. The observed microstructure of the service-exposed reformer tube is typical of centrifugally cast reformer tubes exposed to high temperatures [1, 2, 3]. The optical and electron microscopic studies revealed the presence of voids and micro-cracks in the service-exposed sample. The cracks have been formed by the linkage of voids and followed the interdendritic zones. The observed voids and micro-cracks are typical of creep damage [1, 2, 3]. The presence of creep voids and fissures in the service-exposed tube indicates that the failure of the service-exposed reformer tube was due to creep embrittlement.

The fine and uniformly distributed secondary carbides provide the required high-temperature strength, while carbide precipitation at grain boundaries strengthens grain boundaries [3]. High-temperature strength and creep resistance are important properties required for reformer alloys. As the temperature of exposure of reformer tubes increases, the carbides coalesce. At higher temperatures, the coalescence of carbides becomes reversible. It has been reported that M₂₃C₆ carbides are stable up to 1250°C in some high-temperature alloys [3, 39]. However, the coalescence of fine carbides due to exposure to high temperatures negatively impacts the creep resistance and strength. The creep voids predominantly initiate within the primary carbide network and eventually, the initiated creep voids link to form fissures and cracks. The type of precipitated carbides and the mismatch between the precipitated carbides and the matrix plays an important role in the initiation of creep voids in reformer tubes [6, 8, 9, 10]. The present results indicate that carbide precipitates coalesced, which affected the creep property of the alloy. The reason for the coalescence of carbides is the exposure of the alloy to higher temperatures. The presence of voids, formation of cracks by the linkage of aligned creep voids, and coalescence of carbide, as in the failed reformer tube are the typical features of reformer tubes failed due to overheating and the resultant creep damage [1, 2, 3, 4]. The premature failure of the alloy due to overheating and creep damage points to the need to adhere to the design and operational conditions to avoid the reoccurrence of such failures. The failure investigation also points to the need to further enhance the capabilities of material research to develop high-temperature alloys to withstand much higher temperatures so that accidental overheating is not leading to catastrophic failures in critical services.

4. Conclusions

The investigation conducted on the service-exposed reformer tube sample revealed the presence of voids and fissures along the interdendritic boundaries. The voids were initiated predominantly within the dendritic primary carbide network. The carbides were found precipitated along the interdendritic boundaries and also inside the grains in the service-exposed tube, while in the new tube, the carbides were precipitated along the interdendritic areas only. The carbides precipitated in the service-exposed tubes were M₂₃C₆, M₇C₃, and NbC. The precipitated secondary carbides were also found coalesced. The microstructural change as a result of the high-temperature exposure marginally increased the strength, while dropping the ductility. The investigation conducted on the service-exposed reformer tube indicated that the failure of the tube was due to creep embrittlement. The microstructural changes of the service-exposed reformer tube and creep embrittlement in a short duration further indicate that the reformer tube was subjected to overheating, which resulted in creep embrittlement. Adherence to the design and operational parameters is critical in mitigating the premature failure of reformer tubes.