**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.

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.


**Table 1.**

*The chemical composition of the reformer tube samples.*

**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

#### *Failure Analysis – Structural Health Monitoring of Structure and Infrastructure Components*

#### **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.*

carbides have been reported in the literature [1–3, 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**).

**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

**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.*

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 [1–4]. The creep voids typically initiate at the primary carbide-matrix interface.

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.

#### **Figure 8.**

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

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, 22–25]. 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 9** shows the XRD spectrum obtained for the phases extracted from the service-exposed reformer tube sample. The phases that precipitated in the serviceexposed 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 [36–38]. 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].

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.

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


#### **Table 2.**

*The hardness of the reformer tube samples.*


#### **Table 3.**

*The tensile properties of the reformer tube samples.*

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.

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 [1–3]. 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 [1–3]. 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 hightemperature 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, 8–10]. 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 [1–4]. 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.
