**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 [1–3], while the hardness measurements were made to understand the effect of secondary phase precipitation on the hardness [1–3] of the alloy. The specimens were mounted and metallographically prepared for the examination of the crosssections. 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).
