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1 Overview of the first-stage converter: The first-stage converter is a key equipment in ammonia synthesis plants, and the furnace tubes are the most important components of this converter. After desulfurization, the feedstock is mixed with steam and preheated to 500°C in the convective section of the conversion furnace, after which it enters the conversion furnace tubes where the conversion reaction takes place under the action of a nickel catalyst and at high temperatures. The furnace tubes operate continuously under conditions of a wall temperature as high as 900°C and a pressure of 2.1 MPa; they are also subject to erosion by the medium inside the tubes as well as by the high-temperature flue gases outside, resulting in extremely harsh operating conditions. The furnace tubes are made of expensive high-chromium-nickel alloy steel (25‑20 centrifugally cast), which not only requires large investment but also faces a shortage of resources. Therefore, extending the lifespan of the furnace tube is of great significance for the use of a converter. The structure of this conversion furnace is a double-tube, double-sided radiant side-firing type. The upper part of the furnace tubes is supported by the roof support beams, while the upper and lower sections are connected to the collection pipes via pigtail tubes. The specifications of these furnace tubes are φ152 mm×15 mm, with a length of 11 m; the outer surface of these centrifugally cast tubes has not been mechanically processed. 2 Factors Affecting the Service Life of Furnace Tubes 2.1 Creep Damage When the temperature of a metal rises to a certain level, even if the stress remains unchanged and the initial strain is still a small elastic strain, over time the material structure deteriorates, leading to the formation of creep holes and creep cracks. The metal then undergoes slow, continuous deformation – a plastic deformation that cannot be reversed – ultimately resulting in fracture. The characteristics of creep damage in HK-40 centrifugally cast tubes are similar to those of conventional cast heat-resistant steels under high-temperature creep failure. Pores tend to form preferentially at dendritic grain boundaries, grain boundary inclusions, the interfaces between carbides and the matrix, the interfaces between the σ-phase and the matrix, as well as at the interfaces between precipitates. These pores then propagate along the interfaces between precipitates and the matrix or along grain boundaries, forming microcracks that further develop into larger cracks. It has been shown that the main factors affecting creep are temperature and stress. When a section of the heated furnace tube overheats, it leads to accelerated creep and a sharp decline in its sustained strength. The diameter of the furnace tube increases while its wall thickness decreases, which in turn speeds up the rate of creep swelling. This results in the furnace tube turning red or developing localized red spots, ultimately leading to its rupture. On the other hand, there is the effect of stress: the temperature difference between the inside and outside of the converter generates thermal stress, and the working stress exerted on the furnace tubes due to internal pressure, along with the stress caused by their own weight, leads to an increase in the creep strain rate as stress increases. 2.2 Local overheating of the furnace tubes: By monitoring the wall temperature of the conversion tubes, it was found that the service life of HK-40 conversion furnace tubes is closely related to temperature. As the wall temperature of the tube decreases, its service life is significantly extended ； When the wall temperature remains high for an extended period, the service life is significantly reduced. In the past, the majority of furnace tube failures occurred within a range of 2 to 7 meters from the furnace top; the cracks were all axial in nature and faced the burner, with the failures taking place in the high-temperature areas. Converting furnace tubes are used at high temperatures; as time goes by, the fine and dispersed carbides in the austenitic structure gradually grow larger and aggregate, while the diameter of these carbides decreases as the temperature rises over time. The growth of these carbide aggregates reduces the creep strength of the steel; moreover, being hard and brittle, these carbides are the origin points for crack formation or the propagation of cracks. Under high-temperature conditions, voids nucleate and grow at the creep grain boundaries, thereby developing into cracks that cause damage to the furnace tubes. Studies have shown that when the wall temperature of the furnace tube reaches 880 °C, significant oxidation and carbonization occur on its inner surface. The furnace tubes exhibit the following characteristics: ① A layer of porosity forms on the inner wall to varying degrees, with a maximum depth of up to 3 mm; moreover, the structure within this porous layer also changes ； ②Carbonide accumulation increases in the middle and lower parts of the furnace tube, with rod-shaped and angular carbonides appearing ； ③Small particles of secondary carbides precipitate on the inner surface planes, and the carbonides at the grain boundaries aggregate and become significantly thicker. The precipitation temperature range for the σ phase is 600–900°C, and the sensitive temperature is 750–800°C, which exactly corresponds to the temperature of the furnace inner wall. Therefore, the secondary carbides within the tube can easily transform into the σ-phase, causing a sharp decline in the material’s impact strength at room temperature and high temperatures, as well as its strength, elongation, and reduction of area; this leads to creep fracture of the material and a significant reduction in its service life. The reasons for localized overheating of the furnace tubes are as follows. (1) The feed gas contains sulfur, which poisons the catalyst. Coking of the catalyst reduces its activity, or even causes it to lose all of its activity, which in turn leads to overheating of the furnace tubes. (2) Improper catalyst loading leads to bridging. (3) The steam contains high levels of salts and silicon, which can easily cause salt deposition on the catalyst. If the operation is not proper and the water-to-carbon ratio is off, it can cause coking and pulverization of the catalyst, an increase in gas resistance, uneven distribution of the feed gas. In areas with lower gas flow, the heat absorption decreases, leading to an increase in the wall temperature of the furnace tubes and overheating. (4) Misalignment or damage of the burner, as well as changes in the oil, liquid hydrocarbons, water, and gas components present in the fuel, can all cause the burner to exhibit issues such as large flames, backflow, a red flame tip, or clogging of the nozzle due to carbon deposition. This leads to uneven furnace temperature, as well as corrosion and contamination of the furnace tube surface; in particular, direct exposure to intense flames causes localized overheating on the furnace tube. (5) When the raw gas enters the furnace tube, uneven resistance due to piping issues, combined with the improper use of throttle plates, leads to an uneven distribution of airflow and causes localized overheating in the furnace tube. 2.3 Stress (1) Stress caused by internal pressure: The internal pressure varies from the top to the bottom of the furnace tube, ranging approximately between 3.17 and 3.55 MPa; triaxial stress exists on the tube wall. Among them, the circumferential stress is the greatest, while the axial and radial stresses are only about half of the circumferential stress. (2) Stress caused by the temperature difference between the inner and outer walls The temperature difference between the inner and outer walls at various parts of the furnace tube ranges from 28.4 to 101°C; assuming a temperature difference of 50°C, the maximum instantaneous thermal stress can reach 74–85 MPa, with the axial stress being slightly lower than the circumferential stress. This thermal stress is tensile on the inner half and compressive on the outer half. (3) Thermal stress relaxation: The stress caused by internal pressure remains constant, whereas the thermal stress resulting from temperature differences can relax over time and with changes in temperature; the higher the temperature, the faster the stress relaxes. (4) Impact of starting and stopping: At the moment of startup, tension is applied to the inside of the tube, while pressure is applied to the outside of the tube ； But it’s the opposite when parking. This causes tensile and compressive fatigue in the material, and due to sudden changes in temperature that result in large temperature differences over short periods, stress concentration occurs in certain areas of the material. (5) Differences in material properties caused by large temperature differences between the inside and outside over a long period of time: Even if the material type is the same, if the temperature difference exceeds 50°C, the sustained strength of the material can vary by nearly one order of magnitude, which leads to changes in the actual strength of the material. (6) Residual stresses also exist in the welding and casting of furnace tubes. (7) Deformation resulting from long-term use or additional stress caused by restricted expansion. Based on the above analysis, if the triaxial stress is simplified to the direction of the principal stresses, the hoop stress becomes the maximum value; in this case, failure of the furnace tube should occur primarily through axial cracks, which is consistent with the conditions observed on site. Due to the combined effect of various stresses, the stress is highest at a depth of about 1/3 of the wall thickness from the inner surface of the furnace tube, and cracks generally originate there. Generally, the area 2 to 7 m above the furnace top is not only a high-temperature zone but also a high-stress zone. Therefore, the vast majority of furnace tubes are first damaged in this area. 2.4 Corrosion of inner and outer surfaces: The outer surface of the furnace tubes is primarily subject to oxidation and decarburization corrosion. The inner surface of the furnace tube is primarily affected by the medium, leading to carburization and oxidation. After carburizing, carbides dominated by chromium are formed, followed by oxidation; carbon continues to carburize inward, but this type of corrosion has little effect on the base material. At the weld area, the corrosion layer can penetrate into the effective wall thickness of the furnace tube; however, as observed on-site, it has not yet reached a level that affects the furnace tube’s lifespan. In addition to the above reasons, substandard quality of the furnace tubes also affects their service life. 3 Methods to Extend the Service Life of Furnace Tubes 3.1 Controlling Overheating of Furnace Tubes To prevent damage to the furnace tubes caused by overheating, the following control measures can be taken to address various causes. (1) Strict management and scientific loading are required when loading the catalyst. First, place the catalyst in a long cloth bag, and slowly move the bag along the wall of the tube toward the bottom of the furnace tube; then pull the bag out of the furnace tube. This prevents the catalyst from bridging together or becoming powdered. Before and after catalyst loading, differential pressure measurements were taken for each furnace tube to ensure that its pressure drop remained within ±5% of the average pressure drop of the furnace tubes, thereby guaranteeing uniform fluid distribution. (2) Control the steam-to-carbon ratio to be appropriately stable, maintaining it at its upper limit as much as possible, with a value not less than 3. A high steam-to-carbon ratio can suppress the carbon deposition reaction on the catalyst and prevent methane and light hydrocarbons from depositing as carbon inside the tubes. (3) Maintain the activity of the catalyst. This not only directly affects the conversion rate but also is directly related to the service life of the conversion tube. To a large extent, the wall temperature is the most direct and reliable indicator of the catalyst’s condition. When the catalyst has poor activity, it should be replaced as soon as possible to avoid relying on raising the furnace temperature to compensate for its shortcomings. (4) Strengthen burner management. Keep the temperature difference across a single tube within 30°C, and carry out desulfurization, degreasing, and dehydration treatments before introducing gas into the furnace to reduce corrosion of the furnace tubes by the gas. The burner should be kept \"flat, short, and even,\" with large flames being strictly avoided; regular inspection, maintenance, and checking are necessary. (5) The design of the conversion tube should be reasonably configured according to specific conditions. (6) Strictly control various process parameters to ensure stable production and minimize the number of start-up and shutdown cycles, in order to prevent thermal fatigue. 3.2 Quality inspection of purchased furnace tubes: Proper inspection should be carried out on purchased furnace tubes. It is recommended to purchase pipes with unprocessed outer walls to ensure an excellent casting structure. At the same time, the entire wall thickness of the pipe segment should consist of a dense microstructure; the inner surface must be carefully processed to ensure that there are no casting defects such as inclusions or shrinkages, in order to prevent carburization. 3.3 Replacement of furnace tubes: By altering the chemical composition of the furnace tubes, their high-temperature resistance and creep resistance are improved. New improved alloys such as HK-40VH and HP40-Nb have been developed in China, whose performance is significantly better than that of HK-40 furnace tubes.