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What is hydrogenation? Hydrogenation technologies include hydrocracking and hydrofining. Hydrocracking cracks heavier crude oils into lighter oils (gasoline, diesel, kerosene, and feedstocks for olefin production, etc.). Hydrorefining removes harmful impurities such as sulfur, oxygen, and nitrogen from oils by converting them into hydrogen sulfide, water, and ammonia. Through hydrogenation technology, we convert degraded and heavy crude oil into high-quality, light crude products. Since hydrogenation must be carried out under harsh conditions of high temperature, high pressure, and exposure to hydrogen, and since some feed streams contain corrosive substances such as hydrogen sulfide and ammonia, the equipment is very prone to damage. Today, I will explain in detail what the types of damage are in hydrogenation equipment, the mechanisms behind such damage, the influencing factors, and the preventive measures... What types of hydrogenation equipment are there? Due to the special operating conditions of hydrogenation units, the equipment is prone to certain specific types of damage. To prevent these destructive damages, not only proper design and material selection are necessary, but it also depends greatly on correct manufacturing processes and proper operation and maintenance. What are the main types of damage to hydrogenation equipment? High-temperature hydrogen corrosion HA—Hydrogen Attack: There are two forms of high-temperature hydrogen corrosion. Surface decarburization does not cause cracks; its effects are generally mild, with a slight decrease in the strength and hardness of the steel while its ductility increases. Internal decarburization and cracking: Internal decarburization occurs as a result of hydrogen diffusing into the steel and reacting to form methane; namely: https://mmbiz.qpic.cn/mmbiz_png/vSABwyURAVia57gurjPG3cRnCic2ibE3KibSCvIGibj4T5QUHdwJCPTS6p*bdGTv1LL1SZK8yiaHB6e7rFT0qLm28UlA/640?wx_fmt=png Methane accumulates near grain boundary voids and inclusions, creating high local stresses that cause the steel to develop cracks or bulges, thereby significantly reducing its strength and toughness. Since this damage is the result of a chemical reaction, it is irreversible, also known as permanent embrittlement. The actual process involves methane bubbles nucleating at the grain boundaries, growing, and connecting with each other to create intergranular microcracks; eventually, these microcracks come together to form fracture pathways. During the incubation period, the concept of an “incubation period” (or latent period) needs to be introduced here. The period during which methane bubbles are forming, but their growth rate is slow and there is no interaction between them, and during which the mechanical properties of the steel do not change significantly, is referred to as the \"incubation period\". “The concept of the \"incubation period\" is very important for engineering applications. It can be used to determine the approximate safe service life of the steel used in equipment and pipelines. “The length of the \"incubation period\" depends on many factors, including the type of steel, hydrogen pressure, temperature, degree of cold working, content of impurity elements, and applied stress. Main factors affecting high-temperature hydrogen corrosion ● Effects of temperature, pressure, and exposure time: The higher the operating temperature, the more severe the hydrogen corrosion of steel. Below about 200°C, steel generally is not prone to hydrogen corrosion. The higher the operating pressure (hydrogen partial pressure), the more severe the hydrogen corrosion of the steel. When the hydrogen partial pressure is ≤ 0.7 MPa, steel basically does not suffer from hydrogen corrosion. The effects of operating temperature and pressure can be attributed to their impact on the hydrogen equilibrium concentration in steel; that is: https://mmbiz.qpic.cn/mmbiz_png/vSABwyURAVia57gurjPG3cRnCic2ibE3KibSfJibbtaIFwyQdyJ9Ex6qJDwlX3pibofRYMliaWr3bApia3KsPlu8H3GfiaA/640?wx_fmt=png C—hydrogen concentration, ppm ； P — Hydrogen partial pressure ； T — Operating temperature, K. ● Influence of alloying elements and impurity elements: At a hydrogen pressure below 6.9 Mpa and a temperature below 538°C, the effects of trace alloying elements in steel are as follows: 1. Mo’s resistance to hydrogen corrosion is 4 times that of Cr. 2. The hydrogen erosion resistance of Mo is comparable to that of V, Ti, and Nb (when Nb ≤ 0.1%). 3. Si, Ni, and Ca do not enhance hydrogen corrosion resistance. 4. P and S reduce resistance to hydrogen erosion. ● Effects of heat treatment ● Effect of stress Hydrogen embrittlement HE—Hydrogen Embrittlement Characteristics Hydrogen embrittlement is a phenomenon of embrittlement caused by the retention of hydrogen in steel. Steel that has suffered from hydrogen embrittlement shows a significant decrease in elongation and reduction of area. Hydrogen embrittlement is reversible and is also known as primary embrittlement. The temperature at which hydrogen embrittlement occurs ranges from room temperature to about 150°C. As the temperature rises, the hydrogen embrittlement effect decreases, and it is less likely to occur when the temperature exceeds 71°C to 82°C. Therefore, in actual hydrogenation units, hydrogen embrittlement damage usually occurs during the low-temperature phases of unit startup and shutdown. How to prevent hydrogen embrittlement? 1. The sensitivity to hydrogen embrittlement generally increases as the strength of the steel increases. Therefore, it is required that the strength of the steel used not exceed the specified value, and PWHT should be properly applied to eliminate residual stresses and control the hardness in the heat-affected zone. 2. The microstructure also has an impact on hydrogen embrittlement; for example, untempered materials and pearlitic structures are more sensitive to hydrogen embrittlement. 3. The degree of hydrogen embrittlement in steel is closely related to the hydrogen content in the steel. At temperatures where hydrogen embrittlement may occur, there exists a hydrogen concentration that does not cause subcritical crack propagation; this is known as the safe hydrogen concentration. It is related to the strength level of the steel, the magnitude of the tensile stress at the crack tip, and the geometric dimensions of the crack. 4. Eliminate macroscopic defects as much as possible through non-destructive testing. 5. Control the processes of temperature and pressure increase during startup and shutdown, as well as their decrease during these same phases. 6. In hydrogen-handling equipment such as hydrogenation reactors, hydrogen embrittlement of stainless steel also occurs (some cases are accompanied by σ-phase embrittlement), with the affected areas mostly found at the fillet welds of the reactor’s catalyst support rings as well as at the corners of the groove bottoms in the flange ladder-shaped groove seals. To prevent such damage, the following measures should be taken in terms of structural design, manufacturing process, and production operations: a) minimize the strain amplitude, reduce thermal stress, and avoid stress concentration (by increasing the radius of curvature of the sealing grooves and support protrusions). b. Try to ensure that the surfacing metal or welded metal at Tp.347 has high ductility (the surfacing layers on the sealing grooves and support bosses should be heat-treated after welding before further surfacing and machining). c. When the plant is shut down, try to release the hydrogen absorbed in the steel. d. Try to avoid unplanned emergency shutdowns. High-temperature hydrogen sulfide + hydrogen corrosion: In hydrogen-related installations, where high-temperature hydrogen sulfide and hydrogen coexist, corrosion of equipment and pipelines occurs at temperatures above 204°C, and this corrosion is more severe than that caused by hydrogen sulfide alone on steel. Its corrosion rate generally increases as the temperature rises. Main influencing factors: temperature, hydrogen, hydrogen sulfide concentration, and alloy composition. Stress corrosion cracking of polythiosulfates: cracking characteristics and causes. The stress corrosion cracking of polythiosulfates (H2SxO6, X=3–6) also falls under the category of sulfide stress corrosion cracking, and it generally manifests as intergranular cracks. The formation of polythiosulfates is due to the generation of iron sulfide when the equipment operates in an atmosphere containing high temperatures of hydrogen sulfide; when the equipment stops operating or is taken out of service for maintenance, this iron sulfide reacts with the moisture present and with the oxygen in the air that enters the equipment. Prevention measures: 1. Appropriate materials should be selected in the design. At the same time, structural design should strive to avoid configurations with stress concentration. 2. In manufacturing, efforts should be made to eliminate or reduce residual stresses caused by cold working and welding, and care should be taken to design the structure in such a way that stress concentrations do not occur or are minimized as much as possible. 3. Its main use is to moderate environmental conditions. Temper Embrittlement in chromium-molybdenum steel: Due to their excellent resistance to high-temperature hydrogen corrosion and superior overall mechanical properties, Cr-Mo steels are widely used in hydrogenation reactors. However, after long-term use, temper embrittlement of varying degrees occurs. Phenomenon and its characteristics: The temper embrittlement of Cr-Mo steel is a phenomenon in which, when the steel is maintained at approximately 343°C to 593°C for an extended period of time, or when cooled slowly from this temperature range, the toughness of the material is degraded due to metallurgical changes. It occurs because harmful impurity elements in steel (such as P, Sn, Sb, As) and certain alloying elements (such as Si, Mn) segregate to the boundaries of the original austenite grains, thereby reducing the cohesive strength of these grain boundaries. Temper brittleness has almost no effect on tensile strength and elongation; significant changes can only be observed during impact resistance tests. Once the material develops temper brittleness, the transition temperature between toughness and brittleness shifts toward higher temperatures. The main factors affecting temper brittleness are numerous, such as chemical composition, heat treatment conditions during manufacturing, thermal state during processing, strength level, plastic deformation, morphology of carbides, as well as the temperature and time under which the material is used. Moreover, some factors are interrelated, making the situation rather complex. Effects of chemical composition: Impurity elements in the chemical composition (such as P, Sn, As, Sb) and certain alloying elements (such as Si, Mn, etc.) have a significant impact on temper brittleness. In engineering applications, two empirical formulas related to chemical elements are commonly used to describe the degree of temper brittleness. Effects of heat treatment processes: The effects of heat treatment processes are mainly that the austenitizing temperature and the cooling rate after austenitization have a significant impact on the susceptibility to temper brittleness. 1. Raising the austenitization temperature makes temper embrittlement more likely to occur. 2. Increasing the cooling rate at the austenitizing temperature increases the susceptibility to temper brittleness. (This is contradictory to the requirements regarding mechanical properties; therefore, it is necessary to find an optimal heat treatment process.) Evaluation of temper brittleness: 1. Isothermal aging treatment, that is, treatment for inducing temper brittleness. 2. Step cooling or step cooling method, which is widely used in engineering. The so-called stepped cooling method is a technique in which the specimen of the test material is kept at temperatures within the range causing temper embrittlement and then cooled in a stepped manner (usually with 5 steps), thereby inducing temper embrittlement. The hydrogen-induced disbonding phenomenon in austenitic stainless steel surfacing layers: Its characteristics. Macroscopically, the path of disbonding extends along the interface between the surfacing layer and the base metal; a disbonding state exists between the stainless steel surfacing layer and the base metal, which is why it is referred to as disbonding. Microscopically, the typical patterns of crack formation include those that occur along the chromium carbide precipitation zones formed at the fusion line, as well as those that extend along the growing austenite grain boundaries. The main reasons for the peeling phenomenon: 1. Due to the different hydrogen solubilities and diffusion rates of the Cr-Mo steel used in manufacturing the reactor body and the austenitic stainless steel used for the cladding layer, a very high hydrogen concentration occurs on the cladding layer side in the transition zone between the two layers. 2. Due to the significant difference in linear expansion coefficients between the base material and the surfacing layer material, considerable residual stress exists at the interface. 3. During the surfacing process, coarse crystals that grow along the fused layer may form on the boundary layer. In addition to the factors related to the metal material itself, environmental conditions and manufacturing processes also have an impact on hydrogen-induced delamination of the surfacing layer. ● Environmental conditions: operating temperature, hydrogen partial pressure, cooling rate, and the number of cycles of repeated heating and cooling. ● Manufacturing process: mainly the influence of welding methods, welding conditions, and post-weld heat treatment. Measures to prevent hydrogen-induced delamination of the surfacing layer ● Reduce the hydrogen concentration at the interface (material selection, control of cooling rate). ● Reduce residual stress. ● Strive to make the microstructure near the weld seam of the surfacing layer have a lower susceptibility to hydrogen embrittlement. ● Strictly follow the operating procedures to minimize unplanned emergency stops. ● During normal shutdown, shutdown conditions should be adopted that enable hydrogen to be released from within the vessel walls as much as possible.