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Temperature effects: Some pressure vessels, such as hot-wall hydrogenation reactors, operate at high temperatures for extended periods of time； Another example are liquid hydrogen or liquid oxygen storage tanks, which operate at low temperatures. The properties of steel at high and low temperatures differ from those at room temperature, and at high temperatures they are often related to the duration of exposure. Effects under short-term static loading conditions: At high temperatures, the influence of temperature on the mechanical properties of low-carbon steel is shown in the figure below. The elastic modulus E and yield strength ReL decrease as the temperature rises, while the tensile strength Rm increases at first with rising temperature, but then drops rapidly once the temperature reaches a certain level. Therefore, at higher temperatures, it is not sufficient to determine the allowable stress based solely on the tensile strength and yield strength of the steel at normal temperatures; generally, the yield strength of the material at the design temperature should also be taken into consideration. Image Image At low temperatures, as the temperature drops, the strength of carbon steel and low-alloy steel increases, while their toughness decreases. Austenitic stainless steels maintain good toughness even as their strength increases. For carbon steels and low-alloy steels, when the temperature is below 20°C, the allowable stress at 20°C is typically used ； For austenitic stainless steels, a higher allowable stress can be adopted at low temperatures than at room temperature. When the temperature falls below a certain threshold, the impact absorption capacity of steel decreases significantly, and it shifts from a ductile state to a brittle state; this temperature is commonly referred to as the ductile-brittle transition temperature or the transition temperature without ductility. The phenomenon of steel becoming brittle at low temperatures is a common issue encountered by pressure vessels operating in low-temperature conditions. There are various methods and approaches to determine the ductile-brittle transition temperature of steel. In a series of impact tests at different temperatures, the critical temperature at which the impact absorption energy changes sharply or the fracture toughness undergoes a significant change is the ductile-brittle transition temperature of the steel. It should be noted that not all metals exhibit significant brittleness at low temperatures. Generally speaking, metals with a body-centered cubic crystal structure, such as carbon steel and low-alloy steel, exhibit significant brittleness at low temperatures ； Metals with a face-centered cubic crystal structure, such as copper, aluminum, and austenitic stainless steel, exhibit little change in their impact absorption capacity with temperature, and retain high toughness even at very low temperatures. The influence of high temperature and long-term static loading: At room temperature, a continuous load has little effect on the mechanical properties of steel. However, at high temperatures, properties such as the strength of steel change not only with rising temperature but also are closely related to time. The phenomenon in which metal undergoes slow plastic deformation under high temperature and constant load over a long period of time is known as creep. Creep occurs only when the temperature reaches a certain level. Creep becomes significant when the temperature of carbon steel exceeds 300–350°C, that of low-alloy steel exceeds 400°C, that of chromium-molybdenum low-alloy steel exceeds 450°C, and that of high-alloy steel exceeds 550°C. The result of creep is the occurrence of creep embrittlement, stress relaxation, creep deformation, and creep fracture in pressure vessel materials. Therefore, measures should be taken to prevent creep failure during the design of high-temperature pressure vessels. ①Creep curve: When temperature and stress are specified, the relationship between the strain of a metal material and time can be represented by the creep curve shown in the figure below. A typical creep curve can generally be divided into three stages: decelerating creep, constant-velocity creep, and accelerating creep. In the diagram, the Oa segment represents the instantaneous strain of the specimen after loading; only the strain that occurs over time starting from point a constitutes creep. The slope at any point on the creep curve represents the creep rate at that point. In the image above, ab represents the first stage of creep, namely the unstable phase of creep. The creep rate gradually decreases over time; hence, it is also referred to as the deceleration phase of creep ； BC is the second stage of creep; during this stage, the material deforms at a nearly constant creep rate, which is why it is also referred to as the constant-velocity stage of creep ； CD is the third stage of creep, during which the creep rate continues to increase until fracture occurs. The shape of the creep curve for the same material varies at different stresses at a given temperature, or at different temperatures under a given stress. When the stress is low or the temperature is very low, the duration of the second phase is long, and there may even be no third phase ； Conversely, when the stress is high or the temperature is high, the duration of the second stage is short, or it even disappears entirely. ②Creep limit and endurance strength: The creep limit is the material’s resistance to deformation under long-term high-temperature loading. It is generally expressed as the stress value that causes a certain amount of total creep elongation in the specimen within a given temperature and specified time. The creep limit is commonly denoted by Rnt, and it refers to the average stress at which 1% deformation occurs after 100,000 hours of operation or testing at the design temperature. ‘ Persistent strength is the stress value at a given temperature that causes a material to fracture after a specified period of time; it represents the material’s ability to resist fracture under prolonged high-temperature loading. Persistent strength is commonly expressed as RDt, which refers to the average maximum stress at the design temperature after 100,000 hours of operation or testing without fracture. For components that operate at room temperature, once elastic deformation occurs, if the total amount of deformation remains constant, the stress within the component will also remain constant. However, under the combined action of high temperature and tensile stress, over time, if the total amount of deformation remains constant, the plastic deformation that increases due to creep will gradually replace the original elastic deformation, thereby reducing the stress within the component. This phenomenon in which the total deformation of a component remains unchanged while the stress decreases spontaneously over time under long-term high-temperature stress is known as stress relaxation. The property of a material to resist stress relaxation is known as relaxation stability. For example, the connection bolts in high-temperature pressure vessels may cause leaks in the vessel due to stress relaxation. Deterioration of material properties at high temperatures: At normal temperatures, the microstructure and mechanical properties of steel are generally quite stable and do not change over time. However, at high temperatures, steel behaves differently from it does at room temperature; its microstructure and mechanical properties change, resulting in a deterioration of its material properties. For steel that operates at high temperatures for extended periods of time, in addition to the creep embrittlement mentioned earlier, the main degradation of its mechanical properties includes spheroidization of pearlite, graphitization, temper embrittlement, hydrogen corrosion, and hydrogen embrittlement. ①Spheroidization of pearlite: Carbon steels and low-alloy steels used for pressure vessels generally have a microstructure consisting of ferrite and pearlite at room temperature. In a normal pearlitic structure, flake-shaped cementite is evenly distributed throughout a ferritic matrix. At higher temperatures, these flake-shaped cementite particles gradually aggregate into spherical forms, which reduces the material’s yield strength, tensile strength, impact toughness, creep limit, and endurance strength. This phenomenon is known as pearlite spheroidization. For example, moderate spheroidization can reduce the strength of carbon steel at room temperature by 10% to 15% ； It decreases by 20%–30% during severe spheroidization. Steel that has already undergone spheroidization can have its original structure restored through heat treatment. ②Graphitization is the phenomenon in which, under the action of high temperatures over a prolonged period, the cementite within pearlite decomposes to form graphite: Fe3C -> 3Fe + C (graphite). This is known as graphitization or graphite precipitation. The first step in graphitization is the spheroidization of pearlite; graphitization is the ultimate result of the decomposition of carbides in steel under prolonged high-temperature conditions. Graphitization renders steel brittle, reducing its strength and ductility, and further decreasing its impact toughness. When carbon steel and carbon-manganese steel are used for extended periods at temperatures above 425°C, the tendency of the carbide phases in the steel to graphitize must be taken into account. Measures that can be adopted in the design include changing the material, such as using Cr-Mo steel suitable for pressure vessels operating under medium-temperature conditions ； Reduce the designed service life of the container ； Appropriately increase the thickness of the container’s shell and reduce the stress levels on the compressed components, etc. ③Temper embrittlement refers to the phenomenon in which, after Cr-Mo steel subjected to tempering remains in the embrittlement temperature range (300–600°C), there is a significant decrease in the room-temperature impact energy of the material and the welded joints, or an increase in the transition temperature between toughness and brittleness. The main factors affecting temper embrittlement are chemical composition and heat treatment conditions. The greater the amount of impurity elements such as P, Sb, Sn, and As, the higher the austenitization temperature, and the more sensitive Cr-Mo steel is to temper embrittlement. ④Hydrogen corrosion and hydrogen embrittlement: Hydrogen can cause various types of performance degradation in materials. Ⅰ. Hydrogen corrosion refers to the chemical reaction in which hydrogen reacts with carbon in steel to form methane under high temperature and pressure; it is also known as hydrogen erosion. Based on the stage of development and severity, hydrogen corrosion can be divided into two stages: in the first stage, hydrogen combines with carbon on the surface of steel to form methane, causing decarburization of the steel surface and deteriorating its mechanical properties ； Second, hydrogen penetrates into the steel and reacts with dissolved carbon or carbides to produce methane. The generated methane cannot diffuse away and accumulates at the grain boundaries, forming bubbles with high pressure, which leads to decarburization and the formation of microcracks within the steel. The main factors affecting hydrogen corrosion include: temperature, hydrogen partial pressure, time, alloy composition, stress, etc. Under normal conditions, carbon steel may suffer from hydrogen corrosion in a high-pressure hydrogen environment at temperatures above 200°C. Adding elements to steel such as chromium, vanadium, titanium, and tungsten, which can form stable carbides, can enhance the steel’s resistance to hydrogen corrosion. Austenitic stainless steel can resist hydrogen corrosion well. Currently, in hydrogen corrosion environments, steel grades are generally selected based on the Nelson curve. According to this curve, for carbon steel, the allowable operating temperature is approximately 250 when the hydrogen partial pressure is less than 3.45 MPa℃ ； The allowable operating temperature for 1.25Cr-0.5Mo steel at a hydrogen partial pressure of less than 6.9 MPa is approximately 520°C. When determining the design temperature, a temperature safety margin of over 20°C should be reserved. II. Hydrogen embrittlement refers to the phenomenon in which steel loses its toughness due to the absorption of hydrogen, and it belongs to hydrogen-induced environmental embrittlement. There are two sources of hydrogen: one is internal hydrogen, which refers to the hydrogen absorbed by steel during processes such as smelting, welding, and pickling ； The second is external hydrogen, which refers to the hydrogen absorbed by steel when it is used in a hydrogen-rich environment. In pressure vessels operating under high temperature and high hydrogen partial pressure conditions, hydrogen infiltrates into the steel in atomic form and is dissolved and absorbed by the steel matrix. When the container cools down, the solubility of hydrogen decreases significantly, leading to an accumulation of molecular hydrogen and causing hydrogen embrittlement. Therefore, when such containers are parked, the pressure should first be reduced, followed by heat retention and hydrogen removal (at temperatures above 200°C), before the temperature is lowered to normal levels. It is absolutely forbidden to lower the temperature first and then the pressure. It should be noted that high temperature is not a necessary condition for hydrogen embrittlement; when the hydrogen pressure is very high, steel can also become brittle at room temperature, that is, hydrogen embrittlement at room temperature and high pressure. When steel is exposed to high temperatures for an extended period, there is also a redistribution of alloying elements between the solid solution and carbide phases. Those alloying elements that contribute to the strengthening of the solid solution, such as chromium, aluminum, and manganese, gradually dissolve out, resulting in a decrease in the material’s strength at high temperatures. The embrittlement of materials cannot be effectively detected solely through visual inspection and non-destructive testing; as a result, accidents caused by it are often sudden. During the design phase, predicting whether material properties will degrade over time and taking effective preventive measures are of great significance for improving the safety of pressure vessels.