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Characteristics of low-temperature brittle fracture: ① The nominal stress involved in low-temperature brittle fracture is relatively low, generally below the yield strength of the material, and often also below the design stress; therefore, it is sometimes referred to as low-stress brittle fracture. ②Before low-temperature brittle fracture occurs, the component shows no significant plastic deformation, or only minor localized plastic deformation; the fracture always starts at defects (especially weld defects) or at locations where stress or strain concentrations arise due to geometric changes, and it exhibits characteristics of a brittle fracture surface. ③Once brittle failure begins, it progresses at an extremely high speed, generally around 1/3 of the speed of sound; for example, in steel it can reach 1200–1800 m/s. ④Materials that experience brittle fracture under low-temperature conditions originally have low toughness. Notched specimens exhibit greater sensitivity to low temperatures and lower toughness. ⑤For metal materials with different crystal structures, the effect of low temperatures on their toughness varies. Ferritic steel with a body-centered cubic lattice structure has a high ductile-to-brittle transition temperature, poor toughness at low temperatures, and a greater tendency to brittle fracture ； The hexagonal close-packed structure is next ； Austenitic steel with a face-centered cubic lattice does not exhibit significant low-temperature brittleness. ⑥Fracture of materials in low-temperature equipment or components often originates at stress concentration areas. Shape changes, notches, or internal defects (especially cracks) cause local stress concentration. In particular, the stress concentration in the triaxial stress state at the crack tip causes the stress level to quickly reach or exceed the material’s yield strength, leading to brittle fracture. Therefore, the design of cryogenic vessels should minimize stress concentration by selecting materials with low notch sensitivity. ⑦The metallurgical effects of steel plate thickness and constraint stresses lead to an increase in notch fragility ； Work hardening, welding defects, and residual welding stresses reduce the toughness of the material in the weld zone and heat-affected zone ； When in use, the loading speed increases, and factors such as the frequency and magnitude of temperature changes, as well as the chemical effects of the medium – particularly stress corrosion and hydrogen-induced damage – can cause a shift from ductile to brittle behavior in the material. The impact of these factors is more pronounced at low temperatures, resulting in a significant decrease in the toughness of metal materials. Mechanical property characteristics of low-temperature steels: Many cases of brittle fracture at low temperatures are closely related to the mechanical behavior of materials at such temperatures. Under tensile loading, as the temperature decreases, both the yield strength and the tensile strength of steel increase; however, the yield strength increases more rapidly. The elongation rate begins to decrease slowly, and drops sharply once a certain temperature is exceeded. At this temperature, the yield strength is almost equal to the tensile strength, and the material undergoes a transition from ductile to brittle behavior. It can be seen that at low temperatures, the ratio of the tensile strength to the yield strength of steel allows for a simple assessment of the material’s brittle behavior at that temperature; the closer this ratio is to 1, the greater the likelihood of brittleness. Therefore, the ratio Rm(σb)/ReL (σs) is a mechanical property indicator that must be considered in the design of components for low temperatures. Welding properties of low-temperature steels: Welding heat cycles often reduce the toughness and ductility of the heat-affected zone in welded structures, while residual stresses and welding defects can cause low-temperature fracture of the material at the weld site. When selecting steel for low-temperature applications, the carbon equivalent of the steel, as well as the appropriate welding materials and welding procedures, must be taken into consideration. The welding performance of a material can be determined through brittle fracture tests on welding test plates. For chromium-nickel austenitic stainless steel pressure vessels designed for temperatures below -100°C but not below -196°C, the base material shall be chromium-nickel austenitic stainless steel with a carbon content of ≤0.10%, and the impact absorption energy (KV2) of its welded joints at temperatures not exceeding the design temperature shall be no less than 31 J. Low-temperature containers shall not use stamp markings for welder codes.