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The actual operational conditions in engineering applications are highly complex; various combinations of media, medium temperature, medium pressure, and other operating conditions result in countless material selection options. Given the common material selection criteria, it is unrealistic to provide the selection conclusions for each of them here; this is precisely the issue that design institutes or engineering companies have been striving to address. Here, a different approach will be adopted, focusing on materials as the core, and applying metal theory, corrosion theory, and engineering theory to determine the usage limitations of various common materials. In engineering, when selecting materials for pressure pipelines, it is necessary to determine not only the material grade but also the material standard, as different material standards have varying requirements regarding material quality. 1 General constraints When selecting engineering materials, the following principles should be followed first. 1.1 Meeting the requirements of operating conditions a. Determine whether the pipeline is a pressure pipeline based on the operating conditions, and identify which category it belongs to. Different categories of pressure pipelines have varying degrees of importance, and the severity of the hazards resulting from accidents differs as well; therefore, their material requirements also vary. Generally, high-class pressure pipelines (such as Class 1 pressure pipelines) have higher requirements than low-class pressure pipelines, ranging from the material smelting process to the inspection and testing of the final product. b. The requirements for material selection due to operating conditions should be considered. Different materials have varying corrosion resistance against the same corrosive medium. In corrosive environments, the materials chosen should prevent the occurrence of catastrophic forms of corrosion such as stress corrosion cracking. As for uniform corrosion, it should generally be limited to a \"corrosion-resistant\" level, meaning that the maximum annual corrosion rate should not exceed 0.5 mm ； c. Medium temperature is also an important parameter in selecting materials. This is because changes in temperature can cause a range of changes in the properties of materials, such as brittleness at low temperatures, and issues like graphitization and creep at high temperatures. Many forms of corrosion are closely related to the temperature of the medium, and even constitute the basic conditions for corrosion to occur. Therefore, the material selection for pressure pipelines must meet the temperature constraints. 1.2 Meeting the requirements of material processing techniques and industrial production: a. An ideal material should be easily available, that is, it should possess good processability and welding properties. For instance, in certain corrosive environments, it is undoubtedly cost-effective to use pressure pipelines and their components made from a composite of carbon steel and stainless steel instead of pure stainless steel materials. However, due to the inadequate composite manufacturing processes employed by many factories, numerous problems arise during usage, thereby imposing limitations on the application of composite materials. In particular, for composite plates made from carbon steel and 0Cr13, it is difficult to ensure satisfactory welding quality on-site; as a result, they are rarely or not at all used in engineering applications. b. The application of materials in engineering is serialized and standardized. It’s not like in the laboratory, where it’s possible to use small amounts of idealized materials. Standardizing and serializing materials facilitates large-scale production, reduces the variety of materials used, which in turn helps to save costs in various stages such as design, manufacturing, installation, and use; it also **lowers production costs**. Therefore, in engineering projects, standard materials should be chosen first. For new materials that must be used, complete technical evaluation documents are required, and they must undergo technical appraisal organized by provincial or higher-level regulatory authorities; only after passing such appraisal can they be utilized. For materials that must be imported, detailed technical requirements such as specifications, performance characteristics, material grades, material standards, and application standards should be specified. These materials must undergo re-inspection in accordance with relevant domestic technical requirements before they can be used. 1.3 Meeting the requirements of being both applicable and economical. This is a very fundamental issue; in practice, it’s quite complicated to implement. It requires materials engineers to make a comprehensive judgment by applying knowledge from engineering, materials science, corrosion science, and other related fields. Such problems can sometimes be quantified, while at other times they cannot. Under normal circumstances, the following aspects should be considered: a. Corrosion aspects 1) For localized corrosion, if other measures (such as process-based anti-corrosion measures) can prevent or control the occurrence of localized corrosion, especially sudden and catastrophic cases of it, then materials with lower costs can be used. Otherwise, it is necessary to choose high-grade but expensive materials. 2) In the case of uniform corrosion, under harsh corrosive environments, if a lower-grade but cheaper material is chosen, its corrosion rate may be very high, necessitating a replacement of the material in a short period of time ； On the other hand, using materials with better corrosion resistance but higher costs may result in a lower corrosion rate, thereby extending the production cycle. A comprehensive technical and economic evaluation is conducted; in this case, using advanced materials may be more economical. Conversely, if the corrosion environment is milder, using lower-grade materials may be more economical in this case; although their corrosion rate is higher, their cost is low, and after conducting economic calculations, it might be more cost-effective to opt for such materials. In short, the selection of this type of material should be based on economic considerations. 3) For the same corrosive environment, when advanced materials are used, the corrosion that occurs may be localized corrosion with high risk, whereas when lower-grade materials are used, the corrosion that occurs may be uniform corrosion with a high corrosion rate. At this point, it is advisable to consider using lower-grade materials along with other anti-corrosion measures. b. Material standards and manufacturing: There is no perfect one-to-one correspondence between the categories of pressure pipelines and their material standards as well as manufacturing requirements; this necessitates material engineers to apply relevant knowledge to make comprehensive considerations. Many material and manufacturing standards include several options for the user to confirm. 1) Among these options, some are general items; when the user does not specify anything, the manufacturer will proceed according to its own* practices. For example, the supply length of steel pipes and their supply condition fall under such categories. 2) Some items are additional inspection items; these inspections are not necessary, and the manufacturer will carry them out only if requested by the user. In other words, users can add additional inspection items depending on the usage conditions in order to better control the internal quality of the materials. However, imposing these special requirements means an increase in the product price; the costs of certain inspection methods, such as radiographic testing, are very high. How to add these additional inspection items should be determined by taking into account both the usage conditions and the product’s price; sometimes it is difficult to find the right balance. c. In terms of the application of new materials and new processes, actively adopting new materials and supporting their development and use can effectively reduce construction costs while also meeting the material requirements of production processes. For example: using aluminized carbon steel instead of stainless steel to resist corrosion by sulfur and organic acids ； Replace pure stainless steel materials with composite materials of carbon steel and stainless steel ； Replace seamless steel pipes with welded steel pipes that ensure weld quality ； Wait. 2 Application limitations of common materials 2.1 Cast iron The common types of cast iron are malleable cast iron and ductile cast iron. General restrictions: 1) Used in pressurized or unpressurized pipelines with a medium temperature of -29～343°C ； 2) It shall not be used in pipelines for transporting flammable fluids with a medium temperature higher than 150°C or a gauge pressure greater than 2.5 MPa ； 3) It shall not be used to transport toxic media under any temperature and pressure conditions ； 4) It shall not be used under conditions of cyclic changes in temperature and pressure or when the pipeline is subject to vibration. In fact, malleable cast iron is often used for unpressurized valve handles and underground pipelines ； Ductile iron is often used for the valve bodies in industrial pipes. 2.2 Constraints for ordinary carbon steel: a. Boiling steel 1) should be used only under conditions where the design pressure is ≤0.6 MPa and the design temperature is 0–250°C ； 2) Must not be used for pipelines carrying flammable or toxic fluids ； 3) It must not be used in liquefied petroleum gas media or in environments with stress corrosion ； b. Calming steel 1) is limited to use in a design temperature range of 0–400°C. 2) When used in environments susceptible to stress corrosion cracking, the hardness of the base material and the welds shall not exceed HB200, and 100% non-destructive testing shall be conducted on both the base material and the welds ； c. Boiled steel and killed steel used for pressure pipelines: 1) The carbon content shall not exceed 0.24%. 2) The GB700 standard specifies four common grades of ordinary carbon structural steel, namely: Q235A(F, b), Q235B(F, b), Q235C, and Q235D. Its scope of application is as follows: Q235-A·F steel plates: design pressure P ≤ 0.6 MPa ； Operating temperature: 0–250°C, steel plate thickness: ≯12 mm ； It shall not be used in pipelines for flammable media with moderate, high, or extremely hazardous toxicity. Q235-A steel plate: design pressure P≤1.0 MPa ； Operating temperature: 0–350℃ ； Steel plate thickness ≯16mm ； It shall not be used for pipelines carrying liquefied petroleum gas or media with a high or extremely hazardous toxicity level. Q235-B steel plate: design pressure P≤1.6MPa ； Operating temperature: 0–350℃ ； Steel plate thickness ≯20mm ； It cannot be used for pipelines carrying highly hazardous and extremely dangerous media. Q235-C steel plate: design pressure P≤2.5 MPa ； Operating temperature: 0–400℃ ； Steel plate thickness ≯40mm ； 2.3 High-quality carbon steel: High-quality carbon steel is the most widely used type of carbon steel in pressure pipelines. The relevant material standards include GB/T699, GB/T8163, GB3087, GB5310, GB9948, GB6479, etc. These standards set different quality requirements based on various operating conditions. Common usage restrictions for them: a. When transporting alkaline or caustic media, the possibility of alkali embrittlement must be taken into account; manganese steel (such as 16Mn) shall not be used in such environments ； b. When working in an environment prone to stress corrosion cracking, post-weld stress relief heat treatment shall be carried out, and the hardness of the weld after heat treatment shall not exceed HB200. The welds shall undergo 100% non-destructive testing. Manganese steel (such as 16Mn) is not suitable for use in environments prone to stress corrosion cracking ； c. When operating in an environment with uniform corrosion, economic evaluations should be conducted based on factors such as the corrosion rate and service life. If the evaluation results indicate that carbon steel is a suitable choice, an adequate corrosion margin should be provided, along with other appropriate anti-corrosion measures ； d. When carbon steel, carbon-manganese steel, and manganese-vanadium steel are operated at temperatures of 425°C or higher for extended periods, their carbides may transform into graphite; therefore, their maximum operating temperature should not exceed 425°C (boiler specifications specify this temperature as 450°C) ； e. When operating in the presence of hydrogen, the possibility of hydrogen-induced damage should be taken into account. f. Carbon steel with a carbon content greater than 0.24% is not suitable for use in pipes and their components that are to be welded together ； g. When used at temperatures of -20°C and below, a low-temperature impact toughness test shall be conducted ； h. Carbon steel materials used in high-pressure hydrogen environments and under alternating load conditions should be materials that have undergone extracorporeal refining. 2.4 Chromomolybdenum alloy steels: The common standards for chromomolybdenum alloy steel materials include GB9948, GB5310, GB6479, GB3077, GB1221, etc. The limitations on their use are as follows: a. In carbon-molybdenum steels (C-0.5Mo), the carbides tend to transform into graphite when exposed to temperatures of 468°C for extended periods; therefore, the maximum allowable long-term operating temperature is limited to 468°C℃ ； b. When operating in an environment of uniform corrosion, economic evaluations should be conducted based on the corrosion rate and service life, while also providing an adequate margin against corrosion ； c. When operating in the presence of hydrogen, the possibility of hydrogen-induced damage should be taken into account ； d. When operating in a high-temperature H2+H2S environment, its operating conditions should be determined based on the Nelson curve and the Couper curve ； e. It should be avoided in environments prone to stress corrosion cracking ； f. When operating for extended periods at temperatures between 400–550°C, measures should be taken to prevent temper brittleness. g. Chromium-molybdenum alloy steel should generally be material produced by electric furnace melting or through secondary refining. 2.5 The common standards for stainless heat-resistant steel materials used in pressure pipelines made of such steel include GB/T14976, GB4237, GB4238, GB1220, GB1221, etc. The common usage restrictions are as follows: a. For ferritic and martensitic stainless steels containing 12% or more chromium, when used for extended periods at temperatures between 400–550°C, it is necessary to take measures to prevent temper brittleness at 475°C; this brittleness manifests as a weakening of the material at room temperature. Therefore, when using the aforementioned stainless steel, its bending stress, vibration, and impact loads should be reduced to levels below those that are considered critical, or it should not be used at temperatures above 400°C ； b. During the heating and cooling process of austenitic stainless steel, when it passes through the temperature range of 540–900°C, precautions should be taken to prevent the tendency for intergranular corrosion. When corrosive media with strong reducing properties are present, stable grades (containing stabilizing elements Ti and Nb) or ultra-low carbon grades (C