Thread Content
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 requirements regarding materials 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 corrosion forms 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 readily available, that is, it should possess good processability and welding properties, among other characteristics. For example, in some corrosive environments, it is undoubtedly economical and practical to use pressure pipelines and their components made of a composite of carbon steel and stainless steel as a substitute for pure stainless steel. However, due to the substandard composite manufacturing processes in many factories, various problems arise during use, which limits the application of composite materials. In particular, composite sheets made of carbon steel and 0Cr13 cannot be used on a large scale in engineering applications because it is difficult to ensure the quality of welding at the site. 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 material under ideal conditions. 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 material engineers to use knowledge from fields such as engineering, materials science, and corrosion science to make comprehensive judgments. Such problems can sometimes be quantitatively calculated, 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, materials with lower costs can be used. Otherwise, high-quality but expensive materials must be chosen. 2) For uniform corrosion, in harsh corrosive environments, if lower-grade but cheaper materials are used, the corrosion rate can be very high, requiring the replacement of those materials in a short period of time ； Using materials with better corrosion resistance and higher cost may result in a lower corrosion rate, thereby allowing for a longer production cycle. A comprehensive technical and economic evaluation is conducted; in this case, using advanced materials may be more economical. Conversely, if the corrosive environment is relatively mild, although using lower-grade materials results in a higher corrosion rate, their low cost makes them potentially more economical after conducting a cost-benefit analysis. In short, the selection of this type of material should be based on economic considerations. 3) For the same corrosive environment, when high-grade materials are used, the resulting corrosion may be localized corrosion with a relatively high risk; whereas when low-grade materials are used, the resulting corrosion may be uniform corrosion with a relatively high corrosion rate. At this point, consideration should be given to using low-grade materials supplemented 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. In many material standards and manufacturing standards, there are several options for users to confirm. 1) Among these options, some are general items; when the user does not specify them, the manufacturer will act according to its own *practices. For example, the supplied length and condition of steel pipes all fall under this category of items. 2) Some items are additional inspection items; these inspections are not mandatory, and the manufacturer performs them only when requested by the customer. In other words, users can add several inspection items depending on the usage conditions in order to better control the internal quality of the materials. However, specifying these special requirements means an increase in the product price; the costs of certain inspection items, such as radiographic testing, are quite 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 processes: Actively adopting new materials and supporting the development and application of new materials and processes can effectively reduce construction costs while meeting the material requirements of production processes. For example: using aluminized carbon steel instead of stainless steel to resist corrosion caused by sulfur and organic acids ； Replace pure stainless steel materials with composite materials made of carbon steel and stainless steel ； Replace seamless steel pipes with welded steel pipes whose welding quality is guaranteed ； 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) Applicable to pressurized or non-pressurized pipelines where the medium temperature ranges from -29 to 343°C ； 2) It shall not be used in pipelines for flammable fluids whose medium temperature is higher than 150°C or gauge pressure is 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 where there are cyclic changes in temperature and pressure, or where the pipeline is subject to vibration. In fact, malleable cast iron is often used for valve handwheels that are not under pressure and for underground pipes ； Ductile iron is often used for valve bodies in industrial pipelines. 2.2 Ordinary carbon steel Limitations: a. Killed steel 1) It shall be used only under conditions where the design pressure ≤ 0.6 MPa and the design temperature is between 0 and 250°C ； 2) Not to be used for pipelines transporting flammable or toxic fluids ； 3) It must not be used in liquefied petroleum gas media or in environments with stress corrosion ； b. Calm steel 1) It shall be used only within a design temperature range of 0–400°C. 2) When used in environments susceptible to stress corrosion cracking, the base metal hardness and weld hardness shall not exceed HB200, and 100% non-destructive testing shall be conducted on both the base metal and welds ； c. Boiled steel and killed steel for pressure pipelines 1) The carbon content shall not exceed 0.24%. 2) The GB700 standard specifies four commonly used 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 range: 0–250°C; steel plate thickness ≤12mm ； Not to be used for pipelines transporting flammable media with moderate, high, or extremely hazardous toxicity levels. Q235-A steel plate: Design pressure P≤1.0 MPa ； Operating temperature range: 0–350℃ ； Steel plate thickness ≯16mm ； It shall not be used for pipelines transporting liquefied petroleum gas or media with a high or extremely high level of toxicity. Q235-B steel plate: Design pressure P≤1.6MPa ； Operating temperature range: 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 ≤ 40 mm ； 2.3 High-quality carbon steel High-quality carbon steel is the most widely used type of carbon steel in pressure pipelines. The corresponding 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 considered; 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. Welds shall be 100% non-destructively tested. 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 calculations should be conducted based on factors such as the corrosion rate and service life. If the results of these calculations indicate that carbon steel is a suitable choice, an adequate margin against corrosion 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 considered. f. Carbon steel with a carbon content greater than 0.24% is not suitable for 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 Chromium-molybdenum alloy steel: The common standard materials for chromium-molybdenum alloy steel include GB9948, GB5310, GB6479, GB3077, GB1221, etc. The limitations on their use are as follows: a. In carbon-molybdenum steel (C-0.5Mo), when it is used at a temperature of 468°C for an extended period of time, its carbides tend to transform into graphite; therefore, the maximum allowable 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 Stainless heat-resistant steel: The main standard materials for stainless heat-resistant steel used in pressure pipelines include GB/T14976, GB4237, GB4238, GB1220, GB1221, etc. The common usage limitations are as follows: a. For ferritic and martensitic stainless steels containing 12% or more chromium, when used over extended periods at temperatures between 400–550°C, it is necessary to take measures to prevent temper embrittlement at 475°C; this embrittlement manifests as a decrease in the material’s strength 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, measures 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