Q&A on Pressure Vessels – Materials

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1. What is the difference between carbon steel and normalized steel of grades Q235B and Q235C? What is the scope of application? Answer: The main difference lies in the temperature used for the impact test: Q235B grade is tested for V-notch impact at room temperature of 20°C ； Q235C grade is subjected to a 0°C V-notch impact test. Scope of application: ① Grade Q235B: The design pressure of the container is P≤1.6 Mpa, and the operating temperature of the steel plate is 0~350℃. When used for container shells, the steel plate thickness shall not exceed 20 mm, and it shall not be used in pressure vessels containing highly or extremely hazardous media. ② Grade Q235C: The design pressure of the container is P≤2.5 Mpa, and the operating temperature of the steel plate ranges from 0 to 400°C. When used for container shells, the steel plate thickness shall not exceed 30 mm. 2. What issues should be considered when using carbon steel and carbon-manganese steel at temperatures above 425°C over a long period of time? Answer: GB150 stipulates that for carbon steel and carbon-manganese steel used over long periods at temperatures above 425°C, the tendency of the carbides in the steel to graphitize must be taken into account. In carbon steel and manganese carbon steel under the aforementioned conditions, the cementite in the steel decomposes to form Fe3C—3Fe+C (graphite). This decomposition and graphitization ultimately cause part or all of the pearlite in the steel to disappear, resulting in a decrease in the material’s strength and ductility; the impact value drops even more, and the steel becomes significantly more brittle. 3. When the operating temperature of austenite exceeds 525°C, what precautions should be taken? Answer: According to GB150, when austenitic steel is used at temperatures above 525°C, its carbon content must be at least 0.04%. This is because at usage temperatures of 500–550°C, if the carbon content is too low, the strength and oxidation resistance of the steel will decrease significantly. Therefore, the general guidelines specify the temperature range in which ultra-low carbon (C≤0.03%) austenitic stainless steels can be used: 18-9 grade materials can be used up to around 400°C, while 18-12-2 grade materials can be used up to around 450°C. When the operating temperature exceeds 650°C, abroad it is common to require H-grade 304 and 316 grades, meaning a slightly higher carbon content; this is mainly to ensure better corrosion resistance, as well as heat resistance and thermal strength. 4. What is the operating temperature range for stainless steel composite sheets? Answer: The operating temperature range for stainless steel composite sheets must meet both the requirements regarding the operating temperature of the base material and those of the composite layer. 5. For carbon steel and low-alloy steel used in pressure vessels, at what thickness should they be used in the normalized state? Why? Answer: 20R and 16MnR with a shell thickness greater than 30 mm; other pressure-bearing components (flanges, tube sheets, flat covers, etc.) made of 20R and 16MnR with a thickness greater than 50 mm, as well as 15MnVR with a thickness greater than 16 mm, should be used in the normalized state. This is mainly due to the limitations imposed by domestic rolling equipment; the rolling ratio for thicker plates is low, and the density within the steel plates as well as the quality of the core structure are poor ； Furthermore, normalizing the steel plate can refine the grains and improve its microstructure, thereby endowing the plate with better toughness, plasticity, and overall mechanical properties. 6. Why should tension tests and Charpy (V-notch) impact tests at room temperature or low temperature be conducted on each sheet of carbon steel and low-alloy steel sheets in the quenched and tempered condition, which are used for the inner cylinders of multi-layer wrapped containers? Answer: After quenching and tempering, the yield strength of low-alloy steel increases, but its impact toughness is not stable. In order to accurately assess its overall mechanical properties, tensile and impact tests must be conducted on each individual piece for verification. The inner cylinder of a multi-layer wrapped container is an inner cylinder for equipment that is subject to high operating pressures, with a design pressure of 10~100 Mpa ； At the same time, high-pressure vessels are often subjected to high temperatures as well as corrosion from various media, and their operating conditions are severe; therefore, the requirements for material selection, manufacturing, and inspection of high-pressure vessels are quite high, in order to ensure their safe use. 7. Why are 20R with a nominal thickness δn greater than 25 mm, and 16MnR, 15MnVR, 15MnVNR with δn greater than 38 mm, as well as 18MnMoNbR and 13MnNiMoNbR in any thickness, subjected to Charpy (V-notch) low-temperature impact testing when the design temperature is below 0°C? Why is the sample taken horizontally? What is the indicator for low-temperature impact energy? Answer: For domestic steel grades such as 16MnR, 15MnVR, and 15MnVNR, when their thickness reaches a certain level, or for steel plates of grades 18MnMoNbR and 13MnNiMoNbR with higher strength levels at any thickness, the temperature at which ductility transitions may occur can be in the range of -19.9 to 0°C – a situation that is very dangerous. Yet these materials are not treated as low-temperature materials. To avoid this problem, V-notch impact tests on the base material and test plates must be conducted within the aforementioned temperature range to determine whether they meet the design requirements.    Due to uneven chemical composition or the presence of impurities during the casting of steel ingots, these uneven areas and impurities extend along the direction in which the metal is stretched after hot rolling, resulting in what are known as \"flow lines\" or fibrous structures. As a result, the mechanical properties of the metal exhibit anisotropy: the mechanical properties parallel to the flow lines are higher than those perpendicular to them, with greater differences observed in plasticity and toughness. Therefore, the standards for manufacturing pressure vessel steel plates specify the mechanical properties measured in the transverse direction, which have lower values, as the criteria for impact resistance, in order to ensure the safe use of the material. The indicator for low-temperature impact energy is: Akv at 20R ≥ 18J ； The Akv value for 16MnR and 15MnVR is ≥20 J, while that for 18MnMoNbR and 13MnNiMoNbR is ≥27 J. 8. Under what circumstances should carbon steel and low-alloy steel plates used for container shells be subjected to ultrasonic testing on a sheet-by-sheet basis? Answer: Ultrasonic testing shall be performed on each item if any of the following conditions is met: ① Pressure vessels containing media with extremely high or high toxicity levels. ② Pressure vessels whose filling medium is liquefied petroleum gas and which have a hydrogen sulfide content of more than 100 mg/L. ③ Pressure vessels with a maximum operating pressure of 10 Mpa or higher. ④ Steel plates that require ultrasonic testing on a sheet-by-sheet basis, as specified in Chapter 2 and Appendix C of GB150, GB151 \"Shell and Tube Heat Exchangers\", GB12337 \"Steel Spherical Tanks\", and other **national and industry standards. 9. When low-alloy steel plates are used at temperatures equal to or below -20°C, what requirements must be met regarding their operating conditions and the minimum temperature for impact testing? Answer: Its service condition and minimum impact test temperature shall meet the following requirements: ① For 16MnR in the hot-rolled state: thickness 6–25 mm, minimum impact test temperature is -20°C; in the normalized state: thickness 6–120 mm, minimum impact test temperature is -20°C. ② For 16MnDR in the normalized state: thickness 6–36 mm, minimum impact test temperature is -40°C; in the normalized plus tempered state: thickness 36–100 mm, minimum impact test temperature is -30°C. ③ For 0.9Mn2VDR in the normalized or normalized plus tempered state: thickness 6–36 mm, minimum impact test temperature is -50°C. ④ For 0.9MnNiDR in the normalized or normalized plus tempered state: thickness 6–60 mm, minimum impact test temperature is -70°C. ⑤ For 15MnNiDR in the normalized or normalized plus tempered state: thickness 6–60 mm, minimum impact test temperature is -45°C. 10. What is the sensitization range of austenitic stainless steels? Answer: When austenitic stainless steel is cooled slowly in the range of 400–850°C, high-chromium carbides such as Cr23C6 precipitate at the grain boundaries, resulting in chromium depletion in the adjacent areas and an increased tendency to intergranular corrosion. This temperature range is known as the sensitization range. 11. What is solution heat treatment? What is its effect on the properties of austenitic stainless steel? Answer: The process of heating the alloy to a high temperature and maintaining it at that temperature until the excess elements are fully dissolved into the solvus, followed by rapid cooling to obtain a saturated solvus, is known as solvus treatment. By solution treatment, the high-temperature structure of chromium-nickel stainless steel is stabilized at room temperature, resulting in carbon-saturated austenite in order to improve the corrosion resistance of such stainless steel. It can also improve the plasticity and toughness of chromium-nickel stainless steel. 12. How many measures are currently in use to prevent intergranular corrosion? Answer: ① Solution treatment. ② Reduce the carbon content in steel.    ③ Add elements that stabilize carbon. 13. What is stress corrosion cracking? In which media is austenitic stainless steel prone to stress corrosion? Answer: Stress corrosion is the cracking of metals caused by the combined effect of stress (tensile stress) and corrosion (under certain temperature conditions). Stress corrosion is a complex phenomenon; in the absence of stress, corrosion is minimal ； When stress is applied, metal cracks even when corrosion is not severe. Since the cracking is brittle and occurs without any obvious warning signs, it can lead to catastrophic accidents. The main combinations of metal materials susceptible to stress corrosion and their environments are as follows: ① Carbon steel and low-alloy steel: media include alkaline solutions, nitrate solutions, anhydrous liquid ammonia, wet hydrogen sulfide, acetic acid, etc. ② Austenitic stainless steel: chloride ions, chlorides + steam, wet hydrogen sulfide, alkaline solutions, etc. ③ Molybdenum-containing austenitic stainless steels: alkaline solutions, chloride aqueous solutions, sulfuric acid + copper sulfate aqueous solutions, etc. ④ Brass: ammonia and its solutions, ferric chloride, wet sulfur dioxide, etc. ⑤ Titanium: methanol or ethanol containing hydrochloric acid, molten sodium chloride, etc. ⑥ Aluminum: wet hydrogen sulfide, hydrogen-containing sulfides, seawater, etc. 14. Can ultrasonic testing be used for austenitic stainless steel welds? Why? Answer: Due to the presence of twin boundaries and other factors in austenitic stainless steel that significantly affect the attenuation and propagation of ultrasonic waves, ultrasonic testing is currently not widely used for this type of stainless steel. 15. What requirements must be met when designing pressure vessels using newly developed steel materials? Answer: When designing pressure vessels using newly developed steel materials, complete technical evaluation documents are required, and such documents must be approved by the National Pressure Vessel Standardization Technical Committee. 16. Can the 20g steel plate specified in GB713 “Carbon and low-alloy steel plates for boilers” be used as a substitute for steel plates used in other containers? Answer: The 20g steel plate specified in GB713 \"Carbon and Low-Alloy Steel Plates for Boilers\" can be used as a substitute for the Q235-C steel plate. 17. What requirements must be met when using B-grade steel plates specified in GB712 \"Structural Steel for Ship Hulls\" as a substitute for Q235-C steel plates? Answer: When using Q235-C plates as substitutes, the steel mill must conduct impact tests in accordance with the standards. For Grade B steel plates for which the steel mill did not conduct impact tests with the approval of the ship inspection authority, only Q235-B steel plates can be used as substitutes. 18. For carbon steel and low-alloy steel pipes, when the operating temperature is ≤ -20°C, the operating conditions and the minimum impact test temperature are specified in the table below: Steel grade, Operating condition, Wall thickness, mm, Minimum impact test temperature, °C: 10, Normalized, ≤16, -30; 20G, Normalized, ≤16, -20; 16Mn, Normalized, ≤20, -40; 09MnD, Normalized, ≤16, -50. For pipes for which it is not possible to manufacture small impact test specimens of size 5X10X55 due to dimensional constraints, the impact test is exempted. The minimum operating temperature for pipes of each steel grade is specified in Appendix C. 19. How is the grade of forgings determined? What grade should be selected for carbon steel and low-alloy steel forgings with a nominal thickness greater than 300 mm? Answer: The grade of the forgings is selected in accordance with the provisions of JB4726 \"Technical Requirements for Forgings of Pressure Vessels\". For carbon steel and low-alloy steel forgings with a nominal thickness greater than 300 mm, grade III or grade IV should be selected. 20, 16MnD steel forgings: What is their heat treatment condition and the lowest temperature for impact testing when the operating temperature is equal to or below -20°C?    Answer: The following requirements shall be met: steel grade, heat treatment condition, nominal thickness in mm, minimum impact test temperature in °C. For 16MnD, the condition is normalizing followed by tempering; for quenched and tempered steel, the values are ≤200, 40 >200, 300–300. For low-alloy steel bolts, when the operating temperature is equal to or below -20°C, what are their operating condition and minimum impact test temperature? Answer: The following requirements shall be met: Steel grade, specification (mm), minimum impact test temperature (°C), Akv (J), service condition. 30CrMoA: ≤M56-100, ≥273; 5CrMoA: ≤M56-100, ≥27, ≤M60~M80, -70, ≥27. 1. What requirements must be satisfied when foreign steel is used for the pressure-bearing components of pressure vessels? Answer: When selecting foreign materials, they should be steels permitted for use according to the latest standards for pressure vessels in that country. Their scope of use generally shall not exceed the provisions of such standards, nor shall it go beyond the requirements specified for steels in Chapter 4 on materials and technical requirements of GB150. And it complies with the provisions of the Code of Practice. 22. What requirements must aluminum and aluminum alloys used in the pressure-bearing components of pressure vessels meet? Answer: ① The design pressure should not exceed 8 Mpa, and the design temperature ranges from -269 to 200°C. ② When the design temperature is greater than 65°C, aluminum alloys with a magnesium content of 3% or more are generally not selected. 23. What requirements must titanium and titanium alloys used in the pressure-bearing components of pressure vessels meet? Answer: ① Design temperature: Industrial pure titanium should not exceed 250°C, titanium alloys should not exceed 300°C, and composite sheets should not exceed 350°C. ② Titanium materials used for manufacturing pressure vessel shells should be used in the annealed state. 24. What condition should copper and copper-alloy materials in the pressure-bearing components of pressure vessels be in? Answer: It should generally be in the annealed state.