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Chapter 2 Materials 2-1 How to select steel for pressure vessels? Answer: When selecting steel for pressure vessels, factors such as the operating conditions of the vessel (e.g., design temperature, design pressure, properties of the medium, and operational characteristics), the weldability of the material, the manufacturing process of the vessel, and economic viability should be taken into consideration. Under normal circumstances, material selection is carried out according to the following principles: (1) When the required thickness of the steel plate is less than 8 mm, carbon steel plates should be preferred over low-alloy high-strength steel plates, except in the case of materials used for multi-layer containers ； (2) In applications where stiffness or structural design is the primary concern, ordinary carbon steel should be preferred as much as possible. In applications where strength design is the primary consideration, steel plates such as Q235-A, Q235-B, Q235-C, 20R, and 16MnR should be selected accordingly, based on usage constraints related to pressure, temperature, and the medium involved ； (3) When the required thickness of stainless steel is greater than 12 mm, structures such as lining, composite materials, or surfacing should be used as much as possible ； (4) Stainless steel should be avoided as a heat-resistant steel for applications with a design temperature of 500°C or less ； (5) Pearlitic heat-resistant steel should be avoided as a heat-resistant steel for applications where the design temperature is 350°C or lower. When pearlitic heat-resistant steel must be used for heat-resistant or hydrogen-resistant applications, the variety and specifications of the steel materials should be reduced and combined as much as possible ； (6) Carbon steel is used for atmospheric and low-pressure vessels with weak medium corrosion, medium-pressure vessels with relatively thin walls, forgings, pressure-bearing steel pipes, non-pressure components, and other applications where the wall thickness is determined by rigidity or structural factors ； (7) Low-alloy high-strength steel is used for pressurized vessels with low medium corrosivity and large wall thicknesses (≥8 mm) ； (8) Pearlite heat-resistant steel is used for resisting corrosion by high-temperature hydrogen or hydrogen sulfide, or as a heat-resistant steel for pressure vessels operating at temperatures of 350–650°C ； (9) Stainless steel is used in applications where the medium is highly corrosive (electrochemical or chemical corrosion), to prevent contamination by iron ions, or in applications requiring heat resistance or low-temperature performance, where the design temperature is greater than 500°C or less than -100°C ； (10) Austenitic stainless steels that contain no stabilizing elements and have a carbon content of more than 0.03% should not be used in environments that may cause intergranular corrosion of the stainless steel when welded or subjected to hot working at temperatures above 400°C. 2-2 What are the differences among the three grades of carbon steel and normalized steel, namely Grade A, Grade B, and Grade C of Q235? Answer: The main difference between them is the impact test temperature: No impact test is conducted on grade Q235A ； Conduct V-notch impact test at room temperature of 20°C for Q235B grade ； Q235C grade is subjected to a 0°C V-notch impact test. 2-3 What effect does carbon have on the weldability of steel? What are the effects of other alloying elements? Answer: When steel is welded, the heat-affected zone of the weld is heated above Ac3, and it becomes hardened after rapid cooling. The higher the carbon content in steel, the greater the tendency for hardening and embrittlement in the heat-affected zone, making it prone to cracking under welding stresses. The effect of the chemical composition of steel on its hardenability is usually expressed as the carbon equivalent, denoted as CE. It is generally believed that the critical carbon equivalent for the weldability of steel is 0.45%. The commonly used carbon equivalent calculation formula for carbon steel and low-alloy steel recommended by the International Institute of Welding is as follows: During welding, the grain growth in the weld area caused by high temperatures increases the tendency for post-weld cracking ； Adding elements to steel that refine grains and inhibit their growth, such as Mo, Ti, and V, along with degassing using Al, helps improve weldability, whereas C, Ni, and Mn increase the risk of cracking. 2-4 What is the scope of application for carbon steel boiling steel plate Q235-A·F? Answer: The design pressure of the container p ≤ 0.6 MPa ； The operating temperature for the steel plate is 0～250℃ ； When used for the housing, the steel plate thickness shall not exceed 12 mm ； Must not be used for pressure vessels containing flammable media or media with moderate, high, or extremely hazardous toxicity levels ； It shall not be used to manufacture pressure vessels directly heated by flames. 2-5 What is the scope of application for normalized carbon steel sheets Q235-A, B, C? Answer: Their scope of application is: a) Q235-A steel plates: for vessels with a design pressure p ≤ 1.0 MPa ； The operating temperature for the steel plate is 0～350℃ ； When used for the housing, the steel plate thickness shall not exceed 16 mm ； It shall not be used in pressure vessels for liquefied petroleum gas media, or for media with a high or extremely hazardous level of toxicity ； It shall not be used to manufacture pressure vessels directly heated by flames. b) Q235-B steel plate: container design pressure p ≤ 1.6 MPa ； The operating temperature for the steel plate is 0～350℃ ； When used for the housing, the steel plate thickness shall not exceed 20 mm ； It shall not be used in pressure vessels containing media with a high or extremely hazardous toxicity level. c) Q235-C steel plate: container design pressure p ≤ 2.5 MPa ； The operating temperature for the steel plate is 0～400℃ ； When used for the housing, the steel plate thickness shall not exceed 30 mm. 2-6 What issues should be considered when using carbon steel and carbon-manganese steel at temperatures above 425°C over a long period of time? Why? Answer: GB150-1998 stipulates that when carbon steel and carbon-manganese steel are used at temperatures above 425°C for extended periods, the tendency of the carbide phases in the steel to graphitize must be taken into account. In such cases, in carbon steel and carbon-manganese steel, the cementite in the steel undergoes decomposition: 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 both the strength and ductility of the material; the impact value drops even more, and the steel becomes significantly more brittle. The American ASME standards also specify the same requirements in this regard. 2-7 What issues should be considered when using austenitic steel at temperatures above 525°C? Why? Answer: GB150-1998 stipulates that when austenitic steel is used at temperatures above 525°C, its carbon content must be at least 0.04%. This is because when austenitic steel is used at temperatures above 500–550°C, if its carbon content is too low, its strength and oxidation resistance will decrease significantly. Therefore, general guidelines are established for the range of applications of ultra-low carbon (C≤0.03%) austenitic stainless steels: 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 (C=0.04–0.1%), primarily to ensure corrosion resistance as well as heat resistance and thermal strength. 2-8 How is the scope of application of stainless steel clad plates determined? Answer: The scope of application for stainless steel composite steel plates shall comply simultaneously with the regulations regarding the application of both the base material and the composite material. 2-9 Which carbon steel and low-alloy steel sheets should be used in the normalized state? Why? Answer: 20R and 16MnR with a shell thickness greater than 30 mm, as well as 20R and 16MnR with a thickness greater than 50 mm used for other pressure-bearing components (flanges, tube sheets, flat covers, etc.), and 15MnVR with a thickness greater than 16 mm, shall be used in the normalized condition. This is mainly due to the limitations imposed by domestic rolling equipment; the rolling ratio for thicker plates is lower, resulting in slightly poorer density within the steel plates as well as lower quality of the central structure ； Furthermore, normalizing the steel plate can refine the grain structure and improve its microstructure, thereby endowing the plate with better toughness, plasticity, and overall mechanical properties. 2-10 Why should carbon steel and low-alloy steel sheets supplied in quenched and tempered condition and used for the inner liners of multi-layer wrapped containers be subjected to tensile testing and Charpy (V-notch) impact testing (at room temperature or low temperature) one by one? 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 specimen individually for verification. The inner cylinder of a multi-layer wrapped container is an inner cylinder for equipment that is capable of withstanding high pressures, with a design pressure of 10–100 MPa ； At the same time, high-pressure vessels often have to withstand high temperatures as well as corrosion from various substances; their operating conditions are severe. Therefore, high standards are required for the material selection and manufacturing inspection of such vessels, in order to ensure their safe use. 2-11 Why must steel plates of 20R with a thickness greater than 25 mm, 16MnR, 15MnVR, 15MnVNR with a thickness greater than 38 mm, and 18MnMoNbR and 13MnNiMoNbR in any thickness, used for shells when the operating temperature is below 0°C, undergo Charpy (V-notch) low-temperature impact tests in batches? Why is the sample taken transversely? What is the indicator for low-temperature impact energy? Answer: For steel plates such as 20R, 16MnR, 15MnVR, 15MnVNR with a thickness reaching a certain limit, and 18MnMoNbR and 13MnNiMoNbR in any thickness, the ductility transition temperature may lie between -19.99°C and 0°C, which is extremely dangerous. Yet they are not treated as low-temperature materials. To avoid this issue, it is necessary to conduct Charpy (V-notch) low-temperature impact tests within the aforementioned temperature range to verify whether the design requirements can be met. Due to uneven chemical composition (referred to as segregation in metallography) or the presence of impurities during the casting of steel ingots, after hot rolling, these uneven areas and impurities extend along the direction of metal elongation, forming what are known as \"flow lines\" or fibrous structures (referred to as banded structures in metallography). As a result, the mechanical properties of the metal exhibit anisotropy: the mechanical properties in the direction parallel to the flow lines (the longitudinal direction) are higher than those perpendicular to them (the transverse direction), with greater differences observed in plasticity and toughness. Therefore, in the standards for manufacturing container steel plates, the lower mechanical properties in the transverse direction are used as the criteria for impact resistance, in order to enhance the reliability of safe use of the material. The indicator for low-temperature impact energy is: AKV at 20R ≥ 18J ； AKV of 16MnR and 15 MnVR ≥ 20J ； The AKV of 15MnVNR, 18MnMoNbR, and 13MnNiMoNbR is ≥27 J. 2-12 Under what conditions should carbon steel and low-alloy steel plates used for manufacturing pressure vessel shells be subjected to ultrasonic testing on a sheet-by-sheet basis? What should its qualified grade be, at least, according to the standards specified in JB4730? Answer: Ultrasonic testing shall be performed on each unit if it meets one of the following conditions: 1. Pressure vessels containing media that are extremely or highly toxic. 2. Pressure vessels whose filling medium is liquefied petroleum gas and which have a hydrogen sulfide content greater 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 4 and Appendix C of GB150, GB151 \"Shell and Tube Heat Exchangers\", GB12337 \"Steel Spherical Storage Tanks\", and other **national and industry standards. 5. Mobile pressure vessels. The qualification grade of the steel plates used for the containers described in paragraphs 1, 2, and 5 above shall be no lower than Grade II as specified in JB4730 ； The qualified grade of the steel plates used for the containers mentioned in paragraph 3 above shall be no lower than Grade III as specified in JB4730 ； The grade of the steel plates used for the containers mentioned in paragraph 4 above meets the requirements of the relevant standards. 2-13 When low-alloy steel plates are used at temperatures equal to or below -20°C, what requirements should be met regarding their operating conditions and the minimum temperature for impact testing? Answer: When low-alloy steel plates are used at temperatures equal to or below -20°C, their service conditions and the minimum impact test temperature are specified in the table below: Steel grade, Service condition, Thickness, mm, Minimum impact test temperature, °C. 16MnR, Hot-rolled, 6–25, -20; Normalized, 6–12, 0. 07MnCrMoVR, Quenched and tempered, 16–50, -20. 16MnDR, Normalized, 6–36, -40; >36–100, -30. 07MnNiCrMoVDR, Quenched and tempered, 16–50, -40. 15MnNiDR, Normalized or normalized plus tempered, 6–60, -45. 09Mn2VDR, Normalized or normalized plus tempered, 6–36, -50. 09MnNiDR, Normalized or normalized plus tempered, 6–60, -70. 2-14. What is the sensitization range of austenitic stainless steels? Answer: When austenitic stainless steel is slowly cooled within the range of 427–870°C, chromium-rich carbides Cr23C6 precipitate along the grain boundaries. This results in chromium depletion in the areas adjacent to the carbides, leading to a tendency for intergranular corrosion. This temperature range is referred to as the sensitization range. 2-15 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 constant temperature in the high-temperature single-phase region, allowing the excess phase to fully dissolve into the solid solution, and then rapidly cooling it to obtain a saturated solid solution is called solution heat treatment. By solution treatment, the high-temperature structure of chromium-nickel stainless steel is stabilized at room temperature to obtain carbon-saturated austenite, thereby improving the corrosion resistance of such stainless steel. Furthermore, it can also improve the plasticity and toughness of chromium-nickel stainless steel. 2-16 What are the main measures currently in use to prevent intergranular corrosion in austenitic stainless steels? What are the environments that can cause intergranular corrosion? Answer: There are roughly three types: ① Solution treatment ； ②Reduce the carbon content in steel ； ③Elements that add stable carbides. An environment that may cause intergranular corrosion is an electrochemical corrosion environment with the presence of an electrolyte. The electrolytes that can cause intergranular corrosion in austenitic stainless steels are mainly acidic media, such as industrial acetic acid, formic acid, chromic acid, lactic acid, nitric acid (except dilute nitric acid at room temperature), oxalic acid, phosphoric acid, hydrochloric acid, sulfuric acid, sulfurous acid, urea reaction media, etc. Chemically pure acetic acid, alcohols, aldehydes, phenols, alkanes, gasoline and other solutions, as well as their gaseous phases, do not cause intergranular corrosion in austenitic stainless steel. Therefore, it is not necessary to conduct tests on the tendency for intergranular corrosion in austenitic stainless steel equipment that comes into contact with these media. Furthermore, for austenitic stainless steel equipment intended to prevent iron ion contamination, intergranular corrosion tendency tests are also not required. 2-17 What is stress corrosion cracking? In which media is austenitic stainless steel prone to stress corrosion cracking? Answer: Stress corrosion cracking is a type of failure that occurs in metals as a result of the combined action of stress (tensile stress) and corrosion, under certain temperature conditions. Stress corrosion is a complex phenomenon; in the absence of stress, corrosion is minimal ； Under stress, metals can crack 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 failure 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. 2-18 Can ultrasonic testing be used for welded joints of austenitic stainless steel? Why? Answer: Due to factors such as twin grain boundaries present in austenitic stainless steels, which significantly affect the attenuation and propagation of ultrasonic waves, ultrasonic testing has not yet been widely adopted in these types of stainless steel. 2-19 Which container steel plates can be used as substitutes for the 20g steel plate specified in GB713-1997 \"Carbon and low-alloy steel plates for boilers\"? Answer: The 20g steel plate specified in GB713-1997 \"Carbon and low-alloy steel plates for boilers\" can be used as a substitute for the Q235-C steel plate. 2-20 When carbon steel and low-alloy steel pipes 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: When carbon steel and low-alloy steel pipes are used at temperatures equal to or below -20°C, their service conditions and minimum impact temperatures are specified in the table below: Steel grade, Service 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 produce small impact test specimens of 5mm×10mm×55mm due to size constraints, impact tests are exempt. The minimum service temperatures for pipes of each steel grade are specified in Table C1 of Appendix C (the appendix to the standard) of GB150. 2-21 When the operating temperature for seamless steel pipes made of carbon steel Grade 10 is ≤ -20°C, which standard should be used for selecting the pipes? Answer: The material used for low-temperature carbon steel seamless pipes is generally grade 10 steel. Currently, there are three standards for grade 10 steel seamless pipes: GB6479-2000 \"High-pressure seamless pipes for fertilizer equipment.\" This standard specifies that the supply condition for grade 10 steel pipes should be normalized, with an impact value guaranteed at -20°C. By agreement between the supplier and the buyer, 10 Number steel pipes can be subjected to Charpy (V-notch) impact tests at -30°C. GB8163-1999 \"Seamless steel tubes for transporting fluids\" specifies that steel tubes made of grade 10 steel shall be supplied in either hot-rolled or heat-treated condition, with no requirement for impact testing. GB9948-88 \"Seamless Steel Tubes for Petroleum Cracking\" specifies that for steel tubes made of grade 10 steel, the supply condition should be hot-rolled tubes after final rolling, or cold-drawn tubes after normalizing; no low-temperature impact test is required. Therefore, when using Grade 10 seamless steel pipes in applications at ≤-20°C, pipes conforming to the GB6479-2000 standard “High-pressure seamless steel pipes for chemical fertilizer equipment” should be selected. Additionally, requirements for delivery in the normalized condition and low-temperature impact testing must be specified. 2-22 How many grades are there for steel forgings used in pressure vessels? What determines the level selected? Answer: Carbon steel, low-alloy steel, and stainless steel forgings for pressure vessels are classified into four grades: I, II, III, and IV. Low-alloy steel forgings for low-temperature pressure vessels are classified into three grades: II, III, and IV. The grade of the forged part is determined by the designer based on its shape, operating conditions, as well as size and weight, and it must be indicated on the drawings (by adding a grade symbol after the steel grade, such as 16MnⅡ). 2-23 When 16MnD steel forgings are used at temperatures equal to or below -20°C, what is their heat treatment condition and the minimum impact test temperature? Answer: It shall comply with the provisions in the table below: Steel grade, heat treatment condition, nominal thickness, mm; minimum impact test temperature, °C. For 16MnD: normalizing followed by tempering, quenching and tempering – ≤200: 40; >200: 300–30; 2–24. What are the regulations regarding the steel grades and heat treatment conditions for studs used in low-temperature environments, as well as the requirements for impact tests? Answer: It shall comply with the specifications in the table below: Steel grade, heat treatment condition, specifications (mm), minimum impact test temperature (°C), AKV value (J). For 30CrMoA after quenching and tempering: ≤M56-100, ≥27; for 35CrMoA after quenching and tempering: ≤M56-100, ≥27; for M60–M80: -70; for 40CrNiMoA after quenching and tempering: M52–M80, -70, ≥31; for M85–M140: -50; 2–25. What requirements must be met when foreign materials are used for the pressure-bearing components of pressure vessels? Answer: 1. Materials that are permitted for use according to foreign pressure vessel codes and for which there are existing examples of use should be selected; their scope of application shall comply with the relevant codes and standards of the country where the material is produced, and a quality certificate for that material must be available. 2. Before first use, the manufacturing unit shall conduct a welding procedure qualification and welder tests, as well as re-test the chemical composition and mechanical properties. Only after these meet the usage requirements can production commence. 3. The technical requirements shall generally not be lower than the technical specifications of corresponding domestic materials. 4. Materials that are used for the first time in China and for which the specified lower limit for tensile strength in the standards is 540 Mpa or higher shall go through the approval procedures as stipulated in Article 7 of the Code for Tolerance. When domestic material manufacturers produce materials of foreign grades, they must follow the smelting methods specified in the foreign standards for those grades. The requirements regarding sample types, sizes, processing specifications, and testing methods for mechanical and bending property tests also need to comply with foreign standards. Before mass production, product certification must be obtained and approval from the relevant safety supervision agency is required; in such cases, the materials can be treated as the aforementioned foreign steel products. 2-26 What requirements must be met when manufacturing pressure vessels using newly developed materials or materials not specified in standards such as GB150? Answer: The test verification data for this material and the third-party inspection reports should be submitted to the National Pressure Vessel Standardization Technical Committee for technical evaluation, in order to obtain a certification document issued by the committee granting permission for trial use (specifying the conditions of use). Approval procedures shall also be followed in accordance with Article 7 of the Pressure Vessel Regulations. 2-27 What requirements must aluminum and aluminum alloys used in the pressure-bearing components of pressure vessels meet? Answer: It shall meet the following requirements: 1. The design pressure should not exceed 8 MPa, and the design temperature range is from -269 to 200°C. 2. When the design temperature is greater than 75°C, aluminum alloys with a magnesium content of 3% or more are generally not selected. 2-28 What requirements must titanium and titanium alloys used in the pressure-bearing components of pressure vessels meet? Answer: It shall meet the following requirements: 1. Design temperature: Industrial pure titanium should not exceed 230°C, titanium alloys should not exceed 300°C, and titanium composite sheets should not exceed 350°C. 2. Titanium materials used for manufacturing pressure vessel shells should be used in the annealed state. 3. The forming of the head of titanium pressure vessels should be achieved through hot forming or cold forming followed by thermal reshaping. Ultrasonic testing should be performed on the formed titanium-steel composite plate heads. 4. Titanium pressure vessels generally do not require heat treatment; however, titanium vessels used in stress-corrosion environments or those fabricated from medium-thickness plates should undergo stress-relief annealing after welding or hot working. After explosive bonding of titanium-steel clad plates, stress-relief annealing should be performed. 5. The following welds in titanium pressure vessels shall be subjected to penetrant testing: (1) the fillet welds connecting the nozzles, flanges, reinforcement rings to the shell or head ； (2) Welds connecting the heat exchanger tube sheet to the tubes ； (3) The lap welds of the cladding welds on the titanium-steel composite plate, as well as the lap welds between the trim plates and the cladding of the composite plate. 2-29 What condition should copper and copper alloys in the pressure-bearing components of pressure vessels be in? Answer: It should generally be in the annealed state