Similarities and differences in the design and manufacturing of low-temperature quenched and tempered high-strength steel spherical tanks and conventional low-alloy steel spherical tanks

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The several steel grade numbers of low-temperature quenched and tempered high-strength steel that are commonly used in spherical tanks include Q490DRL2 (-50°C) and Q490DRL1 (-40°C)； Commonly used low-alloy steel grades include Q345R and Q370R. Here is a brief introduction to the main differences between the two types of steels used for spherical tanks mentioned above. 1. Design method: Considering that the unit price of low-temperature quenched and tempered high-strength steel is generally about 60% higher than that of ordinary alloy steel, when the thickness of the spherical tank exceeds 30 mm, a stress analysis-based design method is usually adopted for such tanks. This not only helps to reduce steel consumption but also enables more accurate and comprehensive overall and local stress analyses, thereby ensuring the safety of the spherical tank. For spherical tanks made of ordinary low-alloy steel, design is still primarily carried out in accordance with standards, resulting in a relatively low threshold for such design. 2. Heat treatment condition of the steel plate: For ordinary low-alloy steel, plates with a thickness of 30 mm or less are generally supplied in the hot-rolled or controlled-rolling condition, while thicker plates over 30 mm are usually supplied in the normalized condition. When the medium is highly hazardous or prone to stress corrosion, a normalized condition for delivery is also required in the design of thinner low-alloy steels. The supply condition of low-temperature quenched and tempered high-strength steel plates is quenched and tempered, that is, they undergo high-temperature tempering after quenching. Quenching and tempering heat treatment places higher demands on the heat treatment furnaces in steel mills; therefore, only large-scale steel mills generally possess the capability to carry out such treatments effectively. Special note: To ensure that post-weld heat treatment does not damage the properties of the quenched and tempered steel plate, and to prevent it from weakening the mechanical properties of the material, it is essential to carefully consider the relationship between the tempering temperature of the steel plate and the maximum temperature during post-weld heat treatment, ensuring that the latter is not higher than the high-temperature tempering temperature of the steel plate. 3. The acceptance criteria for the mechanical properties of steel plates differ: Low-temperature quenched and tempered high-strength steel plates require not only low-temperature impact tests at lower temperatures, but also a drop-weight test, as well as tests on the mechanical properties of the core portion of the steel plate. Moreover, the ultrasonic testing requirements for low-temperature quenched and tempered high-strength steel plates are generally at level T1, which is higher than the level I or II requirements for ordinary steel plates. Ordinary low-alloy steel plates generally only require routine mechanical property tests in accordance with the standards for such plates. 4. The requirements during the manufacturing process are also more stringent: Low-temperature quenched and tempered high-strength steel spherical tanks demand higher precision in the fabrication of the spherical shell plates, as well as stricter requirements for weld grinding. Moreover, the control of temperature uniformity during heat treatment is more rigorous, with it being necessary to ensure that the heat treatment temperature does not exceed the tempering temperature. 5. The requirements for non-destructive testing are also more stringent: Higher strength steels that have been tempered at low temperatures generally require higher standards for non-destructive testing; for example, the technical level for TOFD testing is raised to grade C ； Or raise the qualified level from Grade II to Grade I ； Moreover, low-temperature quenched and tempered high-strength steel spherical tanks require 100% magnetic particle testing both before and after the hydrostatic test. 6. Some other special requirements: Due to their high yield strength ratio, low-temperature quenched and tempered high-strength steels require a higher level of purity in the steel material. For example, impurities such as sulfur and phosphorus in the chemical composition need to be controlled at levels lower than the standard requirements, and control limits also need to be set for non-metallic inclusions. The others also involve the control of carbon equivalent and hardness parameters ； Test requirements for the simulated post-weld heat treatment of quenched and tempered steel plates and associated forgings, etc. 7. Welding material selection: At present, for low-temperature quenched and tempered steel spherical tanks in China, about half still choose imported welding materials from abroad. Given the high prices and long lead times of imported welding materials abroad, many owners are willing to opt for domestic alternatives. In China, several top-tier suppliers of high-strength steel welding materials already possess a high level of technical expertise and extensive practical experience; in fact, some of their product specifications show better performance compared to imported welding materials. 8. Assembly and welding: The precision of assembly has a direct impact on the quality of welding that follows. If the pressing precision is poor, and quality control is not strict during installation, cracks are likely to occur in low-temperature high-strength steel after welding. The construction of low-temperature high-strength steel spherical tanks requires advanced pressing techniques for such steel plates, extensive assembly experience, and a highly skilled welding workforce.