Limitations of GB6479-2000 Grade 20 special fertilizer pipes
Limitations of GB6479-2000 Grade 20 special fertilizer pipes
GB6479-2000 Grade 20 fertilizer special pipe was drafted by two steel companies and has been implemented for 14 years. With the development of society, certain limitations have been exposed, such as the standard "Table 4" Note: "When the final rolling temperature of the hot-rolled pipe meets the normalizing temperature, it can replace normalizing". Normalizing process; for example, the impact energy in Table 5 adopts U-shaped notch impact data with high absorption energy, while the V-notch impact energy sensitive to ductile-brittle transition is "not subject to delivery conditions" in Clause 4.4.2. .
A large number of mechanical performance comparison experiments have verified that 20 steel pipes will have certain safety risks without normalizing treatment, such as 20 produced by a steel pipe factory in Jiangsu (the delivery state is normalized) and STS410 produced by a Japanese company (the delivery state is rolled) ) Ф219mm×35mm for V-shaped and U-shaped notch impact test, the results are shown in Table 4.
Note: STS410 (Japanese Industrial Standard JIS G3455-2005 Carbon Steel Seamless Steel Tube for Boiler) is similar to 20 steel.
The data in Table 4 shows that
1) The data of the domestic pipe is more discrete, and the impact value of the U-shaped notch or V-shaped notch is lower than that of the non-heat-treated Nissan pipe, indicating that the material structure and composition of the domestic pipe are not uniform, and the steel pipe has not been normalized;
2) The impact value of U-shaped notch below 20℃ is much larger than that of V-shaped notch. As long as the test temperature is greater than 0℃, Aku2 can meet the requirements of the standard for GB#479-2000 for special fertilizer tube 20#;
3) The inflection point of the V-notch impact value (ie ductile-brittle transition temperature) of the domestically-made 20 steel pipe and the Nissan STS410 steel pipe that are not normalized are all between 10 and 20 ℃. Before the implementation of GB/T 20801.3-2006, the generally adopted "H31 -67 General design technical conditions for high-pressure pipes, fittings and fasteners" The operating temperature of 20 steel pipes is -50 ~ 200 ℃, even the minimum operating temperature of 20 steel pipes of enterprise standard ZH100-93 is -20 ℃. If the fire-treated 20 steel pipe is used in a low-temperature environment, there will be hidden safety risks.
Therefore, to ensure the safe use of the pipeline, the following suggestions are made:
1) When ordering seamless steel tubes for high-pressure fertilizer equipment, users should use GB 6479-2013 (the standard was implemented on July 1, 2014);
2) The steel pipe produced according to GB#479-2000 special fertilizer pipe #20 shall be sampled according to the batch for the V-notch impact test. If it fails, it shall be normalized;
3) Hot bending technology is preferred for pipe bending. For example, cold bending is required to work above the ductile-brittle transition temperature and the hardening heat treatment must be performed after bending;
4) During loading, unloading, transportation, hoisting, and maintenance, the elbow should not be subjected to large impact loads or forced installation;
5) Strengthen the management of the steel pipes in use, do a good job of protection, avoid external force impacts, do magnetic particle or ultrasonic testing on the elbows and check the thickness and hardness during the maintenance;
6) The low-temperature brittleness failure of high-pressure pipelines should be avoided, and 20 steel high-pressure pipelines with a temperature below 0°C should be normalized or directly replaced with special low-temperature steel pipes.
