| A516 steel is an important low-carbon structural steel which mainly contains the alloy elements of Mn and Si. It is widely used for cryogenic pressure vessel in the energy and petrochemical industries. Cryogenic pressure vessel works in the low temperature and has a complex working condition. Thus, the low temperature toughness is the most important performance indicators. It is well known that phosphorus can cause "cold embrittlement" phenomenon, as grain boundary segregation element. However, dephosphorization is not only increase production costs and production cycle, but also the lower phosphorus content is not easy to meet. In this experiment, cryogenic pressure vessel in the case of relatively high phosphorus content is hope to meet the application requirements by adjusting the heat treatment process. Thereby we can increase production efficiency and reduce production costs.In this study, three kinds of A516 steel containing different amounts of phosphorus conduct subcritical normalizing process and accelerated cooling rate. The analytic methods include tensile test, impact test, metallographic examination, Scanning Electron Microscope (SEM) and so on. This experiment carry out welding test on A516 steel of high phosphorus content in the method of automatic submerged arc welding. Similarly tensile test and impact test carry out in weldment and observe the microstructure.According to the results, the microstructure of experimental steel is ferrite and pearlite. Banded structure of hot-rolled steel plate is very serious, and the grain size between the band structure is not uniform. After subcritical normalizing the banded structure is less, air cooling can eliminate banded structure of plate surface, strong wind cooled treatment can completely eliminate the banded structure of the surface and core.After subcritical normalizing process, the grain size decreases with increasing cooling rate. The grain size up to 9-10 on strong wind cooled conditions. At the same time domain size and lamellar spacing of pearlite reduced, the microstructure of pearlite began to appear short rod-shaped cementite.The different phosphorus content steel with the same process has no significant difference in microstructure. Subcritical normalizing process has little effect on the strength and ductility,but can significantly improve the impact toughness of steel, especially low-temperature toughness. The greater the cooling speed, the better the impact toughness. Anisotropy of steel plate has also been significantly improved. The experimental steel with subcritical normalizing treatment and accelerated cooling, phosphorus content of which is 0.029%, longitudinal and transverse impact energy respectively reaches 116J and 102J in-50℃. On the whole, its mechanical properties meet the engineering requirements.Welding is performed on high phosphorus steel plate, while the welding energy input is 20-56kJ/cm. Weld gets a small amount of eutectoid ferrite in grain boundary+a large number of acicular ferrite in intragranular. The overheated zone of the heat affected zone is tempered sorbite. The Strength of welded specimen and base metal are similar. The impact energy of weld metal is close to 200J at-50℃, better than the toughness of base metal. The impact energy of heat affected zone, although slightly lower, is up to 80J at-50 ℃.The performance of weldment meets the requirements of ASME standard. |
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