Study On Microstructure & Property Of X100 Pipeline Bends

In this paper,the Continuous Cooling Transformation(CCT)curve of X100 pipeline steel is established.The phase transformation,the strength-toughness and the microstructure during the manufacture process of X100 pipeline bends are systematically studied by means of heat simulation technique,mechanical properties test and microscopic analysis method.The results of phase transformation analysis show that when the cooling rate is lower than 0.2℃/s,the microstructure of X100 pipeline steel is mainly made up of PF.QF+GF are formed in a cooling rate range of 0.5～10℃/s.When the cooling rate is higher than 20℃/s,the microstructure of X100 primarily includes BF.And the cooling rate higher than 50℃/s may form LM.By the orthogonal experiment,the optimum hot-working parameters of X100 pipeline bends is chosen,which is as the base of further studies.The research indicates that the strength raises and the toughness decreases with the increasing of heating temperature.When the temperature ranges is 950℃～1050℃,it can get fine grain and BF+GF which can bring excellent strength and toughness for X100.When temperature is higher than 1050℃,the grains of X100 grow very coarse,so the toughness decreases.When temperature is between A_c1～A_c3,the strength decreases because of PF.The testing results show that it can get high strength when the cooling rate is improved. Using water as the cooling medium,it can get BF+AF,and the microstructure of multi-direction distributed BF and fine effective grains ensure excellent strength and toughness for X100.QF formed in cooling medium of air and M formed in cooling medium of saline water can both reduce the strength and toughness of X100.The results also reveal that while the temperature is about 500℃～550℃,it can get fine strength-toughness.And the strength-toughness of straight sections of pipe may be higher than base metal and the strength-toughness of bends may be equivalent with base metal by suitable temper treatment.During the process of temper,the precipitate of carbide-nitride and the recrystallization of partial grains are of benefit to high strength.And the variation of dislocation substructure and the decomposition of M-A components are of benefit to toughness. The studies also demonstrate that the strength and hardeness can be improved and the plasticity and toughness decrease with the increasing of prestrain amount and strain aging temperature,and it can get a higher ratio of yield strength to tensile strength(Y/T)and lower strain hardening exponent(n)which result in lower hardening capacity of pipeline bends.