Effects of tempering temperature on microstructures and properties of V150 casing and tubing with ultra-high strength and toughness

Effects of tempering temperature on microstructures and properties of casing tube with ultra-high strength and good toughness were studied by OM,TEM,SEM and mechanical property tests.The results show that complete tempered-sorbite microstructure can be obtained tempered at 580-700 ℃.After tempering at 580-630 ℃,the microstructure still keeps the exposure and shape of martensite,which demonstrates that the microstructure is of certain stability.Ferrite recrystallization occurs at 640 ℃ and the microstructure begins to coarsen when the tempering temperature is up to 700 ℃.Compared with the hot rolled steel,the ductility and toughness of the steel after quenching and tempering are improved greatly.High temperature tempering embrittlement is not found at 580-700 ℃,and the strength and hardness decrease gradually with the increase of ductility and toughness at elevated tempering temperature.When tempering at 650 ℃,the optimum matching between strength and toughness can be realized,fine microstructures can be gotten,which satisfies the requirement of V150 well tube steel,and therefore,the optimum tempering temperature is 650 ℃.