Effect of tempering process on strength and toughness of oil casing tube for ultra-deep well

Effect of tempering process on strength and toughness of V150 oil casing tube was studied by optical microscopy(OM),transmission electron microscopy(TEM),scanning electron microscopy(SEM) and mechanical property test.The results show that the optimum tempering temperature is 650 ℃ for the experimental steel,and strength and hardness gradually decrease with the increase of tempering time.When the steel is tempered at 650 ℃ in the time range of 15 75 min,strength could meet the requirement of V150 grade steel.When the tempering time is longer than 45 min,toughness can satisfy the requirement of V150 steel grade.In order to obtain an excellent combination of strength and toughness and to save energy and reduce cost,the optimum tempering time should be controlled within the range from 45 to 60 min.Strength of the oil casing tube decreased and toughness increases with the increase of tempering frequency.When the steel is tempered twice at 650 ℃ for 45 min,quenching stress releases completely,and remaining austenite transform into finer tempered structure,which makes the lateral impact energy of the steel exceed 110 J at 0 ℃,and the strength satisfies the requirement of V150 grade steel.