Research On New Low Temperature Oil Well Steel With High Strength And Toughness

The oil well steel used in low temperature environment requires high strength and good low-temperature impact toughness.Most of the existing low-temperature oil well steels possess relatively high Ni content,resulting in relatively high material cost.A series independtly designed low temperature oil well steels with different Ni contents were researched to improve performance and reduce material cost.The effects of heat treatment on the microstructure and mechanical properties were investigated in thermal dilatometer,optical microscope,scanning electron microscope and tensile testing machine and so on.Then optimize the heat treatment process of low temperature oil well steel The main research contents and conclusions are summarized as follows:?1?On the basis of the thermodynamic calculation and kinetic analysis,the phase transformation at thermodynamic equilibrium state and the microstructure evolution during continuous cooling process of LS steel were investigated.The results showed that the precipitates and microstructure evolution are significantly affected by Ni content in LS steel.With the increasing of Ni content,the precipitation temperature of M₇C₃ carbide decreased gradually,and the temperature range for the precipitation of M₂₃C₆ carbide was enlarged.Meanwhile,the austenitizing temperature declineed and the critical cooling rate for the transformation of martensite,bainite and pearlite reduced with the increasing of Ni content.?2?The influence of heat treatment process on the microstructure and mechanical properties of LS steel were studied by varying quenching and tempering temperature.When quenched at 880?,the LS steel could obtain complete martensite microstructure,fine austenite grains as well as excellent combination of strength and toughness.In the tempering temperature range of 200?⁷50?,with the increasing of tempering temperature,the quantity of carbides gradually increased,and the room temperature strength showed a trend of decrease.At the same time,the low temperature impact energy increased at first and then decreased.Additionally,the LS steel exhibited tempering brittlement when the tempering temperature is above600?.According to the above results,the better low temperature oil well steel is LS2 steel,the corresponding better heat treatment process is quenching at 880?and tempering at 600?,After heat treatment,the low temperature impact is 36.37 J,and yield strength and tensile strength are 726.7 MPa and 843 MPa,respectively,to meet the performance requirements of manufacturers.?3?The effect of Ni content on the microstructure and mechanical properties of LS steel was investigated.The results indicated that the austenite grain size changed in a small range and the quantity of carbides gradually increased with the increasing of Ni content.Meanwhile,with the increasing of Ni content,the strength of LS steel appeared a trend of increase,while the low temperature impact energy at-45?increased first and then showed a trend of decrease.When the Ni content is0.022wt.%,the ductile-brittle transition temperature?DBTT?is about-20?.As the Ni content increase,the DBTT increased at first to a peak value of 8?when the Ni content is 1.97wt.%and then decreased to about-14?when the Ni content is2.99wt.%.