| Ultrahigh-strength steel is wildly used in aerospace field due to its excellent performance.A W-Mo compound secondary hardening ultrahigh strength steel with tensile strength level of 2200 MPa has been development and researched,which is strengthen by?Mo,Cr,W?2C carbides.The effect of heat treatment on the mechanical properties and microstructure were systematic researched,and the best heat treatment process is determined.The precipitation processs of M2 C carbides and deformation behavior of the steel were.Density functional theo ry has been performed to research the phase stability,elastic and thermo-physical properties of Hexa-?Mo,Cr,W?2C carbides.The main conclusions obtained are as follow:To quench the steel at 950?1100?,the tensile strength,yield strength and impact energy of the test steel rise firstly,then drop with rising of temperature,.At 1000?,Strength reached a peak when the quenching temperature is 1000?,the impact energy reaches a peak at 1050?.When the temperature is at 950? or 1000?,a few of undissolved M6 C carbides exist in the steel.When the temperature is at 1050?,M6 C carbides are dissolved,and the prior-austenite grains grow up quickly.The optimum quenching temperature is about 1050?.When the drawing temperature reach about 560 ?,the grain boundary enriched lots of N?P?S elements lead to intergranular fracture.When the test steel tempers at 510 ?,with the extension of tempering time,M2 C experiences nucleation,growth and ripening.During 10min30min of tempering,it was found that cementite gradually dissolved as M2 C separated out.The transgranular M2 C carbide was at the nucleation stage to finish the transformation from G-P area to kainotype crystal nucleus.Plenty of M2 C carbides gathered at the crystal boundaries.The gathering of carbides at the crystal boundary decreased with the extension of tempering time.After 5 hours,transgranular M2 C carbide was at the growth stage when cementite dissolved completely,and the gathering of M2 C carbide at crystal boundaries disappeared.After 100 hours,M2 C carbide was at the ripening stage when carbides with two sizes appeared.Under the effect of interfacial energy,large-size carbide grew while the small-size carbide gradually dissolved.At the nucleation and growth stages,M2 C carbides were influenced by diffusion of alloy elements.The number density,size and volume content of carbide particles increased constantly.Under the influence of diffusion of alloy elements,the content of C,Cr,Mo and W increased gradually.At the ripening stage,the size and volume content of M2 C carbides continued to increase while the number density of carbide particles decreased.Affected by the diffusion of alloy elements and stability of carbide,Mo and W that have stronger connection ability with C atoms gradually replaced Cr atom,which leads to the increase of the content of C,Mo and W at the core of carbide and the decrease of the content of element Cr.Density functional theory has been applied to research the phase stability,elastic properties,thermo-physical properties and electrical properties of Hexa-?Mo,Cr,W?2C carbides.The results indicate that Mo6W1Cr1C4 has the highest stability,bulk modulus,shear modulus,elas tic modulus,hardness and good deformation resistance.The mechanical anisotropy of Mo7Cr1C4 is the strongest while that of Mo8C4 is the weakest.From the electronic property analysis,it can be found that the doped Cr and W atoms can not only participate in orbitals hybridizat ion themselves,but also enhance the orbitals hybridization between Mo atom and C atom,which can reinforce the interatomic interactions.The activation energy of the steel developed is 471.28kJ/mol and the hot deformation equation is built in the temperature between 850? to 1200? and the strain rate from 0.01s-1 to 10s-1.The law between average dynamic recrystallization grain size D and Z parameter are obtained.Based on the dynamic materials model,the hot processing maps of this newly developed ultrahigh strength steel are established. |
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