Effects Of Heat Treatment On Microstructure And Carbide Evolution Of 25CrMo48V Ultra-high Strength Steel

With the development of aerospace,military ships,high-speed railways,deep sea and lightweight cars,there are some stringent requirements for comprehensive performance of the ultra-high strength steels,especially the excellent combination of strength and toughness.In order to meet the environmentally friendly and economical demands,countries in world have considered ultra-high strength steels as the key research projects.25CrMo48V steel is a micro-alloyed C-Cr-Mo ultra-high strength steel with minor Nb,V and Ti addition.Until now,the performances of 25CrMo48V steel made in China are still unstable,and most of them require further improvement.The studies on microstructural control and strengthening mechanisms are also not comprehensive,as well as the carbide evolution behavior in 25CrMo48V steel.In this work,effects of different heat treatments and hot deformation on microstructure and properties of 25CrMo48V steel were systematically studied,and the carbide evolution behaviors in various heat treatments are discussed in detail.The key results can be summarized as follows:?1?The grain growth behaviors of 25CrMo48V steel at different austenitizing temperatures were investigated,as well as the types and sizes of carbides.The effects of carbides on austenite grain growth are discussed.The temperature at which grain grows significantly is determined to be 1100 ^?.Based on microstructure examination,the spherical particle was identified as Nb-rich MC,and the cuboidal shape was identified as Ti-rich MC.With austenitizing temperature increasing,both of the two types of MC carbides coarsen or dissolve,pinning the austenite grain boundaries weakly.The pinning contribution of two types of MC carbides was calculated using pinning parameter equation,and the pinning effect of Nb-rich MC is much larger than that of Ti-rich MC.This suggests that fine Nb-rich carbide play a more predominant role in retarding austenite grain growth.?2?The hot deformation behavior of 25CrMo48V steel was investigated by isothermal compression at 900 to 1200 ^? with strain rates from 0.01 s^-1 to 10 s^-1.The constitutive equation was derived on the basis of true stress-true strain curves and the processing maps were also obtained,and the effects of hot deformation conditions on microstructure and precipitates are discussed.It is found that the peak stress of25CrMo48V steel increases with increasing the strain rate and decreasing the deformation temperature.The deformation activation energy Q is determined as about407.29 kJ/mol.The amount of carbides gradually increased with the increase of the strain rates and two safe regions with relatively high power dissipation efficiency are located in(1000 ^?,0.01 s^-1)and(1200 ^?,10 s^-1).?3?The transformation behavior of 25CrMo48V steel during continuous cooling was investigated by Gleeble-1500 thermal mechanical simulator.Thermal dilation curves were measured at different cooling rates,based on which continuous cooling transformation?Countious Cooling Transformation,CCT?curve was established.At cooling rates of 0.2^1?/s,the microstructure primarily consists of polygonal ferrite,pearlite and lower bainite,while at cooling rates of 2¹0 ^?/s,besides dominated lower bainite,small amount of polygonal ferrite can also be observed.At cooling rate higher than 10 ^?/s,martensite lath is obtained.The nano-sized precipitates at 0.2^1?/s include M₂₃C₆,M₂C,M₃C,rich-Nb MC and rich-Ti MC and at cooling rate higher than 1 ^?/s,the type of precipitates were M₃C and two types of MC.The polygonal ferrite transformation start temperatures?Ar₃?decreases with increasing cooling rate.At cooling rates ranging from 0.2 to 1 ^?/s,Vickers hardness of the steel decreases significantly.While at intermediate and higher cooling rates,the Vickers hardness increases gradually with increasing cooling rate.?4?The microstructures,precipitates of 25CrMo48V steel under different tempering conditions were investigated.With the increase in tempering temperature and time,the width of martensitic laths gradually increased,and the dislocation density was decreased.Under different tempering conditions,two types of MC carbides had always been observed.M₃C precipitated when tempering at 200⁴00 ^?,and it was replaced partially by M₂C when the temperature is elevated to 500 ^?,at which M₂₃C₆also precipitated.For a higher temperature,above 600 ^?,M₃C disappeared,and some M₂C transformed into M₇C₃.The transformation sequence of other four types of carbides during different tempering periods can be summarized as follows:M₃C?M₂C?M₇C₃?M₂₃C₆.?5?The evolution of microstructure and precipitates of 25CrMo48V ultra-high-strength steel subjected to Q-P?Quenching-Partitioning,Q-P?and Q-P-T?Quenching-Partitioning-Tempering,Q-P-T?treatment were investigated.After Q-P and Q-P-T heat treatments,with increase in partitioning temperature and time,both the volume fraction of retained austenite and width of martensite lath increased gradually.After Q-P process at 440^? for 5 min,M₃C and two types of MC carbides precipitated.M₂C,M₂₃C₆ and M₇C₃ are formed at the expense of M₃C during tempering treatment.The other two types of MC particles can also exist stably after tempering process.When the steel was partitioned at 440 ^? for 5 min to 10 min,with partitioning time increasing,a considerable portion of M₇C₃ transformed into M₂₃C₆carbides and more M₂C particles precipitated.As partitioning time was increased to15 min,M₇C₃ carbides disappear,while M₂C,M₂₃C₆ and two types of MC carbides exist.