The Carbide Characteristics Of M2High Speed Steel Containing Nitrogen And Rare Earth

As a typical representative of W-Mo series high-speed steels, M2high-speed steel hasexcellent mechanical properties, it has been widely used to manufacture kinds of cuttingtools and molds with high properties. Coarse size and heterogeneous distribution ofcarbides affect the application of HSS seriously. In this dissertation, M2high speed steelwas modified with nitrogen and rare earth. Thermodynamic calculation, DSC, OM, SEM,EDS, TEM and electrolytic extraction were used to study the carbide characteristics of M2high-speed steel containing nitrogen and rare earth as well as the change during heatingand hot deformation. The effects of nitrogen and rare earth on carbide characteristics wereanalyzed. The main conclusions are as follow:During equilibrium solidification, the steel has the main reaction: L→δ, L→MN,L+δ→γ and L→γ+MxC. The carbides in the as-cast structure are formed during eutecticreaction at1206¹249°C.The carbides of M2high-speed steel containing nitrogen and rare earth are networkdistributed, and the average net distance is approximately56.3μm.The main carbides inthe as-cast structure are M₂C、MC and M₆C. The volume fraction of them is58.5%,30.3%,and11.2%, respectively. MC-type carbide, which has the shape of block or strip, locates atthe interface between the eutectic carbides and austenite substrate, and its main alloyingelement is V and C; metastable carbide M₂C has the shape of irregular lamellar, whoselamellar space is about1²Î¼m; M₆C has the shape of fish bone, and its main alloyingelements are Fe, W, Mo and C.During heating at high temperature, some changes occur in M2high-speed steelcontaining nitrogen and rare earth, such as the decomposition of M₂C, disconnection,spheroidizing and dissolution of M₆C and MC. The whole process is affected by heatingtemperature and holding time. Carbide dissolves quickly above1200°C.Hot deformation can not change the carbides type, but can break and refine coarse meshcarbide in the as-cast structure. The refinement is the most obvious when the deformationis more than75%.The addition of nitrogen and rare earth leads MN-typed nitrides（mainly VN） to precipitate from liquid during solidification, reduces the precipitation temperature of δ（about9°C）, increases the starting temperature of peritectic reaction （about15°C）, andpromotes the formation of M₆C. At the same time, the carbides lamellar becomes irregular,the bending degree of lamellar increases, the lamellar space becomes smaller, and thermalstability of M₂C reduces.Because of the addition of nitrogen and rare earth, the distribution of network carbideis improved. The average network space decreases from98.3μm to53.6μm. In addition,the tempered hardness and red hardness of M2high-speed steel are increased. Afterquenched at1200°C and tempered at560°C, the tempered hardness and red hardness ofM2HSS containing nitrogen and rare earth are improved, and the maximum values are66.5HRC and52.3HRC, respectively, which increase by3.21%and2.83%.