Formation Mechanism And Evolution Laws Of Carbides In M42 High Speed Steel

As the representative of high-speed steel,M42 high-speed steel has excellent mechanical properties and is widely used in the manufacture of cutting tools and high load mould for various kinds of intractable workpiece.After quenching,martensite content of M42 high-speed steel can reach more than 20 wt%of the total matrix while the molybdenum,chromium,vanadium and other alloy elements can form a large number of carbides with the carbon in steel.All these above factors can significantly promote the hardness of M42 high-speed steel,especially under a high temperature.However,the coarse size and uneven distribution of carbides in M42 will seriously reduce its plasticity and restrict its further development and application.In this paper,M42 high-speed steel were studied in the processes of electroslag remelting and subsequent heat treatment,properties of carbides in as-cast ingots and their changes in heating process were analyzed while the influences of heat treatment conditions on the microstructure and mechanical properties of M42 were explored.Meanwhile,influences of different nitrogen content on the characteristics of carbides in M42 high-speed steel were discussed.Carbides which precipitated from M42 ESR ingots along the grain boundaries were mainly lamellar and fibrous,which connected in network.The segregation of ESR ingot core was more serious than of the edge for a slower cooling rate,which represented that the dendrite spacing and the average size of carbide were larger than that of the edge.M2C metastable carbides riched in Mo and MC riched in V were the main types.After forging and annealing,the net structure of carbide in M42 high-speed steel was decomposed and broken.Instead,the shape of carbides was mainly square,irregular spherical and small particles while the size was less than 20 ?m.Small compound carbides composed of Cr,V and Mo were the main types of carbide,including M7C3 riched in Cr,MC riched in V and Fe2Mo4C.High temperature confocal scanning laser microscope was used to observe in situ the changes of quenching temperature and cooling process of high speed steel,and the effect of cooling rate on microstructure was analyzed.The effects of austenization temperature and holding time on carbides were studied.The results show that martensite,austenite and carbides are the main tissues after quenching.With the increase of quenching temperature,the number of carbides dissolved in the matrix increased,so the average size of carbides decreased and the volume fraction decreased.Correspondingly,the amount of carbon that plays a role in nailing decreases,resulting in the growth of grain size.Although the volume fraction of carbide in high-speed steel also decreases slowly with the increase of heat preservation time,the reduction is far less than the effect of quenching temperature.When the heat preservation time increases to 45 min,Ostwald maturation will occur in the steel,resulting in the continuous growth of large-size carbide and the dissolution of small-size carbide.The effects of tempering time(1-20 h)on carbide characteristics and growth in M42 high-speed steel were studied.Through a series of mechanical tests on tempered high-speed steel,the heat treatment conditions for obtaining the best comprehensive mechanical properties are discussed.The results showed that after tempering for 4 h,the secondary carbide and grain size of high speed steels were in the best state,and the impact toughness and wear resistance were effectively improved.When tempering time was over 4 h,the carbide in the steels was significantly increased.As the tempering time continues to increase,dislocation density decreases,resulting in lower yield stress of high-speed steel.When the tempering time is 8-20 h,the coarsening and agglomeration of carbide will seriously reduce the impact toughness and wear resistance of steel.The effect of nitrogen on microstructure and carbides of M42 high-speed steel was studied by X-ray diffraction analysis and inclusion analysis.The results showed that the addition of nitrogen promoted the formation of fibrous M2C,while in steel with a nitrogen content of 60 ppm,M2C was mainly laminated.Fibrous M2C is more easily decomposed into stable carbide M6C and MC than lamellar M2C.The change of nitrogen content in steel only affects the morphology and size of carbide,but not the type of carbide.Based on the improvement of the size and distribution of nitrogen to carbide,the impact toughness of high-speed steel is improved.Therefore,it is recommended to add nitrogen in M42 high-speed steel to achieve uniform carbide and improve the impact toughness of steel.