Research Of Microstructure Design And Control On High Strength High Toughness Cold Work Die Steel

It is directed at the ledeburite carbides segregated seriously in solidification of Cr12 die steels (e.g. Cr12MoV or D2 etc.). The morphology characterization of ledeburite carbide is distributed in banding and net with fishbone shapes. Thus the moulds are usually failure with tipping, fracture and collapse in services due to low toughness. In order to improve this situation, a new high strength and high toughness cold work die steel was developed to adapt for the requirement of high strength materials forming. The thermodynamic theory and equilibrium principle of carbon as well as computer aided optimizing design are used in. The alloying ideas are decreasing ledeburite to improve toughness and increasing second precipitation carbides to strength matrix also improve wear resistance. The composition design is to reduce the content of carbon and chromium in order to cut down the segregation of ledeburite carbide; to increase content of molybdenum and vanadium in order to refining grain as well as improve toughness based on Cr12MoV. At the same time, deep cryogenic treatment (DCT) was carried out in order to give full play to proficiency and improve properties.By means of Mechanics Performance Testing, optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometer, DIL805A dilatometer, electric resistance instrument and internal friction system and so on used in research. The alloying regularity and mechanism of toughness have been discovered. And the variation of microstructure as well as mechanism of phase transformation in the process of DCT has been clarified in this dissertation. The main conclusions are summarized as follows:The new developed high strength and high toughness cold work die steel SDC99 have overcome the weakness of Cr12MoV which failed with tipping, fracture and collapse in services due to low toughness. The temper resistance is better than Cr12MoV. The toughness achieves to 80J which is twice as Cr12MoV while the hardness is 62HRC. In addition, the wear-resisting property is as same as Cr12MoV. It can instead of the imported high strength high toughness steel such as DC53, SLD-Magic and ASSAB88 etc. The carbides were obtained by electroextraction in quenched and tempered steel. The distribution of carbides size was analyzed by Laser Particle Size Analyzer (LPSA). The average size of carbide in SDC99 and Cr12MoV are 1.23μm and 11.37μm respectively after tempering. In addition, the statistical method was used to study the carbides distribution in the tempered two steals. There are 45.5% of carbides distributed between 0.25~1μm in tempered SDC99 steel, while 91%of carbides size lager than 5μm in tempered Cr12MoV. The SDC99 steel was made the carbides size distribution better than Cr12MoV and a large number of fine carbides precipitated in the tempering process. It is the reason that higher toughness obtained in SDC99 while the wear resistance is as well as Cr12MoV.Deep cryogenic treatment was carried out in order to improve the wear resistance and dimensional stability. The hardness is increased 1~3HRC after DCT, however, the toughness reduced. The volume of retained austenite is reduced after DCT. The volume of retained austenite in tempering then DCT treated sample is higher than DCT treated after quenching. The reason is that carbon atoms diffuse into retained austenite in the process of tempering to make the retained austenite stabilized. Besides, the carbon content in martensite is lower than tempered sample after DCT. It can be concluding that more carbide was precipitated in the tempering after DCT.It is shown the Snoek peak reduced after DCT treating by means of internal friction. The Snoek relaxation which is associated with the reorientation of interstitial solute atoms(C, N) disappeared almost in the body-centered cubic metals under the application of oscillatory stress. The decreasing of the height of Snoek peak indicates the solute C amount is reduced after DCT treating. However, the Snoek-Kê-K?ster (SKK) peak increased after DCT means the carbon atoms segregate to nearby dislocations and produced strong interactions including the interstitial carbon atom themselves and between the interstitial carbon atoms with time-dependent strain field of dislocations. The interactions between the interstitial carbon atoms in the Cottrell atmosphere are stronger than the long range interactions between interstitial carbon atoms in the perfect bcc lattice. These effects are enhanced with the soaking time in temperature of cryogenic treatments due to more transformation of austenite to martensite and increase the lattice distortion with prolonging soaking time. Effect of deep cryogenic treatment on internal friction behaviors of cold work die steel can be explained by coupling model.The SKK peak decreased with increasing tempering temperature. It is shown that the height of SKK peak in DCT treated sample is lower than non DCT treated sample which were tempered at same temperature with 2 h. This indicates that there is more carbide precipitated from matrix after DCT treated during tempering. The same result is obtained from experiment of electric resistance. DCT promotes the carbon atoms segregated nearby dislocations and the segregated carbon atoms forming clusters as well as the carbides precipitation in the process of tempering. This is an important reason in improvement of wear resistance besides the transformation of retained austenite into martensite after DCT.Moreover, the computer modeling and simulating technology was used to calculate the temperature field, stress and strain field in the process of DCT. It is shown that DCT is helpful for reducing internal stress which produced in quenching process. It is obviously that the lower temperature, the lower internal stress after DCT treating. It is necessary to carry out tempering before DCT because large internal stress produced in the process of DCT. However, cooling speed, cryogenic times and cryogenic time have very little effect of stress and strain from simulations.