Effect Of The Control Of Carbides On The Sharpness Of Knives Made Of 8Cr13MoV Steel

There has a history of more than one thousand years in the production of kitchen knives.Unfortunately,the market of high grade knives is almost monopolized by foreign famous brands.The foreign high grade knives possess high sharpness which can maintain for several years,and the knives can even be free of grinding.The sharpness of knives produced in china is relatively poor,and the cutting edge will become dull after using for one year.The comparative analysis shows that the main difference in the microstructure between knives made in china and abroad is the control level of carbides.There are no primary carbides in foreign high grade knives,and the secondary carbides distribute in martensite matrix,which are fine,homogeneous and dispersed.Primary carbides generally exist in homemade knives,and the secondary carbides are abundant,large in size and distribute unevenly.In this thesis,8Cr13MoV steel was taken for an example,and the mechanism for the precipitation,growing and transformation of carbides was studied.The effect of number,type and size of carbides on the sharpness of knives was studied combining the mechanism of blade passivation.The scientific controlling technology for the carbides was put forward,and the sharpness of knives was improved finally.Results indicated that primary carbides formed in the electroslag remelting process,and they can be broken and dissolved in the hot rolling process.During the solidification process,the carbon and alloying elements atoms were constantly discharged into liquid steel from solidification front.The enrichment of carbon and chromium was the main reason for the precipitation of primary carbides.Carbon content was the determining factor for the precipitation of primary carbides,and chromium content was the main necessary element for the growing of priamary carbides.The segregation degree of chromium determined the total number of primary carbides.The most effective measure to reduce the number of primary carbides was to reduce the segregation of carbon and chromium.The precipitation and growth of primary carbides were affected by the nucleation condition,temperature gradient and chromium concentration gradient.Reducing the melting rate in electroslag remelting can reduce the size of primary carbides and realize uniform distribution of primary carbides.Within certain limits,the decrease in melting rate resulted in shallow metal pool,narrow two phase region and less segregation.The increase in electrode ratio can result in weaken heat convection in slag pool,weaken the erosion of electrode by slag pool and the uniform temperature field.These factors will further result in shallow metal pool,narrow two phase region and less segregation.With the melting rate decreasing from 150kg/h to 133kg/h and the fill ratio increasing from 0.23 to 0.33,the number of primary carbides reduced by 23%.With high temperature diffusion annealing of 1180? for two hours,the total number of primary carbide in hot rolled plates decreased by 92.8%.The mechanism of blade passivation was the increase of friction coefficient at cutting edge and the change in the geometrical morphology of cutting edge.The falling off of primary carbides can not only cause the increase of friction coefficient but also cause the large fluctuation of sharpness which aggravated the wear process.With the decreasing number of primary carbides from 2.37%to 0.17%,the sharpness was increased generally,and the fluctuation and frequency of sharpness decreased.The wear resistance also increased and the initial sharpness and sharp persistence increased by 52%and 81%,respectively.There were two kinds of secondary carbides named M23C6 and M7C3 in 8Cr13MoV steel.According to the first principles,M7C3 carbides possess higher Young modulus and shear modulus,which are in favor of increase the hardness and wear resistance of steel.The knives were heat-treated at 1050? for one hour,and then cooled to 950 ? and kept at 950 ? for two hours followed by air cooling.After this heat treatment,some M23C6 carbides dissolved and many nano-sized M7C3 carbides precipitated in the matrix,which increased the wear resistance by 56%and increased the sharpness persistence by 22%.In the quenching process,the changing austenitizing temperature can affect the sharpness by affecting the volume fraction of carbides.The effect of hardness of steel on the sharpness is not obvious.The micro-sized secondary carbides in 8Cr13MoV steel can easily fall off,which will aggravate the wear process.With the increasing number of secondary carbides,the wear resistance and sharpness of knives both decreased.The new roll-forging heat treatment can not only refine the grain size and primary carbides,but also decrease the fluctuation and frequency of sharpness,which generally increased the sharpness of knives and delayed the wear of knives.After the roll-forging heat treatment,the initial sharpness of knives increased by 56%and the sharp persistence increased by 28%.Compared to ordinary quenching technology,the deep cryogenic process can reduce the amount of residual austentite and make the outline of martensite more obvious.The hardness and wear resistance of steel both increased,and the initial sharpness and sharp persistence increased by 77%and 41%,respectively.