| The requirements for large GCr15 steel bars has increased rapidly in modern industries.As a typical high chromium bearing steel,GCr15 steel has been widely used in many engineering fields.In the process of bar production,water cooling is a commonly used and effective means of controlling cooling to improve the microstructure and properties of bars.With the increase of the diameter of GCr15 steel bars,the heat dissipation of bars during water cooling process changes,which makes the temperature distribution of the section of the large bar becomes inhomogeneous,and the network carbide will be produced at some cooling rates,which leads to the difference in the microstructure and properties of the section.In this paper,GCr15 bearing steel was used as the research object.Firstly,thermal simulation experiment was conducted to study the influence of temperature and cooling rates have on the precipitation mechanism of the network carbide.On water-cooled experimental platform,by using the thermocouples to measure temperature,the temperature of large bars section with the diameter of 90mm,120mm and 150mm was tested in the process of water cooling,and the temperature distribution law of large bar section in the process of water cooling was obtained.Analysis techniques such as optical microscope(OM),micro-hardness test,tensile machine,scanning electron microscope(EBSD)were used to analyze the relationship between mechanical properties and microstructure in different positions of large GCr15 steel bars with different diameter sizes through water cooling.The results show that:In the process of continus cooling of GCr15 bearing steel from 1000 ~oC,most of network carbides began to appear at 850 ~oC.When the cooling rate is 1~oC/s?2~oC/s or3~oC/s during 700~oC-850~oC,network carbides appear a lot.The class of it is 2-4.When cooling rate is 5~oC/s?8~oC/s or 10~oC/s during 700~oC-850~oC,network carbides are hardly existed.The class of it is 0.In the process of cooling,because of internal heat transfer,the temperature of surface is increased first then decreased;The temperature of half radius and center is declining all the time,when the speed of it is increased first then decreased.The cooling rate of surface in bars with different diameter sizes is very fast during 700~oC-850~oC,and final microstructure of it is all martensite.The cooling rates of half radius and center in bars with different diameter sizes are all below 2.44~oC/s,and the final microstructure of it are network carbides and pearlite;The largger the diameter is,the position network carbides appear is more closed to surface.According to the results of EBSD,it is obvious that no network carbide is existed in surface.Besides,carbides are more in half radius than center and network carbides are distributed along the grain boundries.The grade of grain sizes in surface of all diameter sizes is 8 while the grade of grain sizes in center of all diameter sizes is 6.After water cooling,the tensile strength of the bar section increases about 150MPa,meanwhile,the hardness is increased about 50HV,the impact toughness is also improved in some degree,but there is a certain difference between the center and the surface of the bar,and the uniformity is poor.The tensile strength,hardness and impact toughness of the surface are the highest,followed by half radius,and the center is the lowest. |
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