| In this paper,1.4%C ultra-high carbon steels with the content of 2%,4% and 6% Al elements were studied.The effects of Al content on phase diagram,phase transition temperature and original forged microstructure of ultra-high carbon steels were studied.A new process for spheroidizing annealing of ultra-high carbon steel was explored.The quenched and tempered microstructure of ultra-high carbon steel and the effects of Al on tempering stability,temperature resistance and corrosion resistance were studied.All of these provided reference for the development of a new generation of low density,high hardness,high temperature resistance,good corrosion resistance and high fatigue resistant bearing steel materials.The results of the study were as follows:(1)The addition of Al increased the ferrite phase area and shifted the C eutectoid point to the right side,which reduced the amount of the eutecto id carbide precipitation.This was the fundamental reason that Al inhibited the precipitation of grain boundary carbide in ultra-high carbon steel.In addition,the phase transition temperature of the material was greatly increased with the addition of Al,and the phase transition temperature increased by about 25 degrees centigrade by adding 2% Al.(2)The increase of Al content and improper control of O content increased the proportion of oxide inclusions and oxide-sulfide inclusions,and increased the size and number of large particle inclusions.In the same smelting condition,the difficulty of inclusion control increased with the increase of the content of Al in UHCSs,especially when the Al content reached more than 6%.More advanced inclusion control technology need be adopted in the smelting of high aluminum steels.(3)Because of the high carbon content,carbon segregation maked the carbon concentration unevenly distributed after forging.Only a single isothermal annealing process could not make the ultra-high carbon steel completely spheroidizing.Prolonging the isothermal time improved the rate of spheroidization,but it was difficult to make it completely spheroidizing.(4)Al alloying maked carbide networks of ultra-high carbon steels thinner and thinner or even without carbide networks.It was possible to normalize when the temperature was lower than Acm to obtain uniform microstructure.Proper increase of normalizing temperature significantly reduced or even eliminated long bar and short rod carbides in normalizing microstructure,and reduced the number of refractory bars and short rod carbides during annealing,which was conducive to spheroidization.The complete spheroidizing microstructure of ultra-high carbon steel containing Al was obtained by proper temperature normalizing and isothermal annealing,which was made up of ultrafine ferrite and fine spheroidal carbides.(5)The addition of Al significantly improved the temperature resistance a nd tempering stability of ultra-high carbon steels.The main reason was that the addition of Al elements inhibited the transformation of ε-carbide to cementite during tempering,resulting in martensite failing to decarbonization to forming ferrite,which maked ultra-high carbon steels still maintain very high hardness during higher temperature tempering.(6)UHCSs with Aluminum containing had good temperature resistance and tempering stability in a wide temperature range,highe r hardness than GCr15 steel,but the toughness was not as good as GCr15.The further improvement of toughness would provide a great possibility for the industrial app lication of UHCSs with Aluminum containing.In addition,the addition of Al effectively enhanced the corrosion resistance of the materials,which provided a useful idea to develop low density,high hardness and good corrosion resistant bearing steel materials. |
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