| For the problems of high carbon chromium steel mesh carbides,insufficient temperature and high density,a new alloying of GCr15 was carried out.On the basis of the GCr15 bearing steel,Aluminum of 6%and more carbon content are added to develop a new type of lightweight ultra-high carbon bearing steel.High aluminum and high carbon content bring a big impact on heat treatment process and properties and thus its carbide spheroidizing and precipitation behavior,quenching and tempering properties and its fatigue life are studied in this research.The main conclusions are showed as follows:(1)Because of high aluminium and high carbon alloying design,the condition of the proeutectoid carbide network is restrained effectively.But it still has a lot of rough carbide unevenly distributed in the matrix after hot forging,especially the massive grain boundary carbides.The rough carbides can be effectively reduced after the isothermal annealing process.The best spheroidizing annealing process is obtained as follows:The designed steel is firstly heated at 880℃ for 30 min and furnace cooling to 750℃ isothermal for 4 h,then furnace cooling to 600℃,and lastly it was air cooled down to room temperature.During the isothermal process,high aluminum and carbon content could suppress the coarsening of carbides of both M3C and M7C3,but accelerate formation of the AlFe3Cx carbide.(2)The designed steel transforms into a microstructure with high carbon martensite,carbides and residual austenite after austenitizing and quenching.And only two type carbides of M3C and M7C3 were observed in the quenched steel.The prior austenite grain size gradually increased with increasing of the austenization temperature.The average grain size of the prior austenite is only 2.8 m when austenitized at 875 ℃,but 6.0 m at 900 ℃.When the temperature exceeds 900 ℃,the micro structure would be significantly coarsened.After the processing of quenching,cold treatment and 160℃ tempering,the hardness of the steel is high up to 65.1 HRC.But toughness of steel is very poor.(3)An excellent heat resistance is obtained in the designed steel with a high hardness of 63 HRC when tempering at 200～400 ℃ for a long time.The nondestructive impact energy is only 7 J when it tempered at 160℃,and the nondestructive impact energy is 39 J when it was tempered at 400 ℃,which increased 5.6 times.Even though the toughness increased significantly with increasing of tempering temperature,the tensile strength could be improved to 2230MPa or more.Through the processing of quenching,cold treatment and 200～400℃ tempering,the designed bearing steel has high hardness,high strength and good toughness.And thus a comprehensive mechanics performance of the steel is improved significantly.(4)The rolling contact fatigue life of the designed bearing steel can significantly increase by application of medium temperature tempering.The contact fatigue life of the designed steel at 300 ℃ tempering is compared with the 160 ℃ tempering.The L10 life could reach up to 8.83×106,which increased 4.09 times.Also the L50 life can reach up to 5.39×107,which increased 4.18 times.A large number of inclusions of Al2O3 was found in the designed bearing steel,which impaired significantly the rolling contact fatigue and thus need to be studied further. |
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