The Microstructure And Properties Of New Low-Density Bearing Steel With High Strength And Toughness

The main characteristics of bearings are high rolling contact fatigue property and wear resistance.Therefore,the traditional bearing steel(52100 steel)obtains high strength of 2600?3000MPa and high hardness of 58?62HRC to meet the service requirements.To achieve these properties,52100 steel gains martensite and carbides of Cr by quenching and low-temperature tempering.52100 bearing steel with "1C-1.5Cr" has been used for hundred years,but it still have some defects:(1)The formation of network carbide due to hypereutectoid carbon will lead to brittleness and destroy the homogeneity of structure;(2)High carbon of 0.8?1.1 wt.%decreases the Ms temperature then reduces the fraction of martensite,so increasing the carbon content can not further improve the hardness;(3)The decomposition of retained austenite occurs during tempering.A small amount of retained austenite guarantees high dimensional stability but causes low resistance for hydrogen embrittlement and low toughness.The micro-hypereutectoid and high carbon bearing steel"1.45C-1Cr-5Al-3.5Ni-0.5V" improves these defects.In addition,the steel has obtained super high strength and hardness.The bearing steel proposed in this study has characteristics as follow:(1)The density of bearing steel is reduced by 7%through adding 5wt.%Al;(2)The addition of V and the reduction of Cr can eliminate large carbides of Cr,replaced by carbides of V to enhance the toughness and strength.On the basis of not forming other carbides,the addition of Ni can make up for the decrease of Cr to ensure the hardenability;(3)Al can move the eutectoid carbon content to higher.With the effect of other elements,the content of eutectoid carbon can be increased to 1.3wt.%,so only 0.15wt.%carbon is hypereutectoid and adsorbed by V,thus avoiding the formation of network carbide;(4)Al can increase the martensitic transformation starting temperature though higher carbon addition reduces it,leading to high content of martensite and more carbon in it.So high hardness of 66.2±0.4HRC has been obtained after quenching and cryogenic treatment.In order to improve the resistance of hydrogen embrittlement and toughness,at the same time to avoid the dimension change due to TRIP effect in service,a small amount of super-stable retained austenite is needed.So a new low-temperature tempering process including bainite transformation is designed to reduce the content of retained austenite.The size of retained austenite decreases and the carbon content in it increases through splitting bulk austenite and partitioning carbon.The thickness of bainite ferrite is 50-160nm and the thickness of film retained austenite is 15-50nm.Average carbon content in retained austenite has increased to 1.9?2.0wt.%according to XRD analysis and empirical formulas.The compression test proves that the retained austenite has high stability under this tempering process.The hardness after this tempering treatment almost does not decrease.With the strength of 3710±44MPa and the hardness of 65.9±0.5HRC after tempered,the toughness of the steel has also been improved.Compression elongation can reach about 30%,which is nearly 10%higher than 52100 bearing steel.The value of KIC measured by specimen without precrack through three-point bending test mehhod is 63.8MPa·m-1/2,which is 20%?50%higher than 52100 bearing steel.