Study On The Control Of Microstructural Stability And Strength-toughness Improvement In GCr15 Bearing Steel

Recently,with the fast development of high speed railway,precision machine tools and wind electric power projects,it is becoming an urgent demand for high performance bearings.The poor dimensional stability and low rolling contact fatigue life during operation are two main factors to seriously constraint the development of equipment manufacturing industry in our country.For the above-mentioned two problems,from the view of metal materials,this paper adopt sub-zero Celsius treatment,ausforming,Si-Mo alloyed GCr15 bearing steel combining pre-quenching treatment to improve the dimensional stability and strength-toughness in bearing steel.Studied the effect of sub-zero Celsius treatment and tempering on the stability of retained austenite,ausforming to improve toughness in bearing steel,Si-Mo alloyed GCr15 bearing steel combining pre-quenching treatment to improve strength-toughness.Revealed the essential reason of sub-zero Celsius treatment improve the stability of retained austenite;Developed a novel ausforming process for improve toughness in GCr15 bearing steel;Designed and developed a novel Si-Mo alloyed GCr15 bearing steel;Analyzed and discussed the relationship of process,microstructures and mechanical property.The main research contents and results are as follows:?1?Sub-zero Celsius treatment and tempering enhance the mechanical and thermal stability of retained austenite in bearing steel.The experimental results showed that with the austenitizing temperature increasing,the fraction of retained austenite and the carbon content in retained austenite increased due to higher degree dissolution of spherical carbides.Meanwhile,the morphology of retained austenite changed from film-like to blocky.After traditional QT?Quenched&Tempered?heat treatment,the volume fraction of the retained austenite in 860QT,920QT and1050QT sample is 13.2%,21.6%and 23.2%,and carbon content in retained austenite caculated by XRD is 0.78wt.%,0.91wt.%and 1.07wt.%,respectively.Compared with traditional QT treatment,sub-zero Celsius treatment and tempering?QST?obviously decreased the volume fraction of retained austenite and enrich the carbon content in it.The volume fraction of retained austenite in 860QST,920QST and 1050QST sample is 9.1%,11.3%and 12.7%,and carbon content in retained austenite is 0.82wt.%,0.95wt.%and 1.12wt.%,respectively.The mechanical stability of RA is evaluated by the exponent decay law,which is the relationship between the true strain and the change of fraction of retained austenite before and after tensile test.Compared with traditional QT treated samples,the mechanical stability of retained austenite in QST samples was enhanced,which is due to the accumulation of compressive stresses during sub-zero Celsius treatment and tempering,thus inhibit the stress-strain induced martensite transformation during tensile test.In addition,the mechanical stability of film-like retained austenite is higher than that of blocky one.Thermal stability of retained austenite in bearing steel was evaluated through caculate activation energy of retained austenite decomposed during heating processing by differential scanning calorimetry?DSC?.The DSC results shown that both the temperature at which retained austenite start to decompose and peak temperature are keep the same,but the activation energy of retained austenite decomposition slightly increased through sub-zero Celsius treatment and tempering at the same austenitizing temperature,it is indicated that the thermal stability of retained austenite is enchanced.However,both the temperature at which retained austenite starts to decompose and peak temperature are swift toward higher temperature,and activation energy is increased with austenitizing temperature increasing indicate that carbon content in retained austenite is the main factor to determine the thermal stability of retained austenite.?2?Proposed a novel ausforming heat treatment to improve toughness of GCr15bearing steel,this ausforming heat treatment is contain cold deformation,pre-quenching and bainite transformation.Cold deformed GCr15 bearing steel could accerlate dissolution of carbides and refined prior austenite grain during austenitizing and thus decreased Ms?Martensite start?temperature.The prior austenite grain size in non-deformed and 30%cold deformed samples is 14.5?m and 10.3?m and Ms temperature is 232°C and 210°C,respectively.The kinetics effect of cold deformation on bainite transformation after pre-quenched is measured by DIL 805L dilatometer,when the degree of cold deformation less than 15%,the cold deformation accelerate bainite transformation.Inversely,the bainite transformation was retard when the degree of cold deformation more than 15%.The maximum impact toughness⁸7J and fracture toughness³9MPa·m^1/2 was achieved in 30%cold deformed sample after pre-quenching at 200°C and isothermal bainite transformation at 240°C,this is1.8 and 2.6 times the value of QT sample.The hardness is slightly decreased but also satisfies the application scope of 58-63HRC in bearing steel.The mechanical stability of retained austenite and dimensional stability of ausforming sample is obviously higher than that of traditional QT sample.The improved toughness was attributed to the double refined prior austenite grain size and martensite/bainite duplex microstructures,formed enriched carbon filmy retained austenite and high density of boundry.The evolution of microstructures during ausforming process can be describe as follows:The prior austenite grain was refined through recrystallization of cold deformed sample.During pre-quenched at 200°C,the volume fraction of pre-quenched martensite is decreased with the degree of cold deformation increased due to the difference of Ms temperature.Meanwhile,the pre-quenched martensite can divide the austenite grains and reduce the size of untransformed blocky austenite.During the subsequent bainite transformation at 240°C,the carbon atoms in pre-quenched martensite diffuse into austenite?untransformed after pre-quenched?,thereby stabilizing a portion of the austenite?especially those adjacent to pre-quenched martensite?,bainite is prone to nucleate under low carbon conditions and step forward to divide untransformed blocky austenite.Finally,the untransformed blocky austenite after bainite transformation could further divided by martensite transformation during quenched into room temperature and film-like retained austenite retained?<50nm?between pre-quenched martensite and bainite due to enriched carbon.The room microstructures are fomed a large density of boundary and refined duplex martensite/bainite microstructures due to the multistep phase transformation partitioning austenite grain.This novel ausforming heat treatment has a great potential theory and engineering application for the improve quenched deformation of cold ring rolling and rolling contact fatigue life of bearings.?3?We designed and developed a novel Si-Mo alloyed GCr15 bearing steel with composition of 0.99C-0.43Mn-1.31Si-1.5Cr-0.26Mo,combined with pre-quenching and subsequent bainite transformation to improve the strength-toughness,which is due to the formation of a few nanobainite.The effect of pre-quenching temperature on bainite transformation kinetics was measured by DIL 805L dilatometer,the experimental results demostrated that the bainite transformation has the shortest incubation period after pre-quenching at 140°C.The sample after pre-quenching at140°C and subsequent bainite transformation at 200°C obtained the hardness is⁶3HRC and impact toughness is⁷2J,which is equal to the hardness and 1.6 times impact toughness of GCr15 bearing steel after traditional QT heat treatment,achieved the aim of improve strength-toughness in Si-Mo alloyed GCr15 bearing steel and improve productivity.The microstructures of Si-Mo alloyed GCr15 bearing steel was characterized by SEM,TEM and XRD,the results showed that the microstructure was consist of martensite,nanobainite,retained austenite and undissolved spherical carbides.The evolution of microstructures during heat treatment can be described as follows:pre-quenching produce fine martemsite,partitioning the austenite grain and introduce dislocation in austenite nearby pre-quenching martensite.The subsequent nanobainite transformation was accelerated due to the increasing of nucleate site.Retained austenite as film-like exist in nanobainite and the size less than 50nm.The improvement of hardness is attributed that solution strengthening of Si and the like-precipitation strengthening of nanobainite.The improvement of toughness is attributed to soften of marteniste,formed film-like retained austenite and nanobainite.In a word,Si-Mo alloyed GCr15 bearing steel designed in this study may be has great potential application value as a novel bearing steel with further optimizing the composition and process.