Effect Of Tempering Process Parameters On Microstructure And Properties Of Carbide-Free Bainite Steel

A newly developed 20CrSiMn2MoV of carbide-free bainite steel was studied in this paper.The transformation temperature of the steel was measured by temperature-hardness method.The effects of tempering temperature,tempering time and tempering times on the normalizing and quenching microstructure and properties of the experimental materials were studied by heat treatment experiment,mechanical properties testing,XRD phase analysis,metallography and scanning microstructure observation and impact fracture morphology observation.The results show that the Ac1 and Ac3 of 20 CrSiMn2MoV are 710? and 880?,respectively.The tensile strength of the experimental materials peaked at 200?300?tempering through air cooling after austenitizing at 920?,the mechanical properties obtained are Rm=1472-1466 MPa.The impact value peaked at 250? tempering,the mechanical properties obtained is 86J,the fracture morphology of impact specimen is dimple fracture surface,which conforms to ductile fracture mechanism.Tempering at 5 50? has the lowest impact value,and the lowest impact value is 11.2J due to bainite tempering brittleness,the impact fracture morphology is quasi-cleavage fracture,which is brittle fracture mechanism.Experimental materials have the best strength and toughness at 250? tempering after normalizing,and the microstructure is bainite ferrite and retained austenite.The tensile strength decreases with the tempering temperature increasing through water cooling after austenitizing at 920?.Experimental materials have the best strength and toughness at 200? tempering after quenching,and the mechanical properties obtained is Rm=1551 MPa,the peak value of impact value is 73.8J,the fracture morphology of impact specimen is dimple fracture surface,which belongs to ductile fracture mechanism.The impact values of tempering at 400? and 550?are 14.4J and 7.2J due to martensite tempering brittleness,and the impact fracture morphology are intergranular and quasi-cleavage fracture,which is brittle fracture mechanism.Experimental materials have the best strength and toughness at 200? tempering after quenching,and the microstructure is tempered martensite and retained austenite.After austenitizing heating and air cooling at 920?,tempering at 250? for different time,the strength has a tendency to decrease,but the decrease is not obvious,and the tensile strength varies between 1472-1442 MPa.The peak of impact value appears when the tempering time is 8h,and the best strength and toughness can be obtained.The microstructure is bainite ferrite and retained austenite.After austenitizing heating and water cooling at 920?,tempering at 200? for different time,the tensile strength varies between 1628-1574 MPa,the peak of the impact value of the experimental material appeared when the tempering time was 6h,which was the best holding time,the mechanical properties obtained are Rm=1592 MPa,A13.5%,Z59%,AKV81.5J,and the microstructure is tempered martensite and retained austenite.?53mm construction machinery connecting rod material for austenitizing heating and air cooling at 920?,tempering at 250?,with the increase of tempering times,the impact value peaked at the secondary tempering.The secondary tempering can improve the strength and toughness of the experimental materials,and the mechanical properties obtained are Rm1245MPa,A16.7%,Z60.9%,AKv90.5J,and the microstructure is bainite ferrite and retained austenite.The impact value of the experimental material appeared a peak value at tempering with the increase of the number of tempering times,through water cooling after austenitizing at 920? and tempering at 200?.Experimental materials have the best strength and toughness at 200? tempering after quenching,and the mechanical properties obtained are Rm=1453MPa,A11.4%,Z55.6%,AKV66.3J,and the microstructure is tempered martensite and retained austenite.For practical engineering applications,20 CrSiMn2MoV steel can obtain high strength and toughness not only by normalizing and tempering at low temperature,but also by quenching and tempering at low temperature.It is a kind of steel with high applicability of heat treatment.