EffectsofQ&Ptechnologicalparameterson microstructuresand mechanicalproperties of34SiMn2CrNiMosteel

A mediumcarbonsteel34SiMn2CrNiMo wasdesignedandfabricated,andtherelationships between processing, microstructures and mechanical properties were established through microstructurecharacterizationand mechanicalpropertiestestingonthequenchingandpartitioning (Q&P)heat-treatedsamples.Basedontheexperimentresultsofthermalexpansiontransformation behavior,theeffectsofquenchingtemperatureTQ,heatingapproach,partitioningtemperatureTPand partitioningtimetP on microstructuresevolution wereinvestigatedand mechanism ofstrength and toughness wasilluminatedas well.Theresultsshow that multiphase microstructuresof martensite andretainedaustenitecanbeobtainedbymeansofQ&Pprocessing,asaresultthematchofstrength andplasticityisgreatlyimproved.In the optimaltreatmentstate,theretained austenite volume fractionisabout24%,theyieldstrengthis1053 MPa,thetensilestrengthis1607 MPa,theelongationis 24.9%,andtheproductofstrengthandelongation(PSE)is40.0 GPa center dot%.Toavoidformingtheblocky secondarymartensite,thebestquenchingtemperaturehastobesetatleast150 degrees C below martensitic transformationstartingtemperatureMs.Moreretainedaustenitecanbeachievedbyslowheatingthan fastheatinguponpartitioningafterthesteelisincompletely quenched,theplasticityandtoughness areimprovedapparently.The microstructuresandpropertiesof Q&P treated steelkeeprelatively stablewithprolonged duration whenitispartitionedat400 degrees C.However,thevolumefraction of retainedaustenite decreases withtP at450 degrees C,leadingtheyield strength and elongation decrease simultaneously.