Influence Of Cycling Quenching And Step Quenching On Microstructure And Mechanical Properties Of Ultra-high Strength Alloy Steel

Ultra-high strength steel is widely used in manufacturing rocket engine shells, aircraftlanding gear and armor plates due to its ultra-high strength and tenacity. However, along withthe extension application fields is the higher demand for properties, which is beyond thecapability of present processing technique and available steel strength. Considering heattreatment melioration, this paper has firstly determined the austenitizing temperature of theconcerned steels by thermal dilatometer and then optical microscope, scanning electronmicroscope, universal stretch test machine and Rockwell hardness apparatus were used tostudy the impact of quenching and step hardening on the microstructure and mechanicalproperties of steel alloys.The austenitizing onset temperature (A_c1) and termination temperature (A_c3) have beenrecorded as698℃and790℃, respectively. And grain grade of8and theRockwell hardnessof62have also been obtained. For the first cycle oil quenching, when heated at700⁸60℃and within the same holding time, the amount of martensite of the quenched alloy increasedwith higher temperature. When heated at the temperature of880⁹20℃, higher soakingtemperature indicated more retained austenite and coarser martensite in the oil quenched steel.Moreover, given either condition invariant, the grain would be enlarged with holding time andsoaking temperature. By comparative analysis, such conclusions have been drawn that twokinds of heat treatment processing including soaking at860℃for2.5min or3min andsoaking at880℃for2min or2.5min would result in finer acicular martensite and unevenlyrefined grain. The hardness has been increased to some degree.For the second cycle quenching, the higher the soaking temperature was, the quicker theaustenitizing would be completed. Under the same soaking temperature, the average grainsize of the steel alloys increased with longer holding time. Taking the grain size and hardnessinto consideration comprehensively, this paper has determined four kinds of superior cyclicquenching process. After a serious comparision and analysis of the structure and mechanicalproperties of the alloy steels heat treated via those four cyclic queching, we have chosensoaking at860℃for0.5h+oil quenching+soaking at860℃for2.5min+oil quenching+ soaking at860℃for4min+oil quenching as the optimal process. Through this process, thealloy steels was fairly grain refined with the grain grade of12and hardness of HRC63high.Finally the alloy steels heatt reated under the optimal cyclic hardening process was temperedat170℃for2h. Ultimately, a combination of the tensile strenth of about2060MPa and impacttoughness of15J/cm²from the9J/cm²was obtained.When the step hardening temperature was450℃or above, the ultimate phase of thequenched steel would be martensite only and down to below425℃, granular bainite wouldshow up. Given the same holding time, lower temperature led to higher granular bainitevolume fraction and lower hardness. Granular bainite imposes much adverse effect in thealloy steel by decreasing its strength. Therefore, the thermal insulation temperature for stepquenching controlled at450℃would be a good choice because it is not only a guaranteeforthe mechanical properties but also a way to minimize the cracking tendency.