Effect of tempering process on microstructure evolution and mechanical properties of HT9 steel was studied by SEM,TEM,phase analysis,tensile and impact tests.The results show that the tempering process can affect the mechanical properties of the tested steel by influencing the precipitates and dislocations.After tempering,the microstructure of the HT9 steel is lath martensite,and the precipitated phase is M23 C6 carbide,and no other precipitated phase is observed.With the increase of tempering temperature,the matrix gradually recovers,the dislocation tangles are gradually normalized and thinned into dislocation networks,and forming subgrains.The M23C6 carbide undergoes Ostwald ripening,the precipitation strengthening and dislocation strengthening effects are weakened,the strength and hardness decrease,while the toughness increases.Tempered at 770 ℃ for 1-12 h,as the tempering time prolongs,the matrix gradually recovers and the M23C6 phase gradually ripens.The strength and hardness gradually decrease with a certain time dependence.According to the mechanical properties standards,the strength and toughness match well when the heat treatment system of the HT9 steel is quenching at 1040℃for 1 h and tempering at 770℃for 6 h.
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