Effect Of Mo Content And Tempering Temperature On Microstructure And Properties Of High Strength Steel

The mechanical properties of low-alloy martensitic steel will be greatly improved after heat treatment.According to the research status and development trend of low-alloy martensitic high strength steel,three groups of low-alloy martensitic steel with excellent comprehensive properties are designed.Their contents of Mn,Ni,Si,Cr and C are roughly the same,and the contents of alloying element Mo are 0.1%,0.2% and 0.4% respectively The steel with thickness of 40 mm was obtained by melting,ingot casting and forging in vacuum induction furnace.After hardness reduction and annealing,the steel is cut into heat treatment samples by wire cutting;the critical point of transformation is determined by metallographic method and thermal expansion method,and the austenitizing temperature and martensitic transformation point are known.Based on this,the quenching and tempering process is proposed: after 950? heat preservation,quenching,and then tempering at 220? ? 460?;microstructure is observed by metallographic microscope;the microstructure of the test steel is measured The mechanical properties such as Rockwell hardness,impact toughness,tensile strength,yield strength and elongation were tested.The microstructure and fracture morphology of impact specimens were further observed by SEM,and the effect of Mo content on carbide precipitation was studied.The results show that: by combining metallography and thermal expansion,Ac3 =918.9?,Ac1= 715.5?,Ms = 317?;after quenching and tempering,the matrix structure is mainly lath martensite,in addition,there is still a little retained austenite.3# The impact energy,tensile strength,yield strength and hardness of the test steel reach relatively high values,which are 1311 MPa,1385MPa,75.2J and 45.4HRC,respectively,with elongation of 15.4% and reduction of area of 60.9% 1#?2#?3#after being fully austenitized at 950?and then tempered at 260? for 3h.The results show that the addition of Mo can inhibit the precipitation of cementite.By observing the SEM photos,it is known that the steel contains lath tempered martensite structure and film retained austenite structure,which makes the steel have good properties.By analyzing the fracture morphology,it is known that when the tempering temperature is 260?,the dimples are fine and uniform,the ductile fracture and impact toughness are good.Through this test,a low alloy martensitic steel with excellent strength and toughness,and simple process is obtained.