Preparation Of 40CrNiMo Steels With High Strength And Good Ductility And Study On Microstructure-Mechanical Properties Control

As a kind of high quality tempered steel,40 CrNiMo alloy steel is widely used in the manufacture of a variety of engineering machinery parts,such as harmonic reducer flex wheel,drive shaft and turbine shaft,etc.These parts are usually in service with high pressure,alternating load and other extreme conditions in the harsh environment,excellent comprehensive mechanical properties are fundamental to ensure safety.However,most of the current research is focused on conventional heat treatment,or grain refinement,which can only improve the mechanical properties to a certain extent.In order to further explore the performance potential of this steel,improve its service life and expand its application range.In this paper,heterogeneous 40 CrNiMo alloy steels with high strength and high toughness properties were prepared by varying the quenching temperature,rolling temperature and rolling depression rate.The effects of different processes on the organization and properties of the alloy steel were systematically studied using SEM,TEM,EBSD and other characterization means,and the strengthening mechanisms were analyzed.The main contents and conclusions of this paper are as follows.(1)The effect of quenching temperature on the evolution of quenched and tempered organization of 40 CrNiMo alloy steel and its mechanical properties was studied.The results show that when the quenching temperature is higher than 800 ℃,the quenching organization is fully martensite,and the tempering organization is tempered troisite after holding at 450 ℃for 2 h.When quenching in the two-phase zone,as the quenching temperature increases from730 ℃ to 770 ℃,the quenching organization is composed of martensite and massive ferrite,but the proportion of ferrite decreases from 37% to 28%,and the corresponding tempering organization is composed of tempered troisite and different proportions of massive ferrite.The microhardness of the quenched and tempered tissues increased with the quenching temperature and then decreased slightly,with the maximum hardness values of 760 HV and450 HV at 770 °C.The experimental steel was quenched at 770 °C and tempered at 450 °C to obtain a heterogeneous organization of tempered troisite + 32%(area fraction)massive ferrite with the best overall mechanical properties,yield and tensile strengths of 1230 MPa and 1300 MPa,respectively,and total elongation of about 15%.The loading and unloading experiments showed that the good strength and toughness matching and the considerable work-hardening ability were attributed to the hetero-deformation induced(HDI)strengthening effect.(2)The effect of different rolling temperatures(650 ℃,770 ℃ and 900 ℃)on microstructure and properties of alloy steel was studied.The results showed that the microstructure obtained after 80% rolling at 650 ℃ was ferrite + a large amount of carbide;the microstructure obtained after 80% warm rolling at 770 ℃ was mainly martensite with an average slat width of 190 nm and contained a very small amount of ferrite;the microstructure obtained after 80% rolling at 900 ℃ was slat martensite.With the increase of rolling temperature,the yield strength and tensile strength of the experimental steel increase first and then decrease,and the total elongation decreases first and then increases.After warm rolling at 770 ℃,the yield strength and tensile strength were up to 2010 MPa and 2620 MPa,and the elongation after break was 3.6%.(3)The effects of different depression rates(20%,40% and 60%)on the tissue properties of alloy steel during warm rolling at 770 ℃ were investigated,and ultrafine grain ferritemartensitic dual-phase steel was successfully prepared.The results showed that the microstructure of the alloy steel after warm rolling was ferrite + martensite,and with the increase of deformation,the ferrite content in the alloy steel decreased from 27% to 10%,and the size decreased from 800 nm to 500 nm,and the size of slat martensite decreased from 360 nm to 250 nm.The strength and uniform elongation are gradually increased,and the best rolling process parameter is 60% warm rolling at 770 ℃,and the yield strength and tensile strength are 1690 MPa and 2400 MPa,respectively,and the total elongation is 5.7%.The HDI strengthening effect decreases with increasing deformation,mainly due to the decrease in the proportion of soft phase(ferrite),as shown by the loading and unloading experiments.Overall,the HDI stress/total stress ratio was above 38%,indicating that HDI stress strengthening played a non-negligible role..