Effect of vanadium replacing nickel in 40CrNiMo steel on microstructure and low-temperature toughness
The nickel in 40CrNiMo steel is mainly used to improve low-temperature toughness,but the price of nickel is expensive and does not meet the requirement of low cost.A low cost CrNiMo steel with a mass fraction of 0.08％vanadium microalloyed and nickel content of only 0.3％was developed to replace 40CrNiMo steel(the mass fraction of nickel was about 1.5％).The differences of matrix and carbide between test steels were studied by means of SEM,EBSD,TEM and XRD.The tensile test and-40℃ impact test were taken to compare the toughness of test steels under the same strength(10.9 grade).The results show that the prior austenite grain size of 40CrNiMo steel is smaller than that of developed steel,which is(14.5±5.3)μm and(20.6±7.1)μm respectively.The large angle grain boundary density of 40CrNiMo is 0.7/μm high than that of the developed steel due to the fine grain size.Due to the tempering resistance improving effect of vanadium,the tempering temperature of the developed steel is increased by 60℃ compared with 40CrNiMo steel at the same tensile strength,and the recovery degree of the matrix is improved,thus obtaining a higher proportion of large angle grain boundary and a lower dislocation density.The dislocation density of the two test steels is 9.3×1014/m2 and 2.3×1015/m2,respectively.The MC,M2C and M3C carbides are precipitated in the developed steel microstructure after tempering.The MC carbides are spherical with diameter less than 20 nm,while the M2C and M3C carbides are ellipsoidal with maximum length less than 150 nm.The M2C and M3C carbides are precipitated in 40CrNiMo steel.The M2C carbides are ellipsoidal with maximum length of less than 100 nm,and the M3C carbides are strips with length of more than 500 nm.Through the statistics of maximum size of all carbide in observation fields,it is found that the proportion of M3C cementite in developed steel is lower,and the overall size of carbides is smaller.Although the prior austenite grain size of the developed steel is large and its large angle grain boundary density is low,but its sufficient recovery and fine spherical carbide particles got after tempering improve its low-temperature toughness.When the tensile strength is about 10.9 grade,the impact absorb energy of the developed steel reaches 86 J at-40℃,which is higher than 74 J of the 40CrNiMo steel.
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