The effect of vanadium content on hierarchical martensite structure and yield strength of petroleum casing steels

Abstract

In the present study, the influence of V content and tempering temperature on the precipitation behavior of hierarchical martensite and yield strength of petroleum casing steels has been systematically studied. Moreover, the relationship between the precipitation behavior and strengthening mechanisms has been analyzed. It was found that yield strength increased with increase of V content. Especially, the steel with 0.10% V has the most efficient strengthening effect, and its yield strength increased nearly 370 MPa compared with that of steel without V. With increase of tempering temperature, the yield strength of steels with 0.10% V decreased in three stages because of the various precipitation behavior on hierarchical martensite structure. The results of nano-indentation tests showed more precipitates were aggregated preferentially in martensite blocks (MBs) of the high V steels (V10 and V15 steels), while the precipitates of V0 and V5 steels were uniformly distributed in martensite packets (MPs), leading to larger nano-hardness of MBs. Moreover, the precipitates on MBs decreased sharply with increase in tempering temperature, and the residual precipitates aggregated on the boundaries of MBs, leading to a dramatic decrease in the nano-hardness of MBs. Considering the size effect and precipitation behavior on the hierarchical martensite structure under different V content and tempering temperature, the traditional yield strength model was modified. Moreover, it was confirmed that the main strengthening mechanisms of V microalloyed petroleum casing steels were grain refinement strengthening and precipitation strengthening.