Effect of vanadium on the precipitation strengthening upon tempering of a high-strength pipe steel with different initial structure

Abstract

Methods of metallography, scanning electron, and transmission electron microscopies were used to study the structure of two pipe steels (without vanadium and with 0.03% vanadium) subjected to γ → α isothermal transformation at temperatures of 400–600°C (initial structure) and tempering at 600–650°C. It has been found that the addition of 0.03% vanadium intensifies the process of the precipitation of ferrite and contributes to the formation of a granular structure. It has been shown that, in high-strength pipe steels with 0.03% vanadium, which have bainitic granular-type structures, the effect of the precipitation strengthening is effected upon subsequent high-temperature tempering at 600–630°; the addition of vanadium leads to an increase in the hardness by 16 HV. In the presence of bainite of the lath type in the initial structure, the subsequent tempering leads to a softening associated with the processes of the recovery, polygonization, and initial stages of recrystallization in situ, which develops at temperatures above 640°C. It has been found that the hardness of the steel without vanadium upon additional tempering decreases regardless of the morphology of structural constituents in the initial structure.