A quantitative evaluation of microalloy precipitation strengthening in martensite and bainite

The influence of V content on the strengthening mechanisms in martensite and bainite tempered between 500 and 650 degrees C was investigated with scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. A quantitative methodology to measure the volume fraction of microalloy (MX) precipitates was employed to directly calculate the contribution of precipitation strengthening after tempering at 500 and 650 degrees C. Increased V contents result in larger increases in hardness at higher tempering temperatures compared to lower tempering temperatures for both martensite and bainite. The increase in hardness from increased V content is not significantly different between martensite and bainite. Greater volume fractions of MX are observed with higher V contents. The increased MX volume fractions inhibit recovery during tempering as indicated by higher measured dislocation densities in the high V conditions. The majority of the strengthening imparted by increased V content is calculated to be a result of higher dislocation densities rather than greater MX precipitation strengthening after tempering at 500 degrees C. However, greater V contents are calculated to primarily raise the hardness by increased MX precipitation strengthening after tempering at 650 degrees C. In general, higher dislocation densities as well as increased MX precipitation strengthening are both important strengthening mechanisms influenced by V content during tempering in the secondary hardening regime.