Effect of Vanadium Microalloying on the HAZ Microstructure and Properties of Low Carbon Steels

Four Steels, C-Mn-0.05V, C-Mn-0.11V, C-Mn-0.03Nb and C-Mn were subjected to heat treatment to simulate the microstructure of a coarse grained heat affected zone (CGHAZ) and an intercritically reheated coarse grained heat affected zone (ICCGHAZ). This involved reheating to 1350 degrees C, rapid cooling (Delta t(8/5)=24s) to room temperature and then reheating to either 750 degrees C or 800 degrees C. The toughness of the HAZs was assessed using both Charpy and CTOD tests. Microstructural features were characterised by optical, scanning and transmission electron microscopy. Fractographic examinations of the Charpy and CTOD specimens were carried out to understand the micromechanism of fracture under different microstructural and test conditions. The CGHAZ toughness was similar for the steels except that Steel C-Mn-0.05V had a slightly lower ITT compared to the others. The toughness deteriorated in the ICCGHAZ for all the steels, again Steel C-Mn-0.05V had a superior toughness compared to the other three steels in both ICCGHAZ conditions. Raising the level of vanadium to 0.11% caused a decrease in ICCGHAZ toughness. Steel C-Mn-Nb exhibited a greater degradation of impact toughness after the intercritical cycles. The presence of M-A constituents was the dominant factor in determining the toughness of the ICCGHAZs. The size and area fraction of the M-A constituents were the smallest in Steel C-Mn-0.05V. Increasing vanadium level to 0.11% resulted in a greater area fraction of the M-A constituents, larger average and maximum sizes of M-A particles, and significantly more fields containing the M-A. The addition of 0.031% Nb produced the largest M-A particles and the greatest area fraction for the steels tested.