MICROSTRUCTURE AND MECHANICAL PROPERTY DEVELOPMENT IN THE SIMULATED HEAT AFFECTED ZONE OF V TREATED HSLA STEELS

The simulated heat affected zone (HAZ) of the high strength low alloy (HSLA) steels containing 0%, 0.047%, 0.097% and 0.151% vanadium, respectively, were studied with Gleeble-2000 thermomechanical simulator to determine the influence of vanadium addition on the mechanical properties of the HAZ. The HAZ simulation involved reheating the samples to 1350°C, and then cooling to ambient temperature at a cooling rate of 5°C/s ranging from 800 to 500°C(Δt8/5=60s). The mechanical properties including tensile strength and −20°C impact toughness were conducted. The microstructures of the base steel and the simulated HAZs were investigated using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Based on the systematic examination, the present work confirmed that about 0.05% vanadium addition to low carbon low alloy steels resulted in expected balance of strength and toughness of the HAZ. And more than 0.10% levels addition led to detrimental toughness of the HAZ. SEM study showed that the simulated 0.097% and 0.151%V HAZs consisted of more coarse ferrite plates with greater and more M-A constituents along austenite grain and ferrite plate boundaries. The impact fracture surfaces of the simulated 0.097% and 0.151% V HAZs showed typically brittle mode with predominant cleavages. The size of the facet in the fracture surface increased with increasing vanadium level from 0.097% to 0.151%. As a result, the simulated 0.151%V HAZ has the lowest impact toughness of the four specimens.