Effect of tempering temperatures on mechanical properties of a novel ultrahigh strength steel

The effect of tempering temperatures on the mechanical properties of G33 steel is researched in this study; its microstructure is also investigated to understand the strengthening and toughening mechanism. Mechanical property tests, including quasi-static tensile, impact toughness and hardness, were conducted at room temperature, and the microstructures were investigated using a scanning electron microscope, transmission electron microscopy and X-ray diffraction. The results show that two distinct stages can be identified from both the mechanical properties data and the morphology images. The first stage varied from 210 to 560°C; both strengths and hardness maintained high values and varied slightly, while impact toughness decreased with increasing temperature, due to the replacement of ε-iron carbide with acicular cementite as well as the decomposition of retained austenite. The second stage ranged from 610 to 700°C; both strengths and hardness reduced markedly, while the toughness rose a little with increasing temperature, which results from coarse carbides emerging and a loss of coherency with the matrix. Thus, this alloy can possess high strength as well as high toughness at a low temperature tempering condition.