Effect of Al on the continuous cooling transformation characteristic of hot extrusion die steel SDAH13

The continuous cooling transformation (CCT) diagrams of SDAH13 hot extrusion die steels with different mass fractions of Al (0.77% and 1.43%) and without Al were measured by dilatometry. The effects of Al on the CCT diagrams, microstructure and hardness of SDAH13 steels were studied by optical microscopy, scanning electron microscopy (SEM) and Vickers hardness test. The results show that the phase transformation temperature points (Ac₁, Ac₃ and Ms) of SDAH13 steels significantly increase, tile contents of retained austenite in the quenched steels decrease, and the a-y multi-phase region expands with the increase in mass fraction of Al. However, when austenited at 1060 ℃, Al has no obvious influence on the critical cooling rate (0.30 ℃·s^-1) of bainite transformation, but bainite phase region broadens. Meanwhile, the critical cooling rates of pearlite transformation of SDAH13 steels with the Al content of 0.77% and 1.43% are 0.05 ℃·s^-1 and 0.3 ℃·s^-1, respectively, which are higher than the critical cooling rate (0, 02 ℃·s^-1) of pearlite transformation of SDAH13 steel without Al. Pro-eutectoid ferrite appears at the cooling rate from 0.02 ℃·s^-1 to 0.08 ℃·s^-1 when the mass fraction of AI reaches 1.43%. Nevertheless, the quenched hardness decreases with Al addition.