Development of high-toughness high-hardness hypereutectoid steels through optimization of heat treatment and alloying elements

This study designed steels based on hypereutectoid compositions containing 0.8 mass% C, 2.0 mass% Cr and 0.2 mass% Mn to achieve high toughness and hardness. The steels were subjected to two specialized heat treatments: grain boundary amelioration treatment at 810 °C and simple oil quenching process, i.e., full martensitic treatment at 950 °C. The effects of heat treatment, rolling process, and chemical composition on microstructure and toughness were investigated. Adding V formed fine vanadium carbide (VC) particles, which inhibited the coarsening of prior austenite (γ) grains, particularly in the samples subjected to full martensitic treatment. Smaller prior γ grain sizes, particularly those below 20 μm, increased impact values. Grain sizes exceeding 20 μm significantly reduced impact values. The rolling process effectively improved toughness by increasing the nucleation sites during the ferrite (α) to γ transformation in both heat treatments, refining prior γ grains, and reducing coarse carbides and inclusions. Grain boundary amelioration exhibited high impact values exceeding 100 J/cm 2 at 700 HV. Notably, a maximum impact value of 243 J/cm 2 was achieved in the rolled 0.7C-0.3V sample with full martensitic treatment, yielding a high hardness of 700 HV without adding expensive alloying elements. The high toughness and high hardness were simultaneously achieved by optimization of the heat treatment and chemical composition, via the control of the size and morphology of the martensitic grains. This enables selecting the best treatment for various practical applications. • The rolled 0.7C-0.3V FM sample exhibited maximum impact value of 243 J/cm 2 with a high hardness of 700 HV. • The rolling process effectively refined the prior γ grain size and homogenized microstructure, improving the impact value. • The samples with finer γ grain sizes exhibited higher impact values, particularly for γ grain sizes of approximately 10 μm. • High toughness and high hardness were simultaneously obtained via the control of the size and morphology of martensitic grains.