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

Abstract

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/cm² at 700 HV. Notably, a maximum impact value of 243 J/cm² 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.