Plasticity Ratio–based post-processing for predicting hot forging die life: Industrial validation for gas-nitrided AISI H13 tools

The service life of hot forging dies is limited by severe thermomechanical loading that promotes localized plastic deformation, wear, and crack initiation. Conventional numerical assessments are commonly based on equivalent stress fields, which do not explicitly consider the temperature-dependent reduction of tool material strength, often leading to limited correlation between predicted critical regions and actual industrial failure locations. This work proposes a mechanically grounded post-processing methodology based on the Plasticity Ratio, defined as the ratio between effective stress and temperature-dependent yield strength, to evaluate local proximity to plastic yielding. The approach integrates thermomechanical finite element simulations, experimentally measured microhardness profiles, and industrial validation. Applied to gas-nitrided AISI H13 finishing dies, the method incorporates the hardness gradient of the diffusion zone through a resistance increment factor ( kₙᵢₜ ≈ 1.23 ), enabling correction of local mechanical severity without remeshing or constitutive remodeling. The analysis shows that less than 2% of the die surface governs failure initiation. A power-law relationship between loading severity and durability predicted tool life improvement consistent with the experimentally observed 83.6% increase under industrial conditions. The framework provides a computationally efficient strategy for predicting durability gains promoted by surface treatments in forming tools.