Effect of tempering temperature on dynamic impact fracture behaviour of H13 hot work die steel with different notch types

Impact toughness is a crucial mechanical property of hot work die steel that is significantly influenced by tempering temperature. Conventional Charpy impact testing provides only a single toughness value without quantifying the energy distribution between crack formation and extension, while the relationship between different notch types remains unclear. This study examines the dynamic fracture behaviour of H13 steel with unnotched, U-notched, and V-notched specimens tempered at 520–600 using oscillometric impact tests. Results demonstrate that unnotched specimens provide richer eigenvalues in oscillometric curves, better characterising the complete fracture process. Their substantially higher crack formation energy ratio reflects a balanced strength-toughness combination, effectively evaluating both crack initiation and arrest under elastic-plastic conditions. In contrast, notched specimens concentrate impact energy on crack extension due to prefabricated notches, primarily revealing crack sensitivity and plastic toughness while overlooking strength contributions. Notably, V-notched specimens show increasing linear-elastic fracture toughness with rising tempering temperature, governed by enhanced plasticity, while unnotched specimens exhibit decreasing elastic-plastic fracture toughness under the same conditions, dominated by strength reduction. These findings highlight the complementary roles of different notch configurations in material evaluation and provide valuable insights for optimising tempering processes in industrial applications.