Role of Hierarchical Substructure in Strengthening and Toughening Mechanisms of Low Carbon Martensitic Steel

Abstract

Low-carbon lath martensitic steel is widely applied in critical engineering, energy, and transportation fields due to its low cost, simple processing, and excellent strength-toughness balance. To fully harness the performance potential of martensite, a deeper understanding of the mechanisms driving its strength and toughness is essential. A detailed investigation into the hierarchical substructure of low-carbon martensitic steel elucidates mechanisms underlying its exceptional strength and toughness. In this work, we analyze the effects of varying quenching temperatures on the martensitic steel’s microstructural features, identified through the Kurdjumov-Sachs orientation relationship and quantified via EBSD data processed with MTEX software. This study explores effective structural units driving mechanical properties in response to quenching conditions. In situ tensile testing reveals substructural evolution within specific grains, supporting the hypothesis that packets serve as effective strength-determining units, as corroborated by strength contribution models and the Hall-Petch relationship. Additionally, we observe that the boundary length proportion of specific variant pairs fluctuates with microstructure refinement, where impact toughness positively correlates with boundary density per unit area. The predominant misorientation angles lie within 30°–50°, localized to prior austenite grain (PAG) and packet scales. Minute dimples on fracture surfaces, sized 1.5–3 μm, closely align with the calculated critical crack size, suggesting that block width serves as the primary determinant of toughness. This study provides new insights for the microstructural design of low-carbon lath martensitic steel.

Keywords: lath martensite, Hierarchical Substructure, yield strength, Impact Toughness, variants