Room-temperature aging of autotempered low-to-medium carbon martensitic steel

High-strength, low-to-medium-carbon, low-alloy martensitic steel is increasingly used in automobile structural components. Understanding the controlling factors and kinetics of low-temperature aging is crucial for ensuring quality control in sheet steel and automobile part manufacturing. This study investigates the room-temperature aging behavior of as-quenched autotempered Fe-C lath martensitic steels (C: 0.07–0.77 mass%) via hardness change kinetics and interrupted atom probe (AP) analyses to identify the dominant hardening mechanism. Age-hardening at 23 °C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content below 0.25 mass%. Although the maximum hardness increment in lath martensite increased with the initial excess solute carbon (Csol) within the matrix, the increment per unit Csol was below that in carbon-supersaturated ferrite. AP and kinetic analyses of the hardness evolution indicated that carbon cluster growth at dislocations dominated the hardening of the martensite.