Room Temperature Aging of Autotempered Fe–C Martensite

Abstract

High-strength low-to-medium carbon martensitic steel is increasingly used in the automobile industry. This study investigated the room temperature aging behavior of as-quenched autotempered Fe–C lath martensitic steels (C: 0.07–0.77 mass%) using kinetic analysis of hardness change and interrupted atom probe (AP) analysis to clarify the dominating factor of hardening. Age-hardening at 23°C was confirmed in the autotempered lath martensitic steels, including low-carbon steel with a carbon content of less than 0.25 mass%. The AP and kinetic analyses of hardness evolution indicated that the growth of carbon clusters at dislocations dominates the hardening of martensite. The maximum hardness increment in lath martensite increased with initial excess solute carbon C_sol in the matrix, but the increment in unit C_sol was smaller than that in carbon-supersaturated ferrite. The smaller hardness increase in martensite may indicate the concurrent softening due to the relaxation of the residual lattice strain in martensite by carbon clustering. Interrupted AP analysis of the prolonged aging over two years indicated that the transformation from carbon clusters to iron carbides occurs via an in-situ transformation of the clusters. The microscopic heterogeneity in carbon distribution in the order of martensite blocks and the gradual decrease in excess C_sol during room temperature aging were also confirmed by AP analysis. The persistence of the heterogeneity and excess solute carbon in the martensite matrix after aging and tempering is also discussed.