2020 Volume 61 Issue 4 Pages 668-677
The changes in the states of carbon (C) together with hardness and the tensile properties of low C steel (0.045C–0.34Mn in mass%) quenched from 710°C and aged at 50°C were investigated as a function of aging time using TEM and atom probe tomography. Vickers hardness increases at about 1.1 × 104 s, exhibits significant increase at 5.8 × 104 s (16 h) and maintains peak hardness untill 8.6 × 105 s (10 d) followed by a decrease after further aging time. At the start of peak aging, C clusters form with an irregular shape that resembles a sphere about 10 nm in diameter. The number of C atoms is about 700, and the C content is in the range of 1–2 at% at 1.0 × 105 s (28 h), where no enrichment of elements except for C is observed. At the end of peak aging, the plate-shaped precipitates (about 1 nm wide and 12 nm long) having a C content greater than 10 at% are distributed with the {100} habit plane, thus confirming the transition from C clusters to fine carbides. Lower yield strength (LYS) is the lowest for the specimen with solute C, and significantly increases for the specimen with C clusters and fine carbides in this order. LYS is determined presumably by the cutting mechanism for the C cluster specimen and the Orowan mechanism for the fine carbide specimen. The work hardening for the solute C and C cluster specimens is high, while the carbide specimen shows less work hardening. The C cluster is assumed to be decomposed into solute C through shearing by dislocations, causing work hardening and relatively good uniform elongation. Post uniform elongation (l-El) was the lowest for the C cluster specimen followed by the fine carbide specimen with the same strength level. This is because dynamic strain aging caused by solute C promotes the strain localization leading to the deterioration in l-El.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 353–362.
