Changes in States of Carbon and Mechanical Properties with Aging at 50°C after Quenching in Low Carbon Steel

Abstract

The changes in the states of carbon (C) together with hardness and the tensile properties of low C steel (0.045C0.34Mn 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 © 10 4 s, exhibits significant increase at 5.8 © 10 4 s (16 h) and maintains peak hardness untill 8.6 © 10 5 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 12 at% at 1.0 © 10 5 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.

A Mechanism of Carbon-Cluster Strengthening through Atomic Simulations

“…Such observations have also revealed that the carbon clusters are distributed at intervals of approximately 20 nm in the two-dimensional view of APT data and at intervals of approximately 30 nm if a square lattice distribution is assumed using the number density and cluster diameter. 1) Given the observations reported thus far, 1,46) this steel material is understood to exhibit peak aging by transitioning from the homogenous solid-solution state of the solution-treated material to carbon clusters during aging; it subsequently exhibits age-softening by changing its morphology to ¾-carbides, etc. However, why carbon clusters exhibit maximum hardness remains unclear.…”

“…The carbon clusters characterized by APT have a carbon concentration of 1³2 at% and a diameter of approximately 10 nm, and the distance between the centers of the clusters is approximately 20³30 nm. 1) In the present paper, analysis models are constructed on the basis of actual APT measurements and the shape of the carbon cluster is assumed to be a sphere with diameter D. The carbon cluster is modeled by randomly arranging N carbon atoms at stable interstitial sites of the carbons in a spherical range of D diameters in ¡-Fe. The number N of carbon atoms is set to 630³640, referring to the value (700) calculated from the measured carbon concentration of APT.…”

“…We confirm that the carbon concentration decreases with increasing diameter D. Notably, in the present study, the density of the number of carbon clusters is fixed and the number of atoms constituting the carbon clusters is kept constant even though the number density of actual carbon assemblies decreases with aging time, indicating Ostwald ripening. 1) Here, we discuss the reproducibility of the carbon-cluster model used in the present paper by comparing the carbon concentration distributions of the two carbon clusters measured by APT and the carbon-cluster model produced by random arrangement. Figure 2 shows the distribution of carbon concentrations in each direction of the measured carbon clusters and the carbon-cluster model (Fig.…”

“…Fe0.045C (0.21 at%), after solution heat treatment, has been reported to exhibit peak aging when subjected to an isothermal aging treatment at 50°C for approximately 28 h. 1) The hardnesses of the aged material are 140 Hv for the as-solution-treated material, 200 Hv for the specimen aged approximately 28 h, and 180 Hv for the specimen aged approximately 128 days. Observations by atom probe tomography (APT) 2,3) have revealed that carbon atoms at peak aging exist as carbon clusters in the solid-solution state enriched at 1³2 at% in the region approximately 10³20 nm in diameter.…”

Best Papers Awarded in 2021 by <i>Materials Transactions</i>

“…11 (2020) pp. 21392148) 5,13) To investigate the reason why low-carbon steels with carbon-clusters shows the maximum strength during lowtemperature aging, 34) interactions between an edge dislocation and carbon clusters are performed through molecular dynamics (MD) simulations. 35,36) Carbon clusters are modeled based on atom probe tomography (APT) observations.…”