Morphology Dependence on Mechanical Stability of Second-phase Austenite in Martensitic Steels

Abe, Takafumi; Fujikura, Kai; Nagashima, Ryota; Nakada, Nobuo; Yabu, Shouhei

doi:10.2355/isijinternational.isijint-2023-112

Cited by 1 publication

(1 citation statement)

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“…In addition to the size, blocky retained γ transforms into martensite more preferentially than filmy γ in the early stage of deformation 15) . In recent years, vigorous investigations have been conducted into the mechanical stability of γ, including studies on its crystal orientation and deformation direction dependencies 16) , the influence of γ morphology 17) , and the effects of strain localization and γ size 18) . However, grain size or morphology and carbon content of  should be interrelated since carbon enrichment during austempering should be affected by  grain size.…”

Section: Introductionmentioning

confidence: 99%

Thermal Stability of Retained Austenite with Heterogeneous Composition and Size in Austempered Fe-2Mn-1.5Si-0.4C Alloy

Watanabe,

Miyamoto,

Zhang

et al. 2024

ISIJ Int.

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The mechanical properties of TRIP steels depend on heterogeneities of chemical composition and grain size in the retained γ structure, although these heterogeneities have not been characterized in detail. Therefore, in this study, we quantitatively investigate the inhomogeneous carbon concentration and grain size distribution, and its effects on the thermal stability of the retained γ in Fe-2Mn-1.5Si-0.4C (mass%) TRIP steel using FE-EPMA, EBSD, Mössbauer spectroscopy, and in-situ neutron diffraction during bainitic transformation at 673 K. In-situ neutron diffraction experiments detects high-carbon γ evolving during bainite transformation, in addition to the original γ, and the time variation of the volume fraction of high-carbon γ agrees well with the fraction of γ retained at room temperature. Williamson-Hall analysis based on peak width suggests that heterogeneity of carbon content exists even within the high-carbon γ. Compositional analysis using FE-EPMA and three-dimensional atom probe directly revealed that fine filmy γ was highly enriched with carbon compared to larger blocky γ, and the carbon content in blocky γ decreases with increasing blocky γ size. DICTRA simulation qualitatively reproduces the size dependency of carbon enrichment into γ. It was also found that  tends to be retained at higher carbon content and smaller  grain size since the smaller grain size directly improves thermal stability and the smaller γ size further contributes to the thermal stability via enhanced carbon enrichment.

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