Detection and Estimation of Retained Austenite in a High Strength Si-Bearing Bainite-Martensite-Retained Austenite Micro-Composite Steel after Quenching and Bainitic Holding (Q&B)

Abstract

To develop an advanced high strength steel with reasonable ductility based on low alloying concept as well as micro-composite microstructure essentially consisting of bainite, martensite and retained austenite, a Si-bearing, low alloy medium carbon sheet steel (DIN1.5025 grade) was subjected to typical quenching and bainitic holding (Q&B) type isothermal treatment in the bainitic region close to martensite start temperature (Ms) for different durations in the range 5s to 1h. While the low temperature bainite has the potential to provide the required high strength, a small fraction of finely divided austenite stabilized between the bainitic laths is expected to provide the desired elongation and improved work hardening. Various materials characterization techniques including conventional light metallography, field emission scanning electron microscopy FE-SEM, electron backscatter diffraction (EBSD), differential thermal analysis, X-ray diffraction (XRD) and vibrating sample magnetometry (VSM), were used to detect and estimate the volume fraction, size and morphology and distribution of retained austenite in the micro-composite samples. The results showed that the color light metallography technique using LePera’s etching reagent could clearly reveal the retained austenite in the microstructures of the samples isothermally held for shorter than 30s, beyond which an unambiguous distinction between the retained austenite and martensite was imprecise. On the contrary, the electron microscopy using a FE-SEM was not capable of identifying clearly the retained austenite from bainite and martensite. However, the EBSD images could successfully distinguish between bainite, martensite and retained austenite microphases with good contrast. Although the volume fractions of retained austenite measured by EBSD are in accord with those obtained by XRD and color light metallography, the XRD measurements showed somewhat higher fractions owing to its ability to acquisition and analyze the diffracted X-rays from very finely divided retained austenite, too. The differential thermal analysis and vibrating sample magnetometry techniques also confirmed the stabilization of retained austenite finely divided in bainite/martensite micro-composite microstructures. In addition, the peak temperatures and intensities corresponding to the decomposition of retained austenite were correlated with the related volume fractions and carbon contents measured by the XRD analysis.

Heat Treatment Effect on Microstructure Evolution in a 7% Cr Steel for Forging

“…In particular, 1.7% retained austenite content was measured in specimen PT-γ-2 by EBSD (to be compared to 2.5% as measured by XRD) and 1.2% in specimen PT-γ-3 (to be compared to <2.0% as measured by XRD). The low discrepancy obtained by EBSD with respect to XRD is justified by the two different adopted techniques [50]. The agreement between RA contents, as measured by the two different methods, makes the results reported in Table 3 consistent, even if their absolute values are quite low (in the proximity of XRD-technique sensitivity).…”

Laboratory- and Semi-Industrial-Scale Thermomechanical Processing of TRIP-Aided Steel with Acicular Ferrite

“…(SEM) FEI Inspect F) analyses were performed to characterize the RA fraction in the microstructure. The fraction of RA was calculated using a Rietveld method [24,35] and to determine the C content in austenite, the following equation was used [36]:…”

“…crostructure. The fraction of RA was calculated using a Rietveld method [24,35] and to determine the C content in austenite, the following equation was used [36]:…”

“…The development of multi-phase steels in recent years is associated with the partial or complete replacement of silicon [24] by Al or P [25][26][27]. The study of their impact on a fraction of retained austenite obtained under different heat treatment conditions and the mechanical stability of the γ phase during cold deformation is also of scientific interest.…”

Identification of Bainite in a Multi-Phase Microstructure of an Austempered Steel Alloy: A Metallography Approach

“…The use of color etching in multi-phase alloys can facilitate the identification of phases in an alloy [19][20][21][22]. Color etching becomes a choice because it can display different visualizations through contrasting colors in each phase formed.…”