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The qualitative–quantitative approach to microstructural characterization of nanostructured bainitic steels using electron microscopy methods

Aleksandra Królicka
Aleksandra Królicka
, 
Aleksandra Janik
Aleksandra Janik
, 
Andrzej Żak
Andrzej Żak
 et 
Krzysztof Radwański
Krzysztof Radwański
   | 01 oct. 2021
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Publié en ligne: 01 oct. 2021
Pages: 188 - 199
Reçu: 23 juil. 2021
Accepté: 16 août 2021
DOI: https://doi.org/10.2478/msp-2021-0017
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© 2021 Aleksandra Królicka et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Fig. 1

TTT chart of 9XC steel, with indicated isotherm corresponding to the performed heat treatment. TTT, time–temperature–transformation.
TTT chart of 9XC steel, with indicated isotherm corresponding to the performed heat treatment. TTT, time–temperature–transformation.

Fig. 2

Heat treatment diagram of the 9XC steel sample analyzed in this research.
Heat treatment diagram of the 9XC steel sample analyzed in this research.

Fig. 3

PAGS analysis using a conventional picric acid solution. (A) Microphotography, light microscopy. (B) Grain diameter distribution. PAGS, prior-austenite grain size.
PAGS analysis using a conventional picric acid solution. (A) Microphotography, light microscopy. (B) Grain diameter distribution. PAGS, prior-austenite grain size.

Fig. 4

PAGS analysis using the EBSD technique. (A) Inverse pole figure map. (B) Inverse pole figure and image quality map with the indicated misorientation angles in the range of 20°–40°. (C) Misorientation angles in the range of 20°–40°. (D) Inverse pole figure and image quality map with reconstructed prior-austenite grain boundaries based on misorientation angles and bainitic sheaves orientation. EBSD, electron backscatter diffraction; PAGS, prior austenite grain size.
PAGS analysis using the EBSD technique. (A) Inverse pole figure map. (B) Inverse pole figure and image quality map with the indicated misorientation angles in the range of 20°–40°. (C) Misorientation angles in the range of 20°–40°. (D) Inverse pole figure and image quality map with reconstructed prior-austenite grain boundaries based on misorientation angles and bainitic sheaves orientation. EBSD, electron backscatter diffraction; PAGS, prior austenite grain size.

Fig. 5

Grain diameter distribution based on EBSD analysis of PAGS. EBSD, electron backscatter diffraction; PAGS, prior-austenite grain size, green line – cumulative histogram.
Grain diameter distribution based on EBSD analysis of PAGS. EBSD, electron backscatter diffraction; PAGS, prior-austenite grain size, green line – cumulative histogram.

Fig. 6

The microstructure of 9XC steel after isothermal heat treatment. (A) Visible lath morphology of bainite sheaves inside the prior-austenite grain obtained by SEM. (B) Visible austenite with blocky (γb) and film-like (γf) morphologies inside the prior-austenite grain; image obtained by SEM. (C) Bainitic ferrite laths (αb) and film-like austenite (γf) in the area of bainitic sheaves. Images were obtained by TEM (bright field image). SEM, scanning electron microscopy; TEM, transmission electron microscopy.
The microstructure of 9XC steel after isothermal heat treatment. (A) Visible lath morphology of bainite sheaves inside the prior-austenite grain obtained by SEM. (B) Visible austenite with blocky (γb) and film-like (γf) morphologies inside the prior-austenite grain; image obtained by SEM. (C) Bainitic ferrite laths (αb) and film-like austenite (γf) in the area of bainitic sheaves. Images were obtained by TEM (bright field image). SEM, scanning electron microscopy; TEM, transmission electron microscopy.

Fig. 7

(A) The bright field image of bainitic sheaves consisting of bainitic ferrite laths and film-like austenite. (B) The dark field image from the austenite reflex ( 11¯1 1\bar 11 ). (C) Selected area diffraction pattern with solution from the area presented in panel (A). TEM, 150 kV. TEM, transmission electron microscopy.
(A) The bright field image of bainitic sheaves consisting of bainitic ferrite laths and film-like austenite. (B) The dark field image from the austenite reflex ( 11¯1 1\bar 11 ). (C) Selected area diffraction pattern with solution from the area presented in panel (A). TEM, 150 kV. TEM, transmission electron microscopy.

Fig. 8

(A) Selected area diffraction pattern with solution from area presented in panel (B) (B) The bright field image of bainitic sheaves consisting of bainitic ferrite laths, film-like austenite, and cementite precipitations.. TEM, 150 kV. TEM, transmission electron microscopy.
(A) Selected area diffraction pattern with solution from area presented in panel (B) (B) The bright field image of bainitic sheaves consisting of bainitic ferrite laths, film-like austenite, and cementite precipitations.. TEM, 150 kV. TEM, transmission electron microscopy.

Fig. 9

(A) Inverse pole figure map of 9XC grade steel after isothermal heat treatment. (B) Phase distribution map. Ferrite is marked in green, austenite is marked in red. (C) Phase distribution map (ferrite – white, austenite – gray), with the indicated K–S misorientation angles (43° ± 1) in red and N–W misorientation angles (46° ± 1) in blue. (D) Distribution of the misorientation angles. K–S, Kurdjumov–Sachs; N–W, Nishiyama–Wassermann.
(A) Inverse pole figure map of 9XC grade steel after isothermal heat treatment. (B) Phase distribution map. Ferrite is marked in green, austenite is marked in red. (C) Phase distribution map (ferrite – white, austenite – gray), with the indicated K–S misorientation angles (43° ± 1) in red and N–W misorientation angles (46° ± 1) in blue. (D) Distribution of the misorientation angles. K–S, Kurdjumov–Sachs; N–W, Nishiyama–Wassermann.

Fig. 10

Example of an image used for phase refinement analysis. TEM, bright field image. TEM, transmission electron microscopy.
Example of an image used for phase refinement analysis. TEM, bright field image. TEM, transmission electron microscopy.

Fig. 11

The width distribution of film-like austenite and bainitic ferrite.
The width distribution of film-like austenite and bainitic ferrite.

Fig. 12

Box plot of the thicknesses of film-like austenite and bainitic ferrite.
Box plot of the thicknesses of film-like austenite and bainitic ferrite.

Fig. 13

(A) Example of area intended for qualitative analysis. (B) Magnification of bainitic sheaf. Visible film-like austenite and bainitic ferrite. (C) Graphic image editing intended for film-like austenite measurements. (D) Prepared image for blocky austenite measurements.
(A) Example of area intended for qualitative analysis. (B) Magnification of bainitic sheaf. Visible film-like austenite and bainitic ferrite. (C) Graphic image editing intended for film-like austenite measurements. (D) Prepared image for blocky austenite measurements.

Fig. 14

(A) Bright field image of bainitic sheaf (TEM). (B) Prepared image for area fraction measurement of film-like austenite. TEM, transmission electron microscopy.
(A) Bright field image of bainitic sheaf (TEM). (B) Prepared image for area fraction measurement of film-like austenite. TEM, transmission electron microscopy.

Measurement results of retained austenite content considering its morphology.

Method Measurement areas Blocky austenite (γb), % Film-like austenite (γf), % Total austenite (γb + γf), % Ratio, γf/γb
SEM – graphical analysis 3 20.5 ± 3.2 23.7 ± 4.1 44.2 1.16
EBSD – phase distribution map 3 22.3 ± 5.8 – – –
TEM – graphical analysis 5 – 24.8 ± 9.1 – –
TEM + EBSD – 22.3 ± 5.8 24.8 ± 9.1 47.1 1.11

Results of measurement of the thicknesses of film-like austenite and bainitic ferrite.

Phase Count number x̄, nm SD, nm Median, nm Q1, nm Q3, nm ½IQR, nm
Filmy austenite 131 44.6 20.3 40.0 32.5 53.4 10.4
Bainitic ferrite 131 106.8 40.7 102.0 76.3 131.8 27.7

The chemical composition of 9XC steel.

Chemical composition, wt.%
C Si Mn Cr Mo Al Ni P S V
0.94 1.54 0.44 1.05 0.01 0.022 0.18 0.01 0.007 0.006

Comparison of prior-austenite grain size measurements obtained by conventional etching by picric acid solution and by the EBSD technique.

Method Measurement counts x̄, mm SD, mm Median, mm Q1, mm Q3, mm ½IQR, mm
Conventional etching by picric acid solution 100 27.8 10.9 26.4 18.9 34.9 8.0
EBSD analysis using misorientation angles (20°–40°) 100 18.7 9.2 17.8 13.9 21.8 4.0
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Materials Sciences, other, Nanomaterials, Functional and Smart Materials, Materials Characterization and Properties
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