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Decomposition mechanisms of continuously cooled bainitic rail in the critical heat-affected zone of a flash-butt welded joints

Aleksandra Królicka
Aleksandra Królicka
, 
Andrzej Żak
Andrzej Żak
, 
Roman Kuziak
Roman Kuziak
, 
Krzysztof Radwański
Krzysztof Radwański
 and 
Andrzej Ambroziak
Andrzej Ambroziak
   | May 10, 2022
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Published Online: May 10, 2022
Page range: 615 - 625
Received: Mar 11, 2022
Accepted: Apr 03, 2022
DOI: https://doi.org/10.2478/msp-2022-0002
Keywords
© 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

Microstructure of base material of bainitic rails. (A) Visible lath morphology of bainitic ferrite, blocky and film-like retained austenite, and cementite precipitations inside bainitic ferrite laths. Scanning Electron Microscopy, 15 kV, secondary electrons detector. (B) Bright-field image of bainitic ferrite and austenite films. Dark-field image obtained from retained austenite reflex. Transmission Electron Microscopy, 150 kV
Microstructure of base material of bainitic rails. (A) Visible lath morphology of bainitic ferrite, blocky and film-like retained austenite, and cementite precipitations inside bainitic ferrite laths. Scanning Electron Microscopy, 15 kV, secondary electrons detector. (B) Bright-field image of bainitic ferrite and austenite films. Dark-field image obtained from retained austenite reflex. Transmission Electron Microscopy, 150 kV

Fig. 2

The model of flash-butt welded joints (the area subjected to investigations is marked in yellow) and the hardness distribution of the welded joint with the distinguished tested LTHAZ area. Hardness was measured by the Vickers method with a load of 98.1 N (HV10). LTHAZ, low-temperature heat-affected zone
The model of flash-butt welded joints (the area subjected to investigations is marked in yellow) and the hardness distribution of the welded joint with the distinguished tested LTHAZ area. Hardness was measured by the Vickers method with a load of 98.1 N (HV10). LTHAZ, low-temperature heat-affected zone

Fig. 3

The residual stresses’ distribution in the tested welded joint: (+) tensile residual stresses, (−) compressive residual stresses. LTHAZ, low-temperature heat-affected zone
The residual stresses’ distribution in the tested welded joint: (+) tensile residual stresses, (−) compressive residual stresses. LTHAZ, low-temperature heat-affected zone

Fig. 4

Comparison of inverse pole figure maps (A, D) and phase distribution maps (B, E) – retained austenite marked in green, bainitic ferrite marked in red, and misorientation distributions (C, F) between the base material and the LTHAZ indicated. LTHAZ, low-temperature heat-affected zone
Comparison of inverse pole figure maps (A, D) and phase distribution maps (B, E) – retained austenite marked in green, bainitic ferrite marked in red, and misorientation distributions (C, F) between the base material and the LTHAZ indicated. LTHAZ, low-temperature heat-affected zone

Fig. 5

(A) Bright-field image of LTHAZ and (B) corresponding dark-field image from cementite precipitations. (C) The magnification of the area marked in the frame in Figure 5A. Visible Moiré fringes. (D) SAED consistent with the ferrite–cementite Bagaryatskii orientation relationship. TEM, 150 kV. LTHAZ, low-temperature heat-affected zone; TEM, transmission electron microscopy, SAED - selected area diffraction pattern
(A) Bright-field image of LTHAZ and (B) corresponding dark-field image from cementite precipitations. (C) The magnification of the area marked in the frame in Figure 5A. Visible Moiré fringes. (D) SAED consistent with the ferrite–cementite Bagaryatskii orientation relationship. TEM, 150 kV. LTHAZ, low-temperature heat-affected zone; TEM, transmission electron microscopy, SAED - selected area diffraction pattern

Fig. 6

(A) Bright-field image of LTHAZ. Visible coarse cementite precipitations inside bainitic ferrite. (B) Upper bainite. (C) Refined blocky austenite (γb). (D) Cementite precipitations (θ) inside refined blocky austenite. TEM, 150 kV. LTHAZ, low-temperature heat-affected zone; TEM, transmission electron microscopy
(A) Bright-field image of LTHAZ. Visible coarse cementite precipitations inside bainitic ferrite. (B) Upper bainite. (C) Refined blocky austenite (γb). (D) Cementite precipitations (θ) inside refined blocky austenite. TEM, 150 kV. LTHAZ, low-temperature heat-affected zone; TEM, transmission electron microscopy

Fig. 7

(A) Bright-field image of LTHAZ. Visible cementite precipitations inside bainitic ferrite, dislocations, and areas indicated the stress fields. (B) Magnification of coarse cementite precipitations, dislocations, and stress fields. (C) Incoherent cementite precipitation with ferrite matrix in the area marked in the frame in Figure 7B. HRTEM, 200 kV. HRTEM, high-resolution transmission electron microscopy; LTHAZ, low-temperature heat-affected zone
(A) Bright-field image of LTHAZ. Visible cementite precipitations inside bainitic ferrite, dislocations, and areas indicated the stress fields. (B) Magnification of coarse cementite precipitations, dislocations, and stress fields. (C) Incoherent cementite precipitation with ferrite matrix in the area marked in the frame in Figure 7B. HRTEM, 200 kV. HRTEM, high-resolution transmission electron microscopy; LTHAZ, low-temperature heat-affected zone

Fig. 8

Bright-field image of LTHAZ (A) and corresponding dark-field image from carbides (B). (C, D) Magnification of area marked with a frame in Figure 8A and 8B, respectively. HRTEM, 200 kV. HRTEM, high-resolution transmission electron microscopy; LTHAZ, low-temperature heat-affected zone
Bright-field image of LTHAZ (A) and corresponding dark-field image from carbides (B). (C, D) Magnification of area marked with a frame in Figure 8A and 8B, respectively. HRTEM, 200 kV. HRTEM, high-resolution transmission electron microscopy; LTHAZ, low-temperature heat-affected zone

Fig. 9

EDS mapping: (A) reference image, (B) C concentration, (C) Cr concentration, and (D) Si concentration. EDS – energy dispersive X-ray spectroscopy
EDS mapping: (A) reference image, (B) C concentration, (C) Cr concentration, and (D) Si concentration. EDS – energy dispersive X-ray spectroscopy

The comparison of the critical LTHAZ and tempering process of bainitic steels

Microstructural feature or decomposition mechanism LTHAZ (this study) Severe tempering process [30]
Hardness drop Occur Occur
Retained austenite decomposition γRA → α + θ γRA → α + θγRA → α′ | M/AγRA → P
Nature of retained austenite decomposition Heterogeneous (combination of indirect and direct approaches) Direct approach
Dislocation Occur together with Moiré fringes and stress fields Dislocation relaxation
Precipitation coarsening Occur Occur
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