# Microstructure Formation and Carbon Partitioning with Austenite Decomposition during Isothermal Heating Process in Fe-Si-Mn-C Steel Monitored by In Situ Time-of-Flight Neutron Diffraction

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. In Situ Neutron Diffraction

#### 2.2. Analyses

^{3}He position sensitive detectors covering a wide 2θ range [20]. The scattering data acquisition by all the detectors was conducted during the whole process of a heat treatment scheme. The scattering data acquired during the arbitral time span can be exported for the following analyses after the measurement. In the current study, we analyzed the data for every 200 s during the isothermal heating, in addition to the full austenizing step and after the final cooling to room temperature.

#### 2.3. Sample

#### 2.4. Heat Treatment Scheme

#### 2.5. Microstructural Observation

## 3. Results

#### 3.1. Dynamic Change in Phase Fractions and Carbon Distribution

^{−4}of lattice strain. By roughly assuming a Young’s modulus of 200 GPa, and that lattice strain is due to the internal stress, the magnitude of internal stress would be 170 MPa. Since the yield stress of 0.6 C austenitic steel seems to be around 200 MPa [25], the internal stress higher than this can plastically be accommodated. Therefore, even if an internal elastic stress field existed, the error of carbon concentration would be only as high as 0.1 mass%.

#### 3.2. Microstructural Features

## 4. Discussion

#### 4.1. Carbide/Austenite Nano-Films

#### 4.2. Carbon Migration Mechanisms

#### 4.3. Formation of High Carbon Austenite at 673 K

_{cm}, ${T}_{o}$, and WBs curves. Para Ae

_{3}, para A

_{cm}, and ${T}_{o}$ curves were calculated by Thermo-Calc software and the WBs line is based on the experimental fitting by Leach et al. [32]. Both ${T}_{o}$ and WBs are basically lines from left-top to right-bottom, but the para A

_{cm}curve has a positive tangent. The carbon concentrations seen at 573 and 623 K are quite close to the para A

_{cm}curve. Therefore, it is concluded that there is a chemical driving force for the cementite precipitation in this temperature range, even though diffusion of Si is prohibited.

_{cm}concentration. Thus, incomplete bainite transformation took place but the stasis cannot be explained by the displacive theory. Furthermore, nor does the WBs line fit well, although the tangent is somewhat close. In Leach’s study, the coefficients for Mn and Si were determined with very scattered data compared to those for C. Since WBs is the phenomenological quantity, it is difficult to identify what is appropriate or not.

_{3}toward the left in Figure 11. Several sources of energy dissipation are suggested, i.e., solute dragging, intrinsic mobility, element partitioning, and transformation strain. Wu et al. suggested that the contribution of transformation strain energy can be the dominant source of energy dissipation in the case of bainite transformation of Si-added steel [33]. Moreover, it should be noted that the carbon concentrations observed in this study are the averages of certain volumes. It is possible that the local equilibrium is established with a higher carbon concentration in the vicinity of phase boundaries.

## 5. Conclusions

_{cm}curve in the phase diagram.

_{3}curve.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**{100}, {110}, and {111} pole figures showing the initial texture of the tested steel. The directions X (vertical), Y (horizontal), and Z (vertical to the paper) correspond to RD (rolling direction), TD (transverse direction), and ND (normal direction) in the hot-rolling process, respectively.

**Figure 2.**Initial microstructure of the current sample (secondary electron image, acceleration voltage: 15.0 kV, working distance: 15.0 mm). The horizontal and vertical directions are the RD and ND in the hot-rolling process, respectively.

**Figure 4.**The splitting of the 111γ diffraction peak observed during the isothermal heating at 673 K. The images correspond to different time spans, (

**a**) 0−200 s, (

**b**) 200−400 s, (

**c**) 400−600 s, and (

**d**) 600−800 s. The origin of the time is the beginning of isothermal heating. The observed intensities are shown as open circles tied with black lines. The split peak observed at each time span was fitted by pink and blue Gaussian peaks representing the high and low carbon austenite, respectively. The magnitude of the scattering vector, K, is defined as K = 1/d = 2sin θ/λ.

**Figure 5.**The phase fractions and carbon concentrations in high and low carbon austenite during austempering at various temperatures: (

**a**) 573 K, (

**b**) 623 K, (

**c**) 673 K, (

**d**) 723 K, and (

**e**) 773 K.

**Figure 6.**Image quality (IQ) map with overlayed phase coloring, where red regions are FCC austenite. The samples are after the heat treatment at (

**a**) 573 K for 1.8 ks, (

**b**) 623 K for 1.8 ks, (

**c**) 673 K for 1.8 ks, (

**d**) 723 K for 3.6 ks, and (

**e**) 773 K for 1.8 ks.

**Figure 7.**Secondary electron images (20k × magnification) for the samples after the heat treatment at (

**a**) 573 K for 1.8 ks, (

**b**) 623 K for 1.8 ks, (

**c**) 673 K for 1.8 ks, (

**d**) 723 K for 3.6 ks, and (

**e**) 773 K for 1.8 ks.

**Figure 8.**Diffraction patterns near the 110α peak observed before (black) and after (gray) the heat treatment process with austempering at 773 K for 1.8 ks. The cementite peak positions are indicated by “+”.

**Figure 9.**The fraction of carbon in detected austenite (FCDA) during the isothermal heating at (

**a**) 573 K, (

**b**) 673 K, and (

**c**) 723 K.

**Figure 10.**EBSD orientation maps for BCC (ferrite/martensite) and FCC (retained austenite) for the sample after isothermal heating at 673 K for 100 s. The arrows indicate bainitic ferrite sheaves.

**Figure 11.**Para equilibrium phase diagram calculated by Thermo-Calc. The WBs line is drawn using the equation suggested by Leach et al. [31]. The maximum carbon concentrations in austenite determined in the current experiment are plotted together.

C | Si | Mn | P | S | Al | N |
---|---|---|---|---|---|---|

0.61 | 1.90 | 0.98 | 0.008 | 0.001 | 0.033 | 0.04 |

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**MDPI and ACS Style**

Onuki, Y.; Umemura, K.; Fujiwara, K.; Tanaka, Y.; Tomida, T.; Kawano, K.; Sato, S.
Microstructure Formation and Carbon Partitioning with Austenite Decomposition during Isothermal Heating Process in Fe-Si-Mn-C Steel Monitored by In Situ Time-of-Flight Neutron Diffraction. *Metals* **2022**, *12*, 957.
https://doi.org/10.3390/met12060957

**AMA Style**

Onuki Y, Umemura K, Fujiwara K, Tanaka Y, Tomida T, Kawano K, Sato S.
Microstructure Formation and Carbon Partitioning with Austenite Decomposition during Isothermal Heating Process in Fe-Si-Mn-C Steel Monitored by In Situ Time-of-Flight Neutron Diffraction. *Metals*. 2022; 12(6):957.
https://doi.org/10.3390/met12060957

**Chicago/Turabian Style**

Onuki, Yusuke, Kazuki Umemura, Kazuki Fujiwara, Yasuaki Tanaka, Toshiro Tomida, Kaori Kawano, and Shigeo Sato.
2022. "Microstructure Formation and Carbon Partitioning with Austenite Decomposition during Isothermal Heating Process in Fe-Si-Mn-C Steel Monitored by In Situ Time-of-Flight Neutron Diffraction" *Metals* 12, no. 6: 957.
https://doi.org/10.3390/met12060957