# Combined Effects of Deformation and Undercooling on Isothermal Bainitic Transformation in an Fe-C-Mn-Si Alloy

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## Abstract

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## 1. Introduction

^{1/2}[1,2,3]. Unfortunately, the very long duration of low-temperature isothermal transformation limits its commercial application. Aluminum and cobalt are added into high-carbon nanobainite steels to reduce the heat treatment time [4,5,6], and the time can be shortened from several days to ~10 h despite being impractical for industry. Low carbon bainitic steels can be designed to shorten the incubation period and improve weldability and impact toughness [7]. However, decreasing carbon concentration causes a considerable increase of martensite start temperature (Ms), making austempering proceed at relative high temperatures, and thereby coarsening bainite laths. Therefore, a medium-carbon Si-Mn-rich alloy could be a desired alternative to produce high strength nano-structure bainite steel. Actually, many works have already been conducted on transformation, microstructure and property of medium-carbon bainite steels (MCBS) [8,9,10,11,12].

_{0}curve [14,15], which places strict limits on the maximum fraction of transformation that can be achieved theoretically. In fact, both theoretical calculation and experimental results have proven that a lower transformation temperature can lead to an increase of bainite formation [16,17,18]. However, the time for transformation could be prolonged below the nose temperature of the time-temperature-transformation (TTT) curve, which is inversely correlated to the expectation. In the author’s previous works [19,20], an ausforming with small strain at low temperatures could not only accelerate the kinetics of bainitic transformation, but also increase the final volume fraction of bainite. Gong et al. [21,22] also claimed that the dislocation structure introduced in austenite by low temperature ausforming is found to assist bainite transformation with strong variant selection where partial dislocations introduced by ausforming play an important role for bainite transformation. Yang et al. [23] also claimed that a two-steps process can be used to shorten the transformation time from over 60 h to nearly 25 h. He et al. [24] reported that the kinetics of bainitic transformation depends on the competition between the increase in nucleation rate and the decrease in average volume of bainite sheaf after deformation. The increase in nucleation rate overcomes the decrease in the average volume of bainite sheaf, resulting in the increase in transformation velocity and volume fraction after small deformation. In this way, a low-temperature ausforming could be utilized to promote transformation when producing low-temperature carbide-free bainitic steels by rolling processing technology. However, considering that the resistance force can be enhanced by the decrease of deformation temperature, the ausforming conditions should be optimized.

## 2. Materials and Methods

^{−1}was utilized to avoid ferrite and pearlite transformation. After compression deformation at respective temperatures with a strain of 0.1, 0.2 and 0.3 (strain rate is 1 s

^{−1}), respectively, all the samples were isothermally held for 90 min for bainitic transformation. The external compressive stress was kept to a very small value during isothermal holding so that the influence of external compressive stress could be ignored. After isothermal holding, the specimens were air-cooled to ambient temperature. Meanwhile, two non-deformed specimens were tested with the same heating and holding routes. To obtain nano-structured lath bainite, the transformation temperature should be design as low as possible between Ms and bainite starting temperature (Bs). The time temperature transformation (TTT) diagrams of the steel was calculated by MUCG83 (Modified version of MUCG73 and MUCG46, Cambridge, UK) [25,26], as shown in Figure 2. The Bs and Ms are 423 and 256 °C, respectively, indicating that bainite could be formed during austempering at 300 and 350 °C. The variation of transformation temperature was attempted to cause a difference in undercooling.

## 3. Results

#### 3.1. Microstructure

#### 3.2. Kinetics of Bainitic Transformation

## 4. Discussions

^{−1}for 350 °C, −1321 J∙mol

^{−1}for 300 °C) (Figure 7a), leading to more bainite in the non-deformed austenite transformed at 300 °C compared to that at 350 °C. Bainitic formation are embedded in the concept of the ${{T}^{\prime}}_{0}$ curve, which is the locus of points, on a temperature vs. carbon content plot, where austenite and ferrite of the same chemical composition have the same free energy, taking into account the stored energy of the ferrite due to the displacive mechanism of transformation (400 J∙mol

^{−1}) [28]. It can be inferred from Figure 7b that the residual austenite at 300 °C accommodates more carbon partitioned from newly formed bainite. Therefore, a bainitic reaction at a lower temperature could theoretically go further until the carbon concentration reaches the higher level. The kinetics of bainitic transformation can also be affected by alloying elements such as Mn and Al. It was reported that that the transformation stasis phenomenon could occur in the Fe-C-Mn and Fe-C-Mn-Si alloys, while the transformation is approaching paraequilibrium mode with decreasing Mn concentration [29]. The Mn content of the tested steel (2.8 wt. %) is so high that the kinetics of bainitic transformation could also be retarded. In contrast, Al addition can be utilized to accelerate bainite formation due to the fact that it can increase the free energy change for transformation [4].

## 5. Conclusions

## Author Contributions

## Acknowledgments

## Conflicts of Interest

## Abbreviations

RA | retained austenite |

MS | martensite starting temperature |

MCBS | medium-carbon bainite steels |

ICT | incomplete transformation |

TTT | time-temperature-transformation |

Bs | bainite starting temperature |

SEM | scanning electron microscopy |

TEM | transmission electron microscope |

B | bainite |

M | martensite |

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**Figure 3.**SEM micrographs of samples with different ausforming conditions: (

**a**) without deformation, 300 °C; (

**b**) 0.1, 300 °C; (

**c**) 0.2, 300 °C; (

**d**) 0.3, 300 °C; (

**e**) without deformation, 350 °C; (

**f**) 0.1, 350 °C; (

**g**) 0.2, 350 °C; (

**h**) 0.3, 350 °C.

**Figure 4.**TEM investigations of samples at different conditions, (

**a**,

**b**) nondeformed sample isothermal transformed at 300 °C; (

**c**,

**d**) deformed with a strain of 0.3 followed by isothermal transformation at 300 °C. Blocky and film-like austenites are shown in dark (left) and bright (right) images, respectively.

**Figure 5.**Normalized dilatation representing the amount of bainitic transformation during isothermal holding: (

**a**) nondeformed samples; deformed with a strain of (

**b**) 0.1; (

**c**) 0.2; and (

**d**) 0.3.

**Figure 6.**Bainite transformation kinetics in different samples measured using dilatometry: (

**a**) nondeformed samples; deformed with a strain of (

**b**) 0.1; (

**c**) 0.2; (

**d**) 0.3.

**Figure 7.**(

**a**) The free energy change $\Delta {G}^{\mathsf{\gamma}\to \mathsf{\alpha}}$ as a function of temperature; (

**b**) Calculated ${{T}^{\prime}}_{0}$ curve of the tested steel.

**Figure 9.**Effects of ausforming at 300 and 350 °C on kinetics of subsequent isothermal bainitic transformation.

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**MDPI and ACS Style**

Zou, H.; Hu, H.; Xu, G.; Xiong, Z.; Dai, F.
Combined Effects of Deformation and Undercooling on Isothermal Bainitic Transformation in an Fe-C-Mn-Si Alloy. *Metals* **2019**, *9*, 138.
https://doi.org/10.3390/met9020138

**AMA Style**

Zou H, Hu H, Xu G, Xiong Z, Dai F.
Combined Effects of Deformation and Undercooling on Isothermal Bainitic Transformation in an Fe-C-Mn-Si Alloy. *Metals*. 2019; 9(2):138.
https://doi.org/10.3390/met9020138

**Chicago/Turabian Style**

Zou, Hang, Haijiang Hu, Guang Xu, Ziliu Xiong, and Fangqin Dai.
2019. "Combined Effects of Deformation and Undercooling on Isothermal Bainitic Transformation in an Fe-C-Mn-Si Alloy" *Metals* 9, no. 2: 138.
https://doi.org/10.3390/met9020138