**3. Results**

The thermal expansion curves of continuous heating up to 500 ◦C at the heating rate of 10 ◦C/min under different tensile stress is shown in Figure 1. It can be found that the relative dimensional change first increases with increasing temperature linearly. When the temperature exceeds 200 ◦C, the slopes between temperature and relative dimensional change begins to increase and then decline. With the temperature further increasing to 400 ◦C, the slopes no longer change. According to the theory of tempering, the specimens may have gone through Stage 0 and Stage I, thus the slopes remained unchanged when temperature was below 200 ◦C. However, it is known that Stage II leads to expansion and Stage III decreases the dimension. Therefore, the variation of slopes during 200~400 ◦C is closely related to the microstructural evolution in Stage II and Stage III.

**Figure 1.** The thermal expansion curves during the non-isothermal process under different tensile stress.

Based on the above thermal expansion curves (Figure 1), the starting and ending temperatures of slope change was obtained by the tangents method [25] as shown in Table 2. In fact, according to the theory of tempering, the starting temperatures of slope change can represent the beginning of retained austenite decomposition (Stage II), while the ending temperature of slope change can reflect the complete formation of cementite (Stage III). The results listed in Table 2 clearly show that the starting temperature of retained austenite decomposition increases from 238 ◦C to 276 ◦C when the applied tensile stress increases from 0 MPa to 40 MPa. This may be due to the increase of stability of retained austenite induced by applied mechanical stress. However, it can be further found that the ending temperature of cementite formation obviously decreases from 474 ◦C to 418 ◦C with the increase of tensile stress, which may indicate a premature formation of cementite.

**Table 2.** The starting and ending temperatures of slope change under different tensile stress.


Furthermore, the activation energy of microstructural evolution without stress and with 40 MPa tensile stress were obtained by DSC testing and thermo-mechanical simulator experiments, respectively. Figure 2a illustrates the representative DSC curves at heating rate of 5 ◦C/min, where it can be seen that the first run curve (solid line) contains a significant heat flow peak in the range of 200~400 ◦C. Meanwhile, the rerun curve (dash line) had no visible peaks, which was quite necessary to act as a baseline to remove the influence of experimental instruments and environment.

By subtracting the baseline and dividing by the mass of the specimens, the revised DSC curve was obtained as shown in Figure 2b. It is obvious that no heat flow peak can be observed when the temperature is less than 200 ◦C, indicating that QT-treated specimens have gone through Stage 0 and Stage I. In the subsequent non-isothermal process, two heat flow peaks appeared in the range of 200~260 ◦C and 300~400 ◦C, which corresponds to Stage II and Stage III, respectively. Furthermore, by fitting the revised DSC curve, the curves of retained austenite decomposition (Stage II) and carbide transformation (Stage III) were both obtained, as shown in Figure 2b.

**Figure 2.** (**a**) The differential scanning calorimetry (DSC) curves at heating rate of 5 ◦C/min; (**b**) revised DSC curves after subtracting baseline.

Accordingly, the peak temperatures of Stage II and Stage III at different heating rates were listed in Table 3. As the heating rate increases, their peak temperatures gradually shift higher, indicating that higher heating rate will delay the transformation of Stage II and Stage III. According to the Kissinger method, the Kissinger straight lines of two stages can be obtained respectively, as shown in Figure 3. As a result, the activation energy of retained austenite decomposition was calculated to be 109.4 kJ/mol, and the activation energy of cementite precipitation was 179.4 kJ/mol, which is consistent with the report in literature [7].

**Table 3.** The peak temperatures of Stage II and Stage III without applied stress at different heating rates.


**Figure 3.** Kissinger analysis (ln*T*2*P*/∅ versus 1/*TP* × 10−<sup>3</sup> ) for the determination of the individual activation energy of Stage II (**a**) and Stage III (**b**) without applied stress.

Figure 4a indicates the thermal expansion curves under 40 MPa tensile stress at different heating rate. As the most common method for analyzing the thermal expansion curves, the leverage theorem was used in this study and the transformed fraction curves were shown in Figure 4b. It can be observed that higher heating rate will result in an increase of the temperature of phase transformation, which is well consistent with the previous DSC results. In addition, the median temperatures (transformed

fraction = 0.5) were listed in Table 4 as well, which was selected to calculate the activation energy by means of the Kissinger method. Consequently, the activation energy obtained by leverage law was 102.1 kJ/mol. However, considering that the transformed fraction curves calculated by the leverage law could not distinguish Stage II and Stage III, a follow-up calculation was thus performed using the isoconversional method.

**Figure 4.** The thermal expansion curves (**a**) and transformed fraction curves (**b**) under 40 MPa tensile stress at a heating rate of 10, 15, 20 ◦C/min.

**Table 4.** The median temperature under 40 MPa at different heating rates.


The isoconversional method can effectively reflect the change of activation energy during the whole process, which can be considered as a more accurate supplement for the Kissinger method. A detailed activation energy and modified pre-exponential factor (ln[*<sup>A</sup>*· *f*(α)]) with respect to different transformed fraction were estimated using differential isoconversional method, as illustrated in Figure 5. It can be found that the activation energy first increases to 121.5 kJ/mol and then decreases to 72.8 kJ/mol with the increasing transformed fraction. During the early stage (0.2 ≤ α ≤ 0.4) and later stage (0.6 ≤ α ≤ 0.7), there are two platforms whose average activation energy are 121.5 kJ/mol and 94.7 kJ/mol, respectively. In fact, the platforms can be interpreted as different stages as reported by Wang et al. [26]. In the present work, according to the reaction sequence of Stage II and Stage III and kinetics analysis by the Kissinger method, the two platforms can be interpreted as the two stages (retained austenite decomposition and cementite precipitation). Meanwhile, the average value of these two stages was 108.1 kJ/mol, which was close to the activation energy obtained by Kissinger method (102.1 kJ/mol) as before. Compared with the activation energy without stress, it was found that the activation energies of Stage II and Stage III changed significantly. Under the applied tensile stress, the activation energy of the decomposition of retained austenite slightly increase to 121.5 kJ/mol, while the cementite precipitation was significantly accelerated due to the decreased activation energy of cementite precipitation (from 179.4 kJ/mol to 94.7 kJ/mol). Moreover, the results show that the variation of modified pre-exponential factor (ln[*<sup>A</sup>*· *f*(α)]) presented similar trends to those of the activation energies.

**Figure 5.** The activation energy and modified pre-exponential factor with respect to the transformed fraction.

To observe the microstructural evolution during the non-isothermal process, the QT treated specimens were heated to 300 ◦C and then air cooled to ambient temperature for microstructural analysis. Figure 6a shows the FESEM microstructure of the QT treated specimens which comprises undissolved spherical cementite, retained austenite and tempered martensite. Figure 6b–d illustrates the microstructure of the specimens after heating to 300 ◦C with different tensile stress, where it can be seen the matrix is mainly consisted of undissolved spherical carbides and precipitated needle-like cementite. In addition, the region highlighted by the yellow oval is poor of the precipitated cementite, indicating that there is still a large amount of cementite not formed for the specimens without tensile stress. However, more cementite has formed for the specimens with increasing tensile stress, which thus proves that the applied tensile stress can accelerate the formation of cementite efficiently.

**Figure 6.** Typical field-emission scanning electron microscope (FESEM) microstructure of the (**a**) quenching and tempering (QT) treated specimens, and the QT treated specimens after heating to 300 ◦C under tensile stress of (**b**) 0 MPa, (**c**) 20 MPa and (**d**) 40 MPa.

Figure 7 illustrates the TEM micrographs and corresponding dark field of the microstructure of the specimen with and without 40 MPa stress, in which typical morphologies of carbides are observed. As shown in Figure 7a, some nano-size particles were found for the specimens without stress. These particles were identified using selected-area electron diffraction (SAED) to be ε-carbides. For the specimens with 40 MPa stress (Figure 7b), the needle-like precipitates presented in the matrix were identified to be θ-carbides (cementite), which is consistent with the SEM results. It can be inferred that a large number of ε-carbides still exist in the specimens without applied stress, and they have not been transformed into stable cementite at 300 ◦C. However, numerous stable θ-carbides (cementite) have formed for the specimens with 40 MPa stress, demonstrating again that the applied stress can accelerate the formation of cementite during the non-isothermal process.

**Figure 7.** The presence of ε-carbides and θ-carbides for the specimens (**a**) without stress and (**c**) with 40 MPa, respectively. Where (**b**,**d**) are the corresponding dark field of (**<sup>a</sup>**,**<sup>c</sup>**), respectively.

The results of the XRD diffraction pattern (Figure 8) clearly show that the retained austenite has been completely decomposed after heating to 300 ◦C, regardless of the applied tensile stress. In addition, according to the observation of the (110)α diffraction peak (as seen in the insert), it can be seen that the martensite diffraction peak shifts to a higher angle with the increase of applied tensile stress. It has been reported that the diffraction peak information of martensite is closely related to the carbon content in martensite [27]. In the present work, the higher diffraction angle of (110) martensite indicates the lower carbon content of tempered martensite for the specimens subjected to tensile stress [28]. Therefore, it can be inferred that interstitial carbon atoms in martensite diffuse more and participate in the formation of cementite under the applied tensile stress.

**Figure 8.** The X-ray diffraction (XRD) pattern of the specimens after being heated to 300 ◦C under different tensile stress.
