# Effect of Initial State and Deformation Conditions on the Hot Deformation Behavior of M50NiL Steel

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Methods

^{−1}. The exact experimental procedure is as follows. First, two specimens with different initial states were prepared by heating to 1100 °C for 5 min and to 1150 °C for 30 min, respectively, at a constant rate of 20 °C/s. Next, all specimens were cooled to compression temperature at 10 °C/s for 10 s, and then were compressed until reaching a total true strain value of 1 at strain rates of 0.01, 0.1, and 1 s

^{−1}; this was followed by immediate quenching in water. The stress–strain curves obtained during the hot deformation were automatically saved in a computer program. After completion of the hot deformation process, the deformed samples were cut along the longitudinal direction using an electric spark cutting machine. Subsequently, the surface of the samples was ground and polished. Thereafter, the samples were electrolytically etched in a supersaturated solution of oxalic acid to reveal the microstructure of the M50NiL steel. For microstructure characterization, the maximum deformation zone of the samples was observed using optical microscopy, scanning electron microscopy equipped with a backscattered electron (SEM–BSE) detector, and transmission electron microscopy (TEM).

## 3. Results and Discussions

#### 3.1. Flow Stress Behavior of M50NiL Steel

^{−1}are presented in Figure 4. According to the true stress–strain curves, both samples exhibited their peak stress value at the initial deformation stage, which then decreased or reached a steady-state with increasing strain.

#### 3.2. Deformation Energy of M50NiL Steel

^{−1}), σ is the flow stress (MPa), Q is the activation energy (J·mol

^{−1}), R is the universal gas constant (8.314 J·mol

^{−1}·K

^{−1}), and n, ${\mathrm{n}}^{\prime}$, A, ${\mathrm{A}}^{\prime}$, ${\mathrm{A}}^{\u2033}$, β, and α are the material constants. The value of α can be calculated according to ${\mathsf{\alpha}=\mathsf{\beta}/\mathrm{n}}^{\prime}$.

_{p}, was used to calculate the material constant, and then a logarithm was applied to both sides of Equation (1):

_{p}and the slope of ln$\dot{\mathsf{\epsilon}}$ versus σ

_{p}, respectively. The values of ${\mathrm{n}}^{\prime}$ and β were, respectively, determined as 0.054 and 7.27 for the coarse-grained specimens and 0.049 and 7.21 for the fine-grained specimens. Then, α was calculated as 0.007 for both types of specimens.

_{p}) versus 1/T, and n can be determined from the slope of ln $\dot{\mathsf{\epsilon}}$ versus lnsinh(ασ

_{p}). As seen in Figure 6 and Figure 7, the values of s and n were, respectively, determined as 11.3 and 5.25 for the coarse-grained specimens and 10.28 and 5.35 for the fine-grained specimens. Finally, the activation energy for the coarse- and fine-grained specimens was determined to be 493.85 KJ·mol

^{−1}and 457.16 KJ·mol

^{−1}, respectively. Moreover, the obtained activation energy for the coarse- and fine-grained samples was similar to the 481.4 KJ·mol

^{−1}obtained by Ji [16] for M50NiL steel.

#### 3.3. Critical Strain

_{c}= AZ

^{k}

#### 3.4. Evolution of Dynamic Recrystallization Under Different Hot Deformation Conditions

## 4. Conclusions

^{−1}on a Gleeble-3500 testing device. Based on the results and analysis, the following conclusions can be drawn:

- The true stress–strain curve can be considered a macroscopic reflection of the changes in the microstructure of a material. Under most deformation conditions, the flow stress of the fine-grained specimens was higher than that of the coarse-grained specimens, which is attributed to the pinning effect of the fine-grain dispersion on the grain boundary.
- The critical strain and deformation activation energy of the fine-grained samples were lower than those of the coarse-grained samples. It was proven that a microstructure with fine grains is beneficial to the dynamic recrystallization of M50NiL steel.
- The original microstructure with fine grains and dispersed phase is more favorable for the hot deformation of M50NiL steel. This is due to the fact that the dispersed phase can pin the grain boundaries and hinder the growth of recrystallized grains. Consequently, a more uniform and finer microstructure can be obtained after recrystallization.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Optical micrographs of the M50NiL steel samples with coarse and fine grains prior to deformation: (

**a**) coarse-grained sample (obtained by heating at 1150 °C for 30 min); (

**b**) fine-grained sample (obtained by heating at 1100 °C for 5 min).

**Figure 2.**Scanning electron microscopy equipped with a backscattered electron (SEM–BSE) of the M50NiL steel samples with coarse and fine grains prior to deformation: (

**a**) coarse-grained sample; (

**b**) fine-grained sample.

**Figure 3.**Energy dispersive X-ray spectroscopy (EDS) results of (

**a**) the dispersed phase and (

**b**) the matrix of M50NiL steel.

**Figure 4.**True stress–strain curves of the fine- and coarse-grained samples at different temperatures: (

**a**) 950 °C; (

**b**) 1000 °C; (

**c**) 1050 °C; (

**d**) 1100 °C.

**Figure 6.**Relationships between: (

**a**) $\mathrm{ln}\dot{\epsilon}$ and σ

_{p}; (

**b**) $\mathrm{ln}\dot{\epsilon}$ and lnσ

_{p}; (

**c**) $\mathrm{ln}\dot{\epsilon}$ and lnsinh (ασ

_{p}); (

**d**) lnsinh (ασ

_{p}) and 1000/T, for the fine-grained specimens.

**Figure 7.**Relationships between: (

**a**) $\mathrm{ln}\dot{\epsilon}$ and σ

_{p}; (

**b**) $\mathrm{ln}\dot{\epsilon}$ and lnσ

_{p}; (

**c**) $\mathrm{ln}\dot{\epsilon}$ and lnsinh(ασ

_{p}); (

**d**) lnsinh (ασ

_{p}) and 1000/T, for the coarse-grained specimens.

**Figure 8.**(

**a**) θ-σ curves and (

**b**) dθ/dσ-σ curves of the fine-grained specimens at a strain rate of 0.01 s

^{−1}for different deformation temperatures.

**Figure 9.**(

**a**) θ-σ curves and (

**b**) dθ/dσ-σ curves of the coarse-grained specimens at a strain rate of 0.01 s

^{−1}for different deformation temperatures.

**Figure 11.**Optical micrographs of the fine- and coarse-grained specimens at different deformation conditions: (

**a**,

**c**,

**e**) fine-grained samples; (

**b**,

**d**,

**f**) coarse-grained samples; (

**a**,

**b**) high Z; (

**c**,

**d**) intermediate Z; (

**e**,

**f**) low Z.

C | Si | Mn | P | S | Cr | Ni | V | Mo |
---|---|---|---|---|---|---|---|---|

0.13 | 0.19 | 0.29 | 0.005 | 0.002 | 4.14 | 3.4 | 1.24 | 4.07 |

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**MDPI and ACS Style**

Zhang, Y.; Yang, M.; Long, S.; Li, B.; Liang, Y.; Ma, S.
Effect of Initial State and Deformation Conditions on the Hot Deformation Behavior of M50NiL Steel. *Materials* **2020**, *13*, 5367.
https://doi.org/10.3390/ma13235367

**AMA Style**

Zhang Y, Yang M, Long S, Li B, Liang Y, Ma S.
Effect of Initial State and Deformation Conditions on the Hot Deformation Behavior of M50NiL Steel. *Materials*. 2020; 13(23):5367.
https://doi.org/10.3390/ma13235367

**Chicago/Turabian Style**

Zhang, Yan, Ming Yang, Shaolei Long, Bo Li, Yilong Liang, and Shaowei Ma.
2020. "Effect of Initial State and Deformation Conditions on the Hot Deformation Behavior of M50NiL Steel" *Materials* 13, no. 23: 5367.
https://doi.org/10.3390/ma13235367