# Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

_{c1}and A

_{c3}temperatures were measured at 668 and 825 °C, respectively, and the microstructure was found to consist of 40 vol % austenite and 60 vol % ferrite using X-ray diffraction. The tensile properties generated following the primary soaking heat treatment are shown in Figure 2. The primary soaked material was characterized as having an ultimate tensile strength of 1038 MPa combined with a total elongation of 41%. The material underwent localized deformation upon yielding, with the yield point drop and yield point elongation characteristic of many medium manganese steels. This was followed by a region of moderate work hardening and limited post-uniform elongation.

^{−4}s

^{−1}in a method consistent with ASTM E8 [17]. Applied load and crosshead displacement were recorded at a rate of 10 Hz. Sample deformation was monitored using 2D digital image correlation (DIC), a technique which relies on the computer vision approach to extract whole-field displacement data [18]. Images were captured using a CCD camera with a resolution of 2448 × 2048 pixels and a lens with a 75 mm focal length. The physical resolution was 22.5 μm. Images were taken at a frequency of 5 Hz. Before testing, a random pattern was printed onto each specimen surface by first spraying white paint as a background and then black paint to generate random speckles. Using the displacement vectors reported by the VIC 3D software developed by Correlated Solutions (Irmo, SC, USA) and a virtual 25.4 mm (1 in) extensometer, the macroscopic strain for each sample was characterized [19].

^{−4}s

^{−1}, and diffraction patterns were recorded while the sample was held at a constant displacement for approximately 1800 s [17]. The reported stress values were taken as the final value measured during each diffraction hold, and the magnitude of stress relaxation measured during each diffraction hold varied between 20 and 60 MPa following macroscopic yielding for both heat treatment conditions. Sample deformation was monitored using 2D DIC during the in situ neutron diffraction. Due to the increased length of the interrupted neutron diffraction tests, the sampling interval was reduced to 0.0034 Hz. Using the displacement vectors reported by the VIC 3D software developed by Correlated Solutions and a virtual 25.4 mm (1 in) extensometer, the macroscopic strain for each sample was determined [19].

## 3. Results

## 4. Discussion

#### 4.1. Role of Martensite in Yielding Behavior

#### 4.2. Role of Martensite during Plastic Deformation

## 5. Conclusions

- The inclusion of large volume fractions of as-quenched athermal martensite in the microstructure of medium manganese steels promotes continuous yielding in both the FCC and BCC/BCT phases and in the overall macroscopic tensile curve. Following tempering, discontinuous yielding was found to occur in both the BCC/BCT phase and in the overall macroscopic tensile curve. The shift from continuous to discontinuous yielding is believed to be linked to a reduction in mobile dislocation density in the BCC/BCT phase as a result of the tempering heat treatment.
- The work hardening rate of a medium-manganese steel is highly dependent upon the properties of the matrix. An initial matrix composed primarily of as-quenched athermal martensite is effective in producing a sustained high rate of work hardening, primarily due to work hardening in the martensitic phase and the dynamic strain-induced transformation of austenite to martensite.
- The substitution of ferrite with martensite (as-quenched or tempered) in the initial microstructure of a medium manganese steel improves the strength–ductility product of the material.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Schematic of the double soaking and double soaking plus tempering heat treatment. Expected changes in microstructure are indicated: Ferrite (α), primary austenite (${\gamma}_{P}$), secondary austenite (${\gamma}_{s}$), martensite (α’), and tempered martensite (${\alpha}_{T}^{\u2019}$).

**Figure 2.**Room temperature, quasi-staticengineering stress-strain curve for the industrially batch annealed (primary soaked) material.

**Figure 3.**Room temperature engineering and true stress-strain curves for the (

**a**,

**b**) double soaked (DS) and (

**c**,

**d**) double soaked and tempered (DS-T) samples tested in both quasi-static and in situ neutron diffraction testing conditions along with the instantaneous work hardening rate, $\mathrm{ln}(d\sigma /d\epsilon )$.

**Figure 4.**Representative in situ neutron diffraction data during uniaxial deformation for the (

**a**) double soaked and (

**b**) double soaked plus tempered heat treatment conditions. True stresses at each diffraction condition are indicated.

**Figure 5.**Austenite volume fractions as a function of macroscopic true stress for the (

**a**) double soaked (DS) and (

**b**) double soaked and tempered (DS-T) heat treatment conditions.

**Figure 6.**Elastic lattice strain as a function of macroscopic true strain for the $\left\{211\right\}$ orientation of the BCC phases ($\alpha /{\alpha}^{\prime}$) and $\left\{311\right\}$ orientation of austenite for the (

**a**,

**b**) DS and (

**c**,

**d**) DS-T heat treatment conditions.

Heat Treatment | γ | α’ | α | |
---|---|---|---|---|

Sample ID | Heat Treatment Parameters | vol % | vol % | vol % |

DS | 800 °C, 30 s | 28 | $60\text{}\left({\alpha}_{F}^{\u2019}\right)$ | 12 |

DS-T | 800 °C, 30 s + 450 °C, 300 s | 29 | $60\text{}\left({\alpha}_{T}^{\u2019}\right)$ | 11 |

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**MDPI and ACS Style**

Glover, A.; Gibbs, P.J.; Liu, C.; Brown, D.W.; Clausen, B.; Speer, J.G.; De Moor, E.
Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength. *Metals* **2019**, *9*, 761.
https://doi.org/10.3390/met9070761

**AMA Style**

Glover A, Gibbs PJ, Liu C, Brown DW, Clausen B, Speer JG, De Moor E.
Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength. *Metals*. 2019; 9(7):761.
https://doi.org/10.3390/met9070761

**Chicago/Turabian Style**

Glover, Alexandra, Paul J. Gibbs, Cheng Liu, Donald W. Brown, Bjørn Clausen, John G. Speer, and Emmanuel De Moor.
2019. "Deformation Behavior of a Double Soaked Medium Manganese Steel with Varied Martensite Strength" *Metals* 9, no. 7: 761.
https://doi.org/10.3390/met9070761