# On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling

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## Abstract

**:**

## 1. Introduction

- Evolution of the microstructure dependent on the deep rolling temperature affected by the different activated elementary deformation mechanisms upon surface treatment.
- Development of residual stresses dependent on the deep rolling temperature.
- Improvement of the cyclic deformation behavior imposed by microstructure and residual stress state.

## 2. Materials and Methods

^{®}H-Mn LY) provided by thyssenkrupp Hohenlimburg GmbH (Hagen, Germany). The chemical composition is given in Table 1. The electron backscatter diffraction (EBSD) micrograph shown in Figure 1 reveals the presence of recrystallization twins in the homogenous as-received microstructure being characterized by an average grain size of about 20 $\mathsf{\mu}$$\mathrm{m}$. The material was provided as hot-rolled blank with a thickness of 9 $\mathrm{m}$$\mathrm{m}$. Cylindrical specimens were machined by turning. Before, cuboid specimens of about 9 $\mathrm{m}$$\mathrm{m}$ × 121 $\mathrm{m}$$\mathrm{m}$ × sheet thickness were obtained by water jet cutting with the longitudinal axis of all specimens being perpendicular to the rolling direction of the sheet. The final specimen geometry is shown in Figure 2. Tensile testing of the untreated condition at room temperature revealed a relatively low yield strength of about 400 MPa, an ultimate tensile strength of 880 MPa and an elongation to fracture of 52% (see Figure 3). The mechanical properties are similar to those determined by Rüsing [34], who also performed tensile tests at elevated and cryogenic temperatures, showing that the material is characterized by higher tensile strength at cryogenic temperature, and an increase of the yield strength by a factor of two. The average hardness of the as-received material was found to be 250 HV.

## 3. Results and Discussion

#### 3.1. Characterization of the Near Surface Properties

#### 3.2. Fatigue Tests

## 4. Conclusions

- Deep rolling of the TWIP steel improves the monotonic mechanical properties, e.g., the yield strength (from 400 MPa to 550 MPa), without having any negative effect on the elongation at fracture. Furthermore, high compressive residual stresses with a maximum of 800 MPa are generated in the near surface area accompanied by high hardness values up to 475 HV0.1.
- The martensitic phase transformation promoting $\u03f5$–martensite in the near surface area induced by cryogenic deep rolling has a negative impact on the fatigue performance at least at relatively high loading amplitudes. In addition, an unpredictable behavior caused by random premature crack initiation is found at an intermediate stress level. Reasons are thought to be linked to increased internal friction going along with increased plastic deformation during the fatigue tests and/or notch effects in the two-phase region established in the near surface area.
- Superior performance is seen for the TWIP steel deep rolled at elevated temperature. However, in light of findings being present in literature for alternative steel grades, pathways towards further property optimization for the TWIP steel considered could be derived.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Inverse Pole Figure (IPF) map of the as-received condition showing a homogenous microstructure with recrystallization twins and an average grain size of about 20$\mathsf{\mu}$$\mathrm{m}$.

**Figure 3.**Stress–strain diagram for the as-received twinning-induced plasticity (TWIP) steel tested at room temperature. The inset summarizes the most important characteristic values.

**Figure 4.**Residual stress depth profiles in longitudinal direction (

**a**) and near-surface hardness distribution (

**b**) after deep rolling at room temperature applying deep rolling forces between 375 $\mathrm{N}$ and 855 $\mathrm{N}$.

**Figure 5.**Monotonic stress–strain response of the as-received material compared to a specimen deep rolled at room temperature using a deep rolling force of 855 $\mathrm{N}$ (

**a**) and direct comparison of the conditions deep rolled at different temperatures applying the same deep rolling force (

**b**).

**Figure 6.**IPF maps (

**a**–

**c**) and phase map with superimposed image quality (IQ) (

**d**) for differently treated specimens. IPF maps depict the near surface microstructure following deep rolling at room temperature (

**a**), 200 ${}^{\circ}\mathrm{C}$ (

**b**) and cryogenic temperature (

**c**) using a deep rolling force of 855 $\mathrm{N}$. The IQ-phase map for a surface layer after deep rolling at cryogenic temperature is shown in (

**d**). What appears to be twinning in the IPF map clearly is resolved as a $\gamma \to \u03f5$ phase transformation in (

**d**). Color coding in (

**a**–

**d**) is in accordance to the standard triangles and the nomenclature shown to the right.

**Figure 7.**Hardness distribution in the near surface area after deep rolling in a temperature range from $-196{}^{\circ}\mathrm{C}$ (Cryo) to $200{}^{\circ}\mathrm{C}$ at a constant deep rolling force of 855 $\mathrm{N}$.

**Figure 8.**Residual stress profiles and integral width values in longitudinal (

**a**+

**c**) and circumferential (

**b**+

**d**) direction plotted as a function of deep rolling temperature at constant deep rolling force.

**Figure 9.**XRD phase analysis using $CrK\alpha $-radiation revealing a significant martensitic phase fraction upon deep rolling at cryogenic temperature. Peak profiles are plotted as a function of distance from the surface.

**Figure 10.**XRD peaks used for determination of residual stresses in a depth of $0.125$ $\mathrm{m}$$\mathrm{m}$ (

**a**) and $0.500$ $\mathrm{m}$$\mathrm{m}$ (

**b**), respectively, plotted for $\Psi $-angles ranging from $-45$${}^{\circ}$ to 45${}^{\circ}$.

**Figure 11.**Woehler-type S-N plots for conditions deep rolled at high, room and cryogenic temperature applying a constant deep rolling force of 855 $\mathrm{N}$. See text for details.

**Figure 12.**Fracture surfaces of specimen deep rolled at room temperature and cryogenic temperature fatigued at a cyclic loading amplitude of 480 MPa. Overview images of conditions deep rolled at room (

**a**) and cryogenic (

**b**) temperature using a deep rolling force of 855 $\mathrm{N}$ are depicted with encircled crack initiation spots. High resolution micrographs depicting the microstructural features being responsible for crack initiation are shown in (

**c**+

**d**).

**Figure 13.**Fracture surfaces of specimens deep rolled at cryogenic temperature after fatigue tests applying a cyclic loading amplitude of 460 MPa. The overview SEM micrographs depict the respective crack initiation points. The specimens are characterized by fundamentally different fatigue lives, i.e., 500,000 cycles (

**a**) and 25,000 cycles (

**b**).

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

Wt.% | Wt.% | Wt.% | Wt.% | Wt.% | Wt.% |

0.45 | 0.40 | 20.00 | 0.03 | 0.005 | 2.50 |

Cr | Cu | V | Mo | Ti | Ni |

Wt.% | Wt.% | Wt.% | Wt.% | Wt.% | Wt.% |

2.50 | 0.20 | 0.20 | 0.20 | 0.20 | 1.00 |

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**MDPI and ACS Style**

Oevermann, T.; Wegener, T.; Niendorf, T.
On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling. *Metals* **2019**, *9*, 825.
https://doi.org/10.3390/met9080825

**AMA Style**

Oevermann T, Wegener T, Niendorf T.
On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling. *Metals*. 2019; 9(8):825.
https://doi.org/10.3390/met9080825

**Chicago/Turabian Style**

Oevermann, Torben, Thomas Wegener, and Thomas Niendorf.
2019. "On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling" *Metals* 9, no. 8: 825.
https://doi.org/10.3390/met9080825