# Observations on the Relationship between Crystal Orientation and the Level of Auto-Tempering in an As-Quenched Martensitic Steel

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## Abstract

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## 1. Introduction

_{s}). In low-carbon steels, martensite has a lath morphology [2,3,4]. The carbon atoms that are fully soluble in austenite prior to the transformation to martensite become supersaturated in the bcc matrix after the formation of martensite. As the solubility of C in ferrite in equilibrium with cementite is only 0.00022 wt.% at 400 °C, for example, there is a large driving force for the precipitation of cementite following the formation of a martensite lath. The mobility of C in ferrite at the temperatures at which martensite typically forms in low-C steels is so high that even at cooling rates of 1000 °C/s there is time for cementite to nucleate and grow in the martensite laths that form at temperatures close to the M

_{s}temperature [5]. This phenomenon is known as auto-tempering as the resultant microstructure resembles that of tempered martensite. The auto-tempering in martensite occurs when the M

_{s}temperature is sufficiently high, as is often the case in low-carbon steels [2,3]. When compared to steels without auto-tempering, auto-tempered steels exhibit superior toughness [6] and improved formability [7].

_{s}temperature. This is due to the fact that the carbon in supersaturated solution after the transformation of such regions can diffuse the greatest distance during the quench [5]. The low-temperature impact and fracture toughness of ferritic steels depends on the orientation of the {100} cleavage planes with respect to the principal stress below the impact test specimen notch or fracture toughness specimen fatigue crack. In the case of lath martensitic steels, the coherence length on {100} planes that are almost normal to the principal stress determines the cleavage crack length, which affects the toughness [14]. Therefore, a detailed study of the crystal orientations of the different parts of the martensitic structure is of interest.

## 2. Materials and Methods

## 3. Results

#### 3.1. Martensite Morphology

**Coarse auto-tempered regions**. The coarse regions were generally wedged shape and had a high density of auto-tempered precipitates within them, as shown in Figure 2. The coarse laths could simply be thin laths intersected by the specimen surface at a small angle. But Morsdorf et al. [20] showed that the coarse laths are clearly thicker than the surrounding thin laths using 3D sectioning techniques. As stated by Morsdorf et al. [20], the austenite matrix is soft and defect density is low just above the M_{s}temperature. Therefore, just below the M_{s}temperature, there is a low resistance to the formation of the coarse martensitic regions. The degree of auto-tempering is higher within these regions as, during quenching, they form at the highest temperatures where atomic mobility is high and beneficial for the nucleation and growth of cementite. The matrix of the coarse auto-tempered regions was the most highly etched of all the features within the microstructure, as shown by the imaged carbides in Figure 2.**Ridge-like regions**. These features appear as narrow raised bands, i.e., ridges, as shown in Figure 3. They tended to surround the coarse martensite features. After the formation of the coarse auto-tempered martensitic regions, the falling temperature and increasing dislocation density of the austenite resulting from the plastic accommodation of the martensite already formed leads to progressive hardening of the untransformed austenite [20]. It follows that the martensite in the ridge-like regions has had to grow into ever stronger austenite. The ridge-like regions consist of clusters of fine martensite laths, which etch to different degrees thereby creating the ridge appearance. It has been shown ferrite with a higher concentration of carbon in solid solution etches less in nital as does carbon supersaturated retained austenite at the lath boundaries [21]. Within the ridge-like regions, carbides were either absent or smaller in size than the carbides found in the coarse auto-tempered regions.**Untempered regions**. These are unetched featureless regions that with no carbides visible were presumably the last regions to transform into martensite, see Figure 4. As the transformation of these regions takes place at low temperatures, near the martensite finish temperature, it is reasonable to postulate that they have high levels of carbon remaining in solid solution leading to very little etching in nital and the resulting plateau-like topography regions, contrasting from the ridge-like and coarse regions.

#### 3.2. Orientations of the Martensitic Regions

#### 3.3. Martensite Orientation Variants

^{®}running the MTEX [18] crystallography toolbox. Note that while importing to MTEX, the orientations were rotated into alignment with the specimen coordinate system, following the default import settings.

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Scanning Electron Microscopy (SEM) secondary electron (SE) micrograph of the as-quenched steel showing lath martensite and carbides formed as a result of auto-tempering. The rolling direction (RD) is normal to the etched surface.

**Figure 5.**(

**a**) Electron backscatter diffraction (EBSD) inverse pole figure (IPF) map of a large grain (Equivalent circle diameter (ECD): 289 μm) in the as-quenched lath martensite. The colouring represents the crystallographic orientation of the plate normal direction; (

**b**) EBSD and corresponding SEM images of the region indicated by the white box in Figure 5a; (

**c**) SEM image of the red IPF shaded regions marked (I) and (II) and orange IPF shaded regions marked (III) and (IV) as seen in 5b) where it can be seen that red shaded regions are relatively untempered compared to the slight tempering in the orange IPF shaded regions.

**Figure 6.**(

**a**) EBSD IPF and SEM SE image of small grains in another region of the sample shown in Figure 5; (

**b**) Higher magnification EBSD IPF and SE images showing that the IPF colors red and brown-red correspond to untempered and slightly tempered regions.

**Figure 7.**EBSD IPF images of small grains showing the crystallographic orientations of (

**a**) the plate normal direction (ND) and (

**b**) the rolling direction (RD) which is the same as the etched sample normal direction.

**Figure 10.**EBSD band contrast map overlaid with variant coloring following adjacent variant color key). The variants have been indexed following the scheme proposed by Morito et al. [10]. Table 2 shows the indexing scheme. The white boundaries in the image enclose the untempered areas. Black boundaries surround prior austenite grains.

**Table 1.**The mean chemical composition of the studied low-carbon steel (wt.%) [16].

C | Si | Mn | Cr | Ni | Ti | V | Al |
---|---|---|---|---|---|---|---|

0.126 | 0.72 | 1.66 | 0.27 | 0.038 | 0.027 | 0.047 | 0.054 |

**Table 2.**24 variants in martensite generated using the mean representative orientation relationship determined during the reconstruction procedure, as defined by Morito et al. [10].

Variant | Plane Parallel | Direction Parallel | Rotation from Variant 1 | |
---|---|---|---|---|

No. | $\left[\gamma \right]\text{}\Vert \text{}\left[\alpha \prime \right]$ | Axis | Angle [deg.] | |

V1 | $\left(111\right)\mathsf{\gamma}\text{}\Vert \text{}\left(011\right)\mathsf{\alpha}\prime $ | $\left[\overline{1}01\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | - | 0 |

V2 | $\left[\overline{1}01\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.5190 0.5459 0.6577 | 60.2703 | |

V3 | $\left[01\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.5131 0.4719 0.7170 | 59.9978 | |

V4 | $\left[01\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.5285 −0.0000 0.8489 | 4.7482 | |

V5 | $\left[1\overline{1}0\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.4719 0.5131 0.7170 | 59.9978 | |

V6 | $\left[1\overline{1}0\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.7065 0.0408 0.7065 | 55.4056 | |

V7 | $\left(1\overline{1}1\right)\mathsf{\gamma}\text{}\text{}\Vert \text{}\left(011\right)\mathsf{\alpha}\prime $ | $\left[10\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.6098 0.5062 0.6098 | 51.6135 |

V8 | $\left[10\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.6979 0.1607 0.6979 | 9.6813 | |

V9 | $\left[\overline{1}\overline{1}0\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.6625 0.2135 0.7179 | 52.7289 | |

V10 | $\left[\overline{1}\overline{1}0\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.4489 0.5800 0.6798 | 51.3062 | |

V11 | $\left[011\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.5135 0.0557 0.8563 | 12.7631 | |

V12 | $\left[011\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.6610 0.1906 0.7258 | 57.3000 | |

V13 | $\left(\overline{1}11\right)\text{}\Vert \text{}\mathsf{\gamma}\text{}\left(011\right)\mathsf{\alpha}\prime $ | $\left[0\overline{1}1\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.0557 0.5135 0.8563 | 12.7631 |

V14 | $\left[0\overline{1}1\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.5800 0.4489 0.6798 | 51.3062 | |

V15 | $\left[\overline{1}0\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.2437 0.6713 0.7000 | 56.4210 | |

V16 | $\left[\overline{1}0\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.6953 0.1819 0.6953 | 15.5492 | |

V17 | $\left[110\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.6508 0.3912 0.6508 | 51.1570 | |

V18 | $\left[110\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.2734 0.6585 0.7011 | 51.9818 | |

V19 | $\left(11\overline{1}\right)\mathsf{\gamma}\text{}\text{}\Vert \text{}\left(011\right)\mathsf{\alpha}\prime $ | $\left[\overline{1}10\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.2135 0.6625 0.7179 | 52.7289 |

V20 | $\left[\overline{1}10\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.1906 0.6610 0.7258 | 57.3000 | |

V21 | $\left[0\overline{1}\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.1058 0.0000 0.9944 | 17.6963 | |

V22 | $\left[0\overline{1}\overline{1}\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.6585 0.2734 0.7011 | 51.9818 | |

V23 | $\left[101\right]\text{}\Vert \text{}\left[\overline{1}\overline{1}1\right]$ | −0.6713 0.2437 0.7000 | 56.4210 | |

V24 | $\left[101\right]\text{}\Vert \text{}\left[\overline{1}1\overline{1}\right]$ | −0.2427 −0.0000 0.9701 | 18.0592 |

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**MDPI and ACS Style**

Ramesh Babu, S.; Nyyssönen, T.; Jaskari, M.; Järvenpää, A.; Davis, T.P.; Pallaspuro, S.; Kömi, J.; Porter, D. Observations on the Relationship between Crystal Orientation and the Level of Auto-Tempering in an As-Quenched Martensitic Steel. *Metals* **2019**, *9*, 1255.
https://doi.org/10.3390/met9121255

**AMA Style**

Ramesh Babu S, Nyyssönen T, Jaskari M, Järvenpää A, Davis TP, Pallaspuro S, Kömi J, Porter D. Observations on the Relationship between Crystal Orientation and the Level of Auto-Tempering in an As-Quenched Martensitic Steel. *Metals*. 2019; 9(12):1255.
https://doi.org/10.3390/met9121255

**Chicago/Turabian Style**

Ramesh Babu, Shashank, Tuomo Nyyssönen, Matias Jaskari, Antti Järvenpää, Thomas Paul Davis, Sakari Pallaspuro, Jukka Kömi, and David Porter. 2019. "Observations on the Relationship between Crystal Orientation and the Level of Auto-Tempering in an As-Quenched Martensitic Steel" *Metals* 9, no. 12: 1255.
https://doi.org/10.3390/met9121255