Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?
Abstract
:1. Introduction
2. Materials and Methods
2.1. Material
2.2. Fracture Mechanics Specimens
2.3. Quasi-Static Fracture Toughness Testing
2.4. Microscopy
2.4.1. Basic Characterization
2.4.2 Fracture Surface and Crack Propagation
3. Results
3.1. Basic Characterization of The Microstructure
3.2. Fracture Toughness Tests
3.3. Primary Fracture Surfaces
3.4. Secondary Fracture Surfaces
3.5. EBSD Misorientation Analysis
4. Discussion
4.1. General Factors Affecting Secondary Cracking
- Particle anisotropy: Particles are arranged in the direction of the rolling/extrusion and hence facilitate crack propagation by cleavage or by void growth and coalescence depending on the test temperature.
- Crystallographic anisotropy: Due to the <110> texture induced in the direction of the rolling/extrusion, the shear modulus with respect to the longitudinal direction is different from the isotropic transverse direction.
- Grain morphology anisotropy: Coarse grains are elongated in the direction of the rolling/extrusion which results in crack blunting when the propagating crack is obstructed by the perpendicularly elongated coarse grains.
4.2. Understanding of Secondary Cracking in Hot-Rolled ODS Steels
4.2.1. Effect of Crystallographic Texture
4.2.2. Effect of Grain Morphology
4.2.3. Fracture Mechanisms
4.3. Understanding of Secondary Cracking in Hot-Extruded ODS Steels
4.4. Comparison
5. Conclusions
- At temperatures below the DBTT, the occurrence of secondary cracks in hot-rolled materials is assisted by the {100} cleavage planes which are aligned parallel to the rolling plane. In hot-extruded materials, these {100} cleavage planes are distributed uniformly around the extrusion axis, therefore not assisting secondary cracking.
- At temperatures higher than the DBTT, secondary cracking is more frequent in hot-rolled than in hot-extruded materials as the critical fracture stresses in secondary crack planes are lower due to the two dimensionally anisotropic elongated ‘pan-cake’ shaped grains which allows free crack propagation in two directions. In hot-extruded materials, the one dimensional ‘cigar’ like grain morphology with grains elongated in the direction of extrusion only allows free crack propagation in one direction and hence does not assist secondary cracking as much.
- Sub-micron particle arrangement in two directions assists secondary cracking more in hot-rolled material than one dimensional sub-micron particle arrangement in hot-extruded material.
- The constraint induced stress in hot-rolled materials is higher than in hot-extruded materials which also assists a higher degree of secondary cracking.
- In ODS-KIT HR at RT, secondary cracking arise predominantly through transgranular cleavage and intergranular fracture possibly due to segregations at the grain boundaries. Between RT and 600 °C, secondary cracking occurs through transgranular ductile fracture and at or above 600 °C, secondary cracking occurs through intergranular fracture due to the weakening of the grain boundaries.
Supplementary Materials
Supplementary File 1Author Contributions
Acknowledgments
Conflicts of Interest
References
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Material | Plane | Mean Grain Size (µm) | GAR | Area Fraction (%) |
---|---|---|---|---|
HR | TS (0.3–3 µm) | 1 | 2 | 20.4 |
HR | LT (1.4–10 µm) | 3 | 1.6 | 27.3 |
HR | LS (0.4–3 µm) | 1.1 | 2 | 17.6 |
HE | T (0.2–3 µm) | 0.78 | 1.7 | 68.9 |
HE | L (0.2–3 µm) | 0.9 | 1.7 | 39.4 |
Material | Plane | Mean Grain Size (µm) | GAR | Area Fraction (%) |
---|---|---|---|---|
HR | TS (3–13 µm) | 6 | 4 | 79.6 |
HR | LT (10–91 µm) | 21 | 3.3 | 72.7 |
HR | LS (3–16 µm) | 6 | 10 | 82.4 |
HE | T (3–21 µm) | 4.7 | 2.2 | 31.1 |
HE | L (3–34 µm) | 7.7 | 10 | 60.6 |
Temperature °C | HE JQ (L-C) kJ/m2 | HE JQ (C-R) kJ/m2 | HE JQ (C-L) kJ/m2 | HR JQ (L-T) kJ/m2 | HR JQ (T-L) kJ/m2 |
---|---|---|---|---|---|
−100 | - | - | - | 49.2 | 45 |
22 | 486.27 | 97.26U | 44.07U | 78.2 | 28.4 |
100 | - | - | - | 45.6 | 15.8 |
200 | 488.52 | 146.17 | 107.54 | 43.4 | 19U |
400 | 300.73 | 95.6 | 78.06 | 50.1 | 9.9U |
600 | 19.69 | 4.04 | 0.95 | 17.1 | 6.5 |
700 | - | - | - | 12.9 | 5.8 |
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Das, A.; Viehrig, H.-W.; Altstadt, E.; Bergner, F.; Hoffmann, J. Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels? Crystals 2018, 8, 306. https://doi.org/10.3390/cryst8080306
Das A, Viehrig H-W, Altstadt E, Bergner F, Hoffmann J. Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels? Crystals. 2018; 8(8):306. https://doi.org/10.3390/cryst8080306
Chicago/Turabian StyleDas, Aniruddh, Hans-Werner Viehrig, Eberhard Altstadt, Frank Bergner, and Jan Hoffmann. 2018. "Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?" Crystals 8, no. 8: 306. https://doi.org/10.3390/cryst8080306