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Metals 2017, 7(11), 466; https://doi.org/10.3390/met7110466
2. Experimental Procedure
2.2. Heat Treatment and Tensile Testing
2.3. Pre-Cracking of CT Specimens
2.4. External Hydrogen Charging
2.5. Hydrogen Concentration Analysis
2.6. Fatigue Testing
2.7. Fracture Toughness Test
3. Results and Discussion
3.2. Mechanical Properties
3.3. Hydrogen Concentration
3.4. Influence of Austempering on the Crack Growth Behavior of Uncharged 4140 Steel
- First, the austempering process has increased the fracture toughness of the material due to the presence of a microstructure containing a large amount of lower bainite. The lower bainitic microstructure increases the fracture toughness of the material [3,4,5]. The higher fracture toughness is indicative of a greater crack growth resistance in this material. This, in turn, causes a lower fatigue crack growth rate and a higher fatigue threshold in the material.
- Secondly, the austempered samples had a very fine-scale microstructure consisting of lower bainite with a limited amount of tempered martensite. A lower bainitic structure has a much finer grain size than upper bainite or pearlite. This creates additional resistance to crack growth since the crack tip encounters large number of fine scale grain boundaries. This, in turn, reduces the crack propagation rate because the crack grows along a longer, more torturous path.
3.5. Influence of Hydrogen on the Fatigue Crack Gowth Behavior of 4140 Steel
4. Conclusions and Future Work
- Austempering in the lower bainitic temperature range has significantly increased the mechanical properties and the fracture toughness of AISI 4140 steel as compared to the as-received (annealed) condition.
- In the absence of any charged hydrogen, the austempered samples had a much lower average crack growth rate and higher fatigue threshold than the as-received (annealed) samples.
- The presence of dissolved hydrogen increased the average crack growth rate in the austempered as well as in the as-received (annealed) samples.
- There is a transition stress intensity factor value of approximately 40–50 MPa√m; below this value, the presence of dissolved hydrogen causes the crack growth rate to be higher in the austempered samples when compared to annealed samples.
- In presence of dissolved hydrogen, above the transition stress intensity factor value, the crack growth rate was increasingly greater in the annealed specimens as compared to the austempered specimens.
- When compared to the as-received (annealed) condition, austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.
Conflicts of Interest
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|Element||Composition (wt %)|
|Set||Material||Externally Charged with Hydrogen?|
|“C”||Austempered (332 °C/1 h)||No|
|“D”||Austempered (332 °C/1 h)||Yes|
|Material Condition||Yield Strength (MPa)||Ultimate Tensile Strength (MPa)||% Elongation||Hardness (HRc)||Fracture Toughness (MPa√m)|
|Annealed||757 ± 20||1031 ± 5||5.6||28 ± 1||65 ± 2|
|Austempered (332 °C/1 h)||1481 ± 12||1646 ± 9||5.5||45 ± 1||72 ± 6|
|Material Condition||Hydrogen Charging Time (Hours)||Measured Hydrogen Content (ppm)|
|Austempered (332 °C/1 h)||150||1.6|
|Material Condition||∆Kth (MPa√m)|
|Austempered (332 °C/1 h)||8.12|
|Material Condition||Paris Law Constant|
|Annealed||1 × 10−10||2.08|
|Austempered (332 °C/1 h)||5 × 10−11||2.14|
|Material Condition||Hydrogen Charging Time (Hours)||Hydrogen Concentration (ppm)||∆Kth (MPa√m)|
|Austempered (332 °C/1 h)||0||---||8.12|
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