Effect of Intercritical Quenching Temperature of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications †
2. Materials and Methods
2.2. Experimental Procedure of Mechanical Properties of the Steel with L Treatment
2.3. Experimental Procedure of Weldability of the Steel
3. Results and Discussion
3.1. Effect of L Treatment Temperature on Mechanical Properties
- The tensile strength (T.S.) of the Q–L–T sample is comparable to that of the Q–T sample. Also, 0.2% yield strength (Y.S.) is slightly decreased by L treatment.
- The toughness is dramatically improved by L treatment.
- The strength–toughness balance was obviously improved by L treatment at 1068 K (near the AC3 point of the steel).
3.2. Mechanism of Improvement of Mechanical Properties by L Treatment
- The strength of the Q–L–T material becomes lower than that of the Q–T material in the cases where the L treatment was carried out in the temperature range from 1003 K to 1053 K (between AC1 and AC3).
- The toughness gradually improves as the L treatment temperature increases.
3.3. Weldability of Cu-Containing Low Alloy Steel
- The strength–toughness balance was obviously improved by L treatment at 1068 K (near the AC3 point of the steel), and an investigation of the mechanical and fracture toughness properties of the overall product revealed that L treatment resulted in high quality characteristics of the forging for use in an offshore structure.
- The in situ EBSD measurement results indicate that a fine and complicated microstructure is formed by L treatment at higher temperatures between AC1 and AC3. The complicated structure seems to indicate that the crystal grain after the L treatment become extremely fine. Moreover, it is clear that the necessary L treatment time is at least 100 min in order to stably obtain the mechanical property-improving effect.
- TEM–EDS analysis shows that coarse Cu precipitates are observed in the not-transformed α phase. Thus, the strengthening effect of the L treatment temperature is relevant to the area ratio of the not-transformed α phase and the transformed γ phase during L treatment. The strength, especially the Y.S., seems to be decreased by the not-transformed α phase acting as a softer phase.
- The EBSD results indicate that the improvement of toughness is due to the refining of the EBSD grain size by the transformed γ phase that is generated during L treatment.
- The long part forgings of Cu-containing low alloy steel have good weldability, since the maximum hardness of HAZ is less than 300 HV, and the HAZ of steel has a good CTOD property with less than 2.3 kJ/mm of heat input using GTAW.
Conflicts of Interest
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(50 kg test ingot)
|0.03||0.35||1.40||2.15||1.27||Cr, Mo, Al, Nb||927||1083|
|0.02||0.33||1.30||2.11||1.23||Cr, Mo, Al, Nb||927||1081|
|Welding Process||Abbreviated |
|Consumable||Maximum Heat Input |
|Submerged arc welding||SAW||A5.23 F9A8-EG-G||3.5|
|Gas tungsten arc welding||GTAW||A5.28 ER100S-G||2.7|
|Heat Treatment Process||L Treatment Temp.|
|Sampling Location||0.2% Y.S.|
|Thin part*||TP||618||617||692||692||28||29||82||82||89||163||208||1.52 (δm)|
|Thick part*||FP||552||547||669||666||30||30||81||81||82||159||228||2.89 (δm)|
|L Treatment Temperature|
|EBSD Grain Size|
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Honma, Y.; Sasaki, G.; Hashi, K. Effect of Intercritical Quenching Temperature of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications. Appl. Sci. 2019, 9, 1705. https://doi.org/10.3390/app9081705
Honma Y, Sasaki G, Hashi K. Effect of Intercritical Quenching Temperature of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications. Applied Sciences. 2019; 9(8):1705. https://doi.org/10.3390/app9081705Chicago/Turabian Style
Honma, Yuta, Gen Sasaki, and Kunihiko Hashi. 2019. "Effect of Intercritical Quenching Temperature of Cu-Containing Low Alloy Steel of Long Part Forging for Offshore Applications" Applied Sciences 9, no. 8: 1705. https://doi.org/10.3390/app9081705