Influence of bainitic transformation temperature on microstructure evolution during isothermal holding and subsequent cooling
Bibliographic record
Abstract
Modified mold steels have recently gained attention as a novel approach in steel grades due to their high-performance applications. In this study, high-resolution dilatometry was used to study the isothermal bainitic transformation of a medium-carbon low-alloy steel after full austenitization at 900 ˚C, followed by austempering for 3 hours at temperatures ranging from 380 to 450 ˚C. The results showed that no bainitic phase transformation occurred at 450 ˚C, while an incomplete transformation was observed at all lower tested temperatures, and martensitic transformation occurred upon cooling at the final stage. Increasing the austempering temperature from 380 ˚C to 425 ˚C resulted in a lower volume fraction of bainite and a higher amount of martensite, whereas at 450 ˚C, a full lath martensite microstructure formed during final cooling. Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) analyses revealed that increasing the isothermal holding temperature led to increased thickness of the bainitic blocks and a higher volume fraction of martensite. Vickers hardness increased from 412 to 500 HV between 380 and 450 ˚C holding temperatures. This increase was attributed to the phase volume fraction, the carbon content of martensite, and the thickness of bainitic ferrite blocks. Nanoindentation tests were conducted on different martensitic and bainitic regions to compare the influence of isothermal holding temperature on the characteristics of the phases. The results indicated that as the isothermal temperature increased, the carbon content of martensite decreased, leading to a reduction in the nanohardness of the martensitic regions from 8.3 GPa at 400 ˚C to 7.8 GPa at 425 ˚C and 6.9 GPa at 450 ˚C. Furthermore, the average block thickness of bainitic ferrite increased from 0.71 μm to 1.98 μm as the isothermal temperature was raised from 380 ˚C to 425 ˚C, resulting in a decrease in nanohardness from 6.3 to 5.4 GPa. These findings are interpreted in terms of the evolution of bainitic block thickness and the carbon content of martensite at different austempering temperatures, as well as their impact on the hardness of the phases.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".