From heat input to microstructure: why cooling curves matter
Cooling rate controls transformation kinetics in the HAZ and weld metal. Faster T8/5 tends to increase hardness and crack susceptibility, while slower T8/5 allows tougher phases to form but may reduce productivity. Understanding how parameters shift the cooling curve lets you tune procedures for both quality and throughput.
Calculating heat input and estimating T8/5 from thermal frames
A common engineering estimate for arc welding heat input (HI) in kJ/mm is:
HI ≈ (60 × V × I × η) / (1000 × travel_speed_mm_min)
Where V is arc voltage (V), I is current (A), and η is thermal efficiency (typical values: ~0.6–0.8 GTAW, ~0.8–0.9 GMAW, up to ~1.0 SAW). Use the value as a comparative indicator rather than an absolute guarantee of properties.
To estimate T8/5 from thermal frames, track a region of interest and measure the time taken to cool from 800 °C to 500 °C. Use a temperature‑calibrated camera where possible, and stabilize readings by averaging over a small ROI and applying noise‑robust interpolation rather than a single pixel. Validate with thermocouples during procedure qualification and after significant changes (materials, wire, gas, preheat).
Correlations: hardness, toughness, and defect likelihood
Relate measured T8/5 to hardness and impact test results by grade using data gathered during PQRs. For higher‑strength steels, very short T8/5 correlates with higher hardness and increased risk; longer T8/5 may mitigate crack susceptibility but can over‑soften certain zones. Keep production limits conservative until field evidence supports tightening.
Process parameters: travel speed, wire feed, shielding gas
Travel speed, current density, and voltage shift heat input and thus the cooling curve. Preheat/interpass temperature and shielding gas composition also affect surface heat exchange. On automated cells, include feedback loops from thermal metrics to suggest parameter corrections before defects propagate.
Calibration and validation: thermocouples vs infrared
Use thermocouples on representative coupons to validate T8/5 derived from IR. Track emissivity, viewing angle, and lens contamination over time; revalidate after maintenance or process changes. Where direct temperature accuracy is limited, use relative thermal metrics (cooling slopes, HAZ width) that are more robust to emissivity drift.
Examples: typical steel grades and expected cooling profiles
In common structural steels, T8/5 often spans tens to hundreds of milliseconds depending on thickness, heat input, and preheat. As section thickness increases or ambient conditions change, revisit your preheat and interpass tables to keep cooling rates within acceptable ranges for the grade and joint category.
Practical limits and uncertainty in field conditions
Field reflections, variable surface conditions, and motion blur add uncertainty. Use robust placement, optics, and filtering; document uncertainty bounds in your procedures and add a safety margin to limits. Where environmental control is weak, emphasize relative indicators and greater sampling in your review plan.
Related:
References
- EN 1011‑2 — Welding recommendations for arc welding of ferritic steels (preheat/interpass concepts)
- ISO 15614‑1 — Specification and qualification of welding procedures for metallic materials (PQR context)
- General engineering practice for heat input calculation and thermal efficiency factors
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