Repair Welding of Nickel-Alloy Reactor Vessel in Petrochemical Plant

Scenario

In-situ repair of a cracked Inconel 625 overlay on a carbon steel reactor vessel (Houston, TX), operating at 350°C service temperature. The repair requires multi-pass GTAW with controlled heat input to prevent HAZ liquation cracking and dilution-induced embrittlement. Space constraints limit access — only one side accessible, no post-weld heat treatment possible. Regulatory oversight (ASME BPVC Section VIII, Div. 1) mandates documented interpass control per PQR-087.

Given Data

• Base Metal Temperature: 112°C (measured at weld toe using embedded thermocouple; residual heat from prior pass + ambient ~32°C)
• Upper Limit Temperature: 180°C (reduced from standard 225°C due to high Nb content in Inconel 625 and risk of Laves phase precipitation)
• Cooling Rate Constant: 0.0065 1/s (validated for 4-mm overlay thickness, 100 mm/min travel speed, and forced-air cooling used between passes)
• Time: 210 s (2 minutes 30 seconds — elapsed since arc termination of previous pass)

Calculation

Using the same Newtonian model:

• T_base = 112°C, T_upper = 180°C, k = 0.0065 s⁻¹, t = 210 s
• ΔT = 112 − 180 = −68°C
• e^(−0.0065×210) = e^(−1.365) ≈ 0.255
• T(210) = 180 + (−68) × 0.255 = 180 − 17.34 = 162.7°C

Since the actual temperature at 210 s is 162.7°C — well below the 180°C upper limit but still within the safe interpass window (120–180°C per procedure qualification) — the tool returns 162.7°C as the maximum allowable interpass temperature at that moment, meaning the next pass must begin before cooling further below 120°C. This value serves as the real-time upper bound to prevent over-aging while preserving ductility.

Result and Decision

The welding supervisor paused the sequence at 162°C (verified via thermocouple + IR cross-check) and initiated the next pass immediately. Forced-air cooling was temporarily halted 30 s prior to restart to minimize thermal shock. All 14 repair welds passed radiography (RT Level 2) and microhardness mapping (no >350 HV spikes in HAZ), confirming absence of Laves phase.

Lesson

For high-alloy repairs where upper limits are conservatively reduced, interpass temperature isn’t just a ceiling — it’s a narrow operational band; the calculator transforms static limits into actionable, time-resolved targets that reconcile metallurgical safety with field practicality.