Postweld Heat Treatment Chart With Two Cold Thermocouples
A shop has finished a 2-1/2 in. (63 mm) thick SA-516 Gr. 70 amine absorber. The vessel holds a U-stamp, and PWHT at 1150 °F (621 °C) is mandatory. The furnace chart carries eight thermocouples: six on the shell read 1148-1155 °F throughout the hold, but two attached near a 12 in. nozzle reinforcement read 1080-1090 °F for the entire 2 hour 40 minute hold. The supporting PQR was qualified with 2 hours at temperature. The Authorized Inspector must sign the Manufacturer's Data Report tomorrow and the vessel is booked for transport. The shop proposes to average all eight readings, argue that the mean falls inside the range, and ship. Nozzle records show no separate thermocouple was placed on the reinforcement and that area was not separately insulated.
1. Is the PWHT record acceptable? State your disposition and the reasoning behind it.
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Not acceptable. UCS-56 sets a holding-temperature range for P-No. 1 material at this thickness (1100-1200 °F), and the requirement is that the metal at every location be held inside that range for the required time. Section VIII contains no averaging provision: a thermocouple is evidence about one location, not a sample of the furnace. Two elements at 1080-1090 °F mean the nozzle reinforcement never reached the lower limit, so the 2 h 40 min hold does not count for that region.
The likely mechanism is thermal lag and mass in the reinforcement, aggravated by the missing insulation and the absence of an element on the part. The reinforcement-to-shell junction is the highest-restraint, highest-hardness region on the vessel, which is exactly where an under-treated condition matters most.
Disposition: reject the chart and withhold signature on the Data Report. Re-heat-treat, either by a full furnace cycle or by an engineered local PWHT (heaters and insulation wrapping the reinforcement, elements on the reinforcement and on the shell about one thickness away), then run a hardness survey across the nozzle weld and heat-affected zone.
2. If the vessel is re-heat-treated, what procedure-qualification and material consequences must be checked before the Data Report is signed?
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Qualification. The PQR covers 2 hours. A second cycle of 2 h 40 min brings the aggregate to roughly 5 h 20 min, well above the 80% threshold in QW-407.2, so a PQR qualified for that aggregate time is required unless the existing PQR already covers about 6 h 40 min or more at temperature.
Test specimens. Under UCS-85 the tension and impact specimens representing the vessel must receive the same heat treatment as the vessel. If the original specimens saw only one cycle, they no longer represent the finished product, and new specimens or additional simulated-cycle specimens are needed before tensile (and any required impact) values can be certified.
Practical checks. Support and brace the vessel against distortion, control heating and cooling rates per UCS-56, keep a thermocouple location map and chart for the new cycle, and confirm the material still meets SA-516 Gr. 70 mechanical properties. Any final machining, dimensional check, or coating that depends on finished geometry must follow the last thermal cycle. Note also that the second cycle is a chance to correct the element placement that caused the problem.
3. What records and hold points should the inspector have controlled so that this situation could not reach the shipping date?
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The failure is as much a surveillance failure as a furnace failure. Before the cycle, the inspector should have approved a written PWHT procedure showing furnace loading, element placement on a sketch of the vessel, insulation of heavy attachments, expected lag at the reinforcement, and the acceptance range; and should have verified thermocouple attachment method, recorder and thermocouple calibration, and furnace survey status.
During the cycle, the chart should have been reviewed at the hold point for stability and spread, not after the fact. The reinforcement is a predictable cold spot and should have had its own element from the start.
On release, the record package should include the chart with element-location map, calibration certificates, the PQR covering aggregate time, UCS-85 specimen heat-treatment records, and a hardness survey map. Signing the Data Report is the last hold point, not a formality: the inspector cannot accept a chart that does not demonstrate the required range at every location, and shipping pressure is not a code basis for acceptance.