Expired Thermocouple During Heat Treatment
During surveillance at a forging supplier, the inspector reviews the heat-treatment records for 40 low-alloy steel bodies. The austenitizing furnace chart shows a 22-minute interruption in the control thermocouple signal, followed by recovery to set temperature. The recorder load thermocouple calibration was due five days before the run. The supplier's procedure requires calibration within 30 days and allows no signal interruption greater than 10 minutes without evaluation. The parts have already been tempered and are awaiting final machining. The supplier proposes to ship the parts because the hardness test results are within specification.
1. Assess whether the heat-treatment records demonstrate conformity, and explain the basis for your conclusion.
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The records do not demonstrate conformity as presented. Two independent problems exist: the load thermocouple calibration was expired at the time of the run, so the recorded temperatures cannot be reliably traced to a calibrated standard; and the 22-minute signal interruption exceeds the supplier's own 10-minute evaluation threshold. Hardness results alone do not cure these defects because hardness is a room-temperature property and does not prove that the entire load reached and held the required austenitizing temperature. The inspector should treat the affected load as suspect, not accepted, until the supplier performs a documented evaluation under its quality system, such as furnace survey data, retest of representative parts, or other technically justified verification.
2. What immediate actions should the inspector require for the 40 bodies, and what evidence should be produced before disposition?
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The inspector should require quarantine or hold of the 40 bodies, identification of the affected heat-treat lot, and a nonconformance report. The supplier should determine the calibration error of the load thermocouple and whether it could have biased the recorded temperature, review the furnace survey and any backup thermocouple data, and evaluate the effect of the signal interruption. Before disposition, the supplier should provide a technically justified evaluation by a qualified person, results of any supplemental testing such as hardness traverse, metallography, or mechanical testing from the same lot, and the proposed disposition. If conformity cannot be demonstrated, re-heat treatment under a qualified procedure with calibrated instrumentation is the typical corrective action, followed by re-verification.
3. What systemic corrective action should be examined, and how does this affect acceptance of future lots?
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The systemic issue is calibration and alarm management for furnace instrumentation. The supplier should review why an expired thermocouple remained in service, whether the calibration recall system failed, and why the signal interruption was not automatically flagged. Corrective actions could include a calibration due-date lockout in the furnace control system, daily verification of thermocouple status before loading, alarm response procedures, and review of other lots processed in the same window. For future lots, the inspector should increase surveillance of calibration records and furnace charts until the corrective action is verified effective. Acceptance of future lots should depend on current calibration evidence and complete chart records, not only on final product test results.