Aerospace special-process providers seeking Nadcap accreditation agree that the process has elevated the level of compliance from procedural accuracy, training, documentation, equipment functionality and process control. For the majority of heat-treat suppliers the Nadcap process is challenging for many reasons.

The Performance Review Institute (PRI) is responsible for the administration of the Nadcap process. PRI has mandated several documents, communications and systems, in addition to customer-invoked specifications, all of which the heat-treat supplier needs to be aware of and incorporate as part of the Nadcap audit preparation process (Table 1).

The dominating discipline for heat-treat suppliers is that of pyrometry, and it is estimated that seven out of 10 nonconformances written during Nadcap audits are pyrometry-specific. This statistic may be due to a combination of a lack of expertise, poor shop-floor discipline and/or weaknesses within the controlling industry standard for pyrometry, AMS 2750.

Evidence suggests that PRI staff engineers provide an interpretation of pyrometry requirements and enforce these interpretations through the root-cause and corrective-action process. Some pyrometry interpretations have been endorsed by the Heat Treat Task Group via audit advisories. Other pyrometry interpretations have been alluded to in the Pyrometry Guide and other unofficial documents. These interpretations provide little or no protection to heat-treat suppliers during Nadcap audits unless due diligence has been applied during the audit preparation process.

Heat-treat suppliers who have experienced Nadcap audits are aware that there is ambiguity and a lack of clarity within AMS 2750 that may only be exposed during the Nadcap audit. Examples shall be provided of selected pyrometry interpretations that have been identified during Nadcap audits and independent pyrometry compliance assessments. Examples shall also be provided of selected pyrometry nonconformances written during Nadcap audits.

Selected Pyrometry Interpretations

Load Sensors

How many load sensors are required, and what should be their position within the qualified work zone during a temperature uniformity survey? Should the load sensors be inserted into a heat sink?

What AMS 2750 States: Not defined

What is the suggested practice?

If load sensors are required during heat treatment by the process or material specification, a minimum of one load sensor shall be acceptable when performing the temperature uniformity survey. If more than one load sensor is used during production heat treatment, a minimum of one load sensor shall be required during the temperature uniformity survey (TUS). The position of the load sensor may be anywhere within the qualified work zone. Since the role of load sensors is to measure part temperature, it would be advisable to insert the load sensor into a heat sink when performing the temperature uniformity survey.

Sensor Type

Is it required that the sensor input type (e.g., N, T and J, etc.) be reported on the control and recording instrument calibration report?

What AMS 2750 States: Not defined

What is the suggested practice?

The sensor input type is only required for the calibration report for secondary-standard and field test instruments. An exception would be if a multichannel recording instrument is being used to record process-sensor temperatures and if different sensor types are used. It would then be advisable to identify the sensor input type on the recording instrument calibration report.

Cool-To Temperature

If a cool-to temperature is stated in the process or material specification that is outside of the equipment qualified operating temperature range, should the control and recording instruments be calibrated at this cool-to temperature?

What AMS 2750 States: Not defined

What is the suggested practice?

The cool-to temperature shall be included as a calibration point only when the cool-to temperature is specifically defined in the applicable process or material specification. Calibration at four points: the minimum; maximum; one point representing the middle third of the qualified operating temperature range; and the cool-to temperature should address this condition.

Uniformity

If the process and TUS tolerance is ±25°F with a minimum qualified temperature of 1000°F, should the control and recording instruments be calibrated at 1000°F or 975°F?

What AMS 2750 States: Calibration shall be performed at the minimum, maximum and one point representing the middle third of the range.

What is the suggested practice?

Intuitively, if the start and end of soak is dependent on the control or recording sensor reaching the lower setpoint temperature tolerance, it would make sense to allow for the tolerance in lieu of the minimum, maximum and midpoint as part of instrument calibration. However, this practice is not acceptable.

Test Instruments

Is sensitivity required to be measured and reported for digital secondary-standard instruments and field test instruments?

What AMS 2750 States: Sensitivity shall be checked during calibration.

What is the suggested practice?

The paragraph arrangement in AMS 2750 appears to imply that the sensitivity is a reporting requirement. However, sensitivity is not required to be reported for digital secondary-standard instruments and field test instruments.

Analog vs. Digital

What is an analog versus digital recording instrument?

What AMS 2750 States: Not defined

What is the suggested practice?

The different types of recording instruments in use confound an already difficult condition. Table 2 may be used as guide.

System Accuracy

Should a system-accuracy test be performed when an offset is introduced or removed from a control or recording instrument?

What AMS 2750 States: A system accuracy test is required when:

• The sensor is replaced.
• The control, monitoring or recording instrument is replaced.
• Recalibration of the control, monitoring or recording instrument when any adjustment has been made.

What is the suggested practice?

A system accuracy test is required when:

• The sensor being tested is replaced.
• The control, monitoring or recording instrument is replaced.
• Recalibration of the control, monitoring or recording instrument when any adjustment has been made, including adjustments made to an in-tolerance as-found condition to return values to their nominal values.
• If an internal offset is introduced, removed or an existing internal offset is adjusted.
• An increase in the maximum qualified operating temperature or decrease in the minimum qualified operating temperature. The system accuracy test shall be performed in the previously untested temperature range.
• After a system accuracy test failure and corrective action has been implemented.

Selected Pyrometry Nonconformances Written during Nadcap Audits

Nonconformance Written by the Auditor

The product literature for the Fluke 743B instrument states that the instrument may not meet the accuracy requirements of a field test instrument in accordance with AMS 2750. The Fluke 743B instrument is used for calibration of control and recording instruments and the system accuracy test. The calibration history of the Fluke 743B confirms that the instrument consistently meets the accuracy requirement of a secondary-standard instrument.

What AMS 2750 States

Secondary-standard and field test instruments shall meet the accuracy requirements of ±0.3°F or ±0.05% of reading and ±1°F or ±0.1% of reading, respectively.

PRI Staff Engineer Response

Objective evidence needed that the field test instrument meets the accuracy requirements of AMS 2750 Table 3.

Nonconformance Written by the Auditor

There is no limitation or restriction noted on the instrument calibration label or instrument calibration report when all channels of a multichannel recording instrument are not used.

What AMS 2750 States

Calibration shall be performed on each channel in use that can be altered or adjusted individually or on each group of channels that can be altered or adjusted as a group. It is recommended that channels not in use be blocked or tagged to prevent unintentional use.

PRI Staff Engineer Response

A comment on the instrument calibration label stating the existence of imitation/restriction label is required. A comment on the instrument calibration report is required when a multichannel instrument is utilized and all channels are not actively used.

Nonconformance Written by the Auditor

Operating parameters used during the TUS do not reflect those used during production heat treatment. The control instrument is programmed to ramp at 10°F/minute during the TUS, whereas the program used during production heat treatment used ramp rates between 10°F/minute and 30°F/minute.

What AMS 2750 States

The TUS shall be performed at any ramp rate used during production heat treatment.

PRI Staff Engineer Response

The TUS shall be performed at any ramp rate between 10°F/minute and 30°F/minute.

Nonconformance Written by the Auditor

The material specification states that a preheat cycle may be used prior to ramping to the required setpoint temperature. The preheat temperature is not within the qualified operating temperature range of the equipment. There was no calibration of control and recording instruments that included the preheat temperature used.

What AMS 2750 States: Not defined

PRI Staff Engineer Response

Since the preheat cycle is suggested, it is not required that control and recording instruments be calibrated at the preheat temperatures.

Nonconformance Written by the Auditor

Instrument calibration is being performed quarterly. The allowable extension for a quarterly test or calibration is three days in accordance with AMS 2750 Table 10. The instrument calibration was consistently being performed quarterly plus three days, and the due date for calibration was based on the quarterly interval and not the quarterly interval plus three days.

What AMS 2750 States: Not defined

PRI Staff Engineer Response

The intent is not to use the allowable interval extension as a convenience but to apply the extension to accommodate equipment availability, holidays, weekends, etc.

These comments provided are the nonbinding opinion of the authors and do not constitute an interpretation by SAE. Such opinions do not substitute for the need to ensure agreement between the supplier, customer and the cognizant engineering organization.

For more information:  Contact Andrew Bassett, president, or Colin Thomas, technical consultant, Aerospace Testing and Pyrometry, 35 Christine Lane, Bangor, PA 18013; tel: 908-217-8722; fax: 610-588-0939; e-mails: abassett@atp-cal.com, colinthomas433@yahoo.com; web: www.atp-cal.com