This article outlines the furnace operation and maintenance requirements of AMS 2750D, with particular focus on how these requirements have changed from AMS 2750C and how proof of compliance can be achieved.

“No way of thinking or doing, however ancient, can be trusted without proof.”

The words of Henry David Thoreau, the 19th century American essayist, poet and philosopher, ring true around the 21st century heat-treatment industry. The heat treatment of metals in order to change their physical properties is one of the oldest industrial processes, with Iron Age man understanding the benefits of heating and quenching his weapons. With the up-issue of AMS 2750 to Revision D, however, proof has become a precious commodity more than ever before. The Nadcap pyrometry audits to the specification AMS 2750D are demanding that heat-treatment suppliers to the aerospace industry provide proof of compliance, leaving many companies facing a list of non-compliances and expensive changes to working practices.

Operation and Maintenance Requirements of AMS 2750D

Crucial to a successful pyrometry audit is the ability to demonstrate to the auditor the adherence to a comprehensive set of procedures for the operation and maintenance of furnaces. AMS 2750D places particular emphasis on the management of a number of key aspects of furnace usage:
  • Management of thermocouples (section 3.1)
  • The specification and calibration of furnace instrumentation (sections 3.2 and 3.3)
  • Temperature uniformity surveys – TUS (section 3.5)
  • System accuracy tests – SAT (section 3.4)
The auditor will be looking for evidence that these operations are carried to the letter of specification and that all operations have been carried out within the correct period of time.

Load Thermocouple Life Monitoring

For a furnace with type A or type B instrumentation, load thermocouples are required for monitoring and recording. Section 3.1.8.5 gives the permitted usage of nonexpendable base-metal load thermocouples – types E, K, J and N. Whereas in revision C these could be used for three months and recalibration was not recommended, under revision D the usage is determined by the furnace operating temperature, and recalibration is prohibited (section 3.1.8.3). The available thermocouple life, measured in number of uses and elapsed usage time, is calculated on a sliding scale. An expendable load thermocouple exceeds its usable life when it exceeds either its maximum number of uses or its maximum elapsed usage time.

Implicit in this section of the specification are the maintenance of thermocouple-usage records and the ability of the supplier to prove that any load thermocouple has not been used beyond its usable life. The decision on how to manage thermocouples is a balance between the cost of monitoring the thermocouple life and the cost of replacing a thermocouple before it has reached the end of its usable life.

Instrument Specification and Calibration

Section 3.2.1 requires that all instrumentation requirements are reviewed, as not all instruments approved for use in revision C will meet the requirements of revision D. All test instruments must meet the requirements of revision D, whereas controlling, monitoring and recording instruments purchased prior to one year after the publication of revision D – prior to September 2006 – may meet the requirements of AMS 2750C.

For test instruments, the principle change in revision D is that all must be digital. For controlling, monitoring and recording instruments, if paper chart recorders are used, Tables 4 and 5 of revision D specify the chart speed, print speed and temperature-resolution requirements for furnace chart recorders purchased later than one year after the release of revision D. All digital controlling, monitoring and recording instruments must have a calibration accuracy of ±2°F (±1.1°C).

TUS

The TUS frequency prescribed by AMS 2750D is a function of both furnace class and instrument type as laid down in Tables 8 (Parts Furnace) and 9 (Raw Material Furnace) of the specification. The determination of furnace class has changed in revision D. It is now being determined by temperature uniformity and no longer by the type of treatment. The number of instrument types has been increased from two (A and AA) to five (A to E). This change is summarized in Table 1 for furnace classes 1-4.

The TUS is the most expensive intervention required by AMS 2750D, with a typical survey downtime of at least one day resulting in a loss of productivity in addition to the cost of the survey itself. However, there is a risk of a far higher cost that can be incurred through a failed TUS or the failure to carry out a TUS within the allotted time. A failure to carry out a successful TUS within the frequencies stated on Tables 8 and 9 of AMS 2750D can lead to the auditor requiring that furnace loads be scrapped or even recalled.

The up-issue from revision C to D has increased the TUS frequency for certain combinations of furnace class and instrumentation type, meaning that the supplier needs to pay close attention to TUS planning. A summary of these changes for parts furnaces – classes 1-4 – is shown in Table 2. Along with changes to the initial frequency come changes to the reduced TUS frequency that can be applied when the conditions of section 3.5.7.1 are met.

Meeting the conditions of 3.5.7.1 can have a rapid positive financial impact. As shown in Table 3, a class 1 or 2 parts furnace with type A instrumentation requires 12 surveys per year. If a reduced frequency is applied, the number of surveys falls to four in the first year and two in subsequent years (moving from monthly to semiannually after two successful surveys). While reducing the TUS frequency brings immediate financial advantages in terms of increased furnace productivity and reduced direct survey costs, it increases the risk of a survey failure. This can be mitigated through the use of high-quality, well-tuned instrumentation with long-term stability and features such as overshoot inhibition. Another way to reduce risk is by running a program of pre-TUS checks such as:
  • Furnace insulation integrity
  • Door-seal integrity
  • Heating-element validation
  • Burner validation
  • Fan-speed validation
  • Thermocouple seal integrity


SAT

As with the TUS, the requirement for SAT is dependent on furnace class and instrumentation type. In AMS 2750C, the maximum allowable SAT interval for any Class 1 parts furnace could be increased to monthly or quarterly if certain criteria are met (section 3.4.1.2). In revision D, however, the maximum allowable interval is dependent on furnace class and instrument type and can be as low as weekly in the case of 1D and 2D furnaces. The higher the instrument type, where type A is the highest, the longer the maximum allowable SAT interval.

Both revision C (section 3.4.1.1) and revision D (section 3.4.3) permit an SAT waiver in the case of certain criteria being met. It is worth noting that revision D requires two recording load sensors in addition to the sensors required by the instrument type.

Fig. 1. Graphical data recorder with tamper-resistant data storage, alarm features and electronic signatures

Proof of Compliance

With the proof of compliance critical, solutions exist to ensure that the heat treater can demonstrate this to the auditor. Graphical data recorders exist that store process data in a tamper-resistant format locally and on centralized servers. The same recorders can be used to log dates of TUSs, SATs and calibrations and provide alarms when the date of the next intervention becomes due (Fig. 1), with electronic signatures stored to provide proof of the time and date of interventions. Thermocouple usage and peak process temperatures can be logged to allow automatic calculation of the remaining thermocouple life, with alarms and messages to ensure that the usage parameters are not exceeded.

Graphical recorders provide a customizable human-machine interface (HMI) with the ability to show process trends, alarms, messages and events as the operator requires. Electronic signatures can be added at any stage of the process to acknowledge events, and batches can be managed via the recorder to enable batch reporting and analysis at completion of the treatment. Graphical recorders can be connected to Ethernet networks to enable the furnace screens to be viewed remotely by managers, technicians and supervisors.

Conclusion

The onus placed on the heat-treatment supplier by AMS 2750D is to provide proof that the furnace maintenance and operation requirements are carried out within the specified deadlines. Failure to prove compliance in these areas can lead to sanctions ranging from corrective-action programs to product recall. Achieving a successful Nadcap pyrometry audit is adding costs to heat treaters, but the long-term cost of failure is potentially far higher.

With TUS and SAT regimes determined by furnace class and instrument type, the decision on instrument type becomes strategic to the heat treater. An increased initial investment in instrumentation leads to greater calibration requirements but in turn reduces the longer-term cost of TUSs and SATs. Whatever the decision, the correct choice of control and data-monitoring instrumentation can provide the heat treater with the furnace control, recording and traceability required to pass the Nadcap pyrometry audit. IH

For more information: Contact Richard Bolton, marketing manager – heat treatment for Eurotherm, Faraday Close Durrington, Worthing BN13 3PL, United Kingdom; tel: +44 (0)1903 837905; U.S.: 703-443-0000 ext 324; e-mail: Richard.Bolton@eurotherm.com; web: www.eurotherm.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: AMS 2750D, systems accuracy, temperature uniformity, digital test instruments, load sensor, graphical recorder