AMS 2750F pyrometric requirements: what changes for the equipment of heat treatment furnaces

In previous articles we have introduced the news in AMS (Aerospace Material Specifications) 2750 revision F and we analyzed the alternative SAT and the comparative SAT.

What changes in the AMS 2750F for the equipment of the instrumentation of the heat treatment furnaces?

The Class and Type of instrumentation remain substantially unchanged, except in the following three points.

First variation: temperature decimals

Decimals have been added to the class list, so we have to get used to writing 6.0°C and not 6°C.

When it comes to class of heat treatment plants, in practice we mean the uniformity of the temperature of the system, as indicated in table 11 of the standard.
The standard indicates 6 classes (from 1 to 6), but for the aerospace industry only the first 5 are applicable.

Table 11 - Furnace class uniformity tolerances

(*) Some design authorities require temperature uniformity tolerances of ±5.0°C and ±7.0°C for class 2 and class 3 respectively.

Second variation: the classes of heat treatment furnaces 2 and 3

Confirmed ±5.0°C for Class 2 and ±7.0°C for Class 3, in fact the new AMS2750F has accepted the declaration in force of Nadcap Task Group of Heat Treatment (HTTG).

This is an important requirement for European manufacturers who were forced to classify the plants to a lower class to satisfy the uniformity required by the technical process specifications and/or by the material specifications.
Many European and non-European manufacturers had previously converted to ±5° C or ±5.5°C a requirement specified in ±10°F (Fahrenheit degrees), the temperature measurement scale used in the United States of America.
Therefore, the correct interpretation is that to satisfy ±5°C or ±5.5°C, a class 1 heat treatment furnace is required and consequently more onerous control frequencies of class 1 heat treatment furnaces must be respected.
Therefore, with revision F of the AMS 2750 higher class control frequencies can be used, but the requirement ±5°C or ±5.5°C must be respected during the temperature uniformity test.

This allows for savings in controls.

The above is also applicable for the ±7°C requirement where class 3 frequencies (±8.0°C) can be used, corresponding to ±15.0°F.

Third variation: types of instrumentation of heat treatment plants

When it comes to the Type of instrumentation of Heat Treatment plants, in practice it indicates the type of equipment of the system, therefore the number of temperature sensors and the number of instruments connected to them. 

The standard indicates 6 types of instrumentation (identified from letter A to E), but the type E instrumentation is not applicable to the aerospace industry.

These listed below are the 6 types of instruments (chillers and extinguishing systems have a separate requirement):

• Type A (the most valuable, also used in vacuum furnaces)
• Type B (mostly used in vacuum furnaces)
• Type C (typically required for TT Al alloys)
• Type D (mostly used)
• Type D+ (new requirement)
• Type E (not used in the aerospace industry)
• Chillers and extinguishing systems

With the AMS2750F a new instrumentation has been introduced, that of type D+, not to be confused with that required by the Safran group standard (PR-0011), from which it differs substantially (users must be able to distinguish on internal procedures).

Instrumentation type D+ according to AMS 2750F requires:

• in addition to the basic instrumentation that characterizes the type D, an additional recording sensor positioned at a distance of at least 76 mm from the control sensor;
• also, the additional recording sensor must be of a different type from the control one. For example, if a type S sensor was used for the control system, the additional recording sensor that characterizes the type D+ instrumentation can be type N, and vice versa. Without any distinction between noble metal sensors and base metal sensors.

It is allowed to use the over-temperature sensor as an additional recording sensor to characterize the instrumentation type D+.

With the AMS 2750 rev. F, all extinguishing equipment must have a temperature recording system. So this means that not only immersion systems need to be equipped with recording, but also other systems such as the shower/spray shutdown.

Finally, the new standard allows an important simplification as regards the application of hot (High) & cold (Low) recording sensors and load sensors for Type A and C instrumentation on multi-zone control systems.
In fact, with the application of the new requirement, in the case of a multi-zone control system, it is no longer required to duplicate the hot (High) & cold (Low) recording sensors and load sensors for the number of control zones, but it is considered a single zone. However, this requirement can be applied for

1. installations with a volume of less than 6.4 m3,
2. the longest dimension of depth/height/width cannot exceed 3 times any other dimension.

In the next article we will illustrate the calibration of the process instrumentation (Control and Registration).