Interpretation of AMS 2759/5F sub zero cool 440C

[KSQuality]

Currently have conflicting interpretations of the timing of a sub-zero step during treatment of 440C to AMS 2759/5F.

Below is reference from the specification:

1. Req. of 440C:
After hardening quench, sub zero cool at -90 °F (-68 °C) or colder for not less than 2 hours.

2. General req. AMS 2759/5F:
After quenching and prior to tempering, subzero cooling may be required as shown in Table 2 [highlight #FCE94F](req. #1)[/highlight]. It is recommended that
subzero cooling begin within 2 hours from end of quench. For subzero treatments, interruptions of the soaking period are
permitted. The total soak time shall not include any time when the temperature is warmer than -90 °F (-68 °C) due to
interruptions.

3. Req. of 440C:
At least two tempers required. For parts requiring maximum dimensional stability over time, a subzero cool to -90 °F (68°C) or
colder for not less than 2 hours is recommended between the first and second temper.

To me the sub zero step should occur within 2 hours of the quench reaching RT, then if dimensional stability is critical, perform another sub zero treatment between 1st and 2nd temper.

I've been told that for 440C specifically, because of req. #3, that a single sub zero treatment can be instead performed between 1st temper and 2nd temper.

I can see how the spec. could be interpreted where this is technically still conforming due to the verbiage (recommended, may...) but interested to see other's thoughts in the community.

 

[EdStainless]

Whoever told you is wrong.
There must be a -90F cycle after quench and before any temper.
And for maximum dimensional stability, add another -90 between tempers.

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P.E. Metallurgy, consulting work welcomed