Short answer: you're in a tough spot.
First off, I have to ask why no one realized the furnace chart wasn't moving. This is ~20 hour aging cycle, and from what you stated, the problem occurred early on, which begs the question that nobody looked at the equipment for 20 hours??
Second, you should have documented procedures that take certain situations into account. In this event, there should at the very least be some sort of documentation stating that there was an equipment (recorder) failure. A "deviation" or "variance" form would be filled out in this case.
Depending on your end user(s) you may be able to notify them that there was a deviation from your standard practice (with the applicable documentation), and if the parts meet all the testing requirements, you might be able to ask for some sort of relief from the record retention requirements in this particular case.
Since you are working with superalloy, and certifying to AMS specs, I imagine you're making parts for the aviation industry, in which case, is there any way to divert or specify to non-critical application? Ask yourself, would you want to fly on a plane that these parts are going into? If it were some other company's parts in a similar situation, would you trust their word and fly on a plane their parts went into?
If you do extensive enough testing, and prove that your parts meet or exceed properties of correctly heat treated parts, and have all the necessary documentation to back it up, you might be alright. Talk to the QA person that deals with the NADCAP auditors (unless you're it), and find out what their thoughts are.
Worst case scenario, re-solution, re-work to smaller size (if possible), and re-age, and make sure your recorder is working. Or don't take the chance and scrap it.
