How large of a part is this?
What product form (casting, forging ...)?
Gr1 processing (full solution anneal) will increase GS a bit, but unless it was already 5 and coarser I don't see you getting 2-4 out of it.
We make tubing that starts as finer than 8, and after full solution anneal we are still finer than 6 with ease.
But our anneal times are measured in minutes.
We have customers that want higher creep resistance and larger grains, it is a pain to get them up to 5-6.
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P.E. Metallurgy, Plymouth Tube