17-4 PH, Anealing, Aging, Thread size

[musashi99]

7-4 PH , solutionizing and H900 aging

Technically, when annealing and aging 17-4 PH Stainless Steel the material will contract. In my tests this contraction is about .0003” per inch.
However these processes seem to have the opposite effect on threads i.e. male threads get larger and female threads get smaller as judged by thread gages. This observation is based on 15+ years of experience with this problem. The effect is enhanced in a part that is annealed and aged as opposed to a part that is aged only.
The degree in change is base on what I call associated mass (the amount of mass around a thread) the more associated mass the more likely the thread will change.
My solution to this problem has been to make most male threads near the low limit and make most female threads near the high limit.
So, can anyone explain why when the material contracts that the opposite happens to threads?
How does everyone else deal with this issue? Is there any reference material out there that describes or deals with this problem?

Thanks,

 

[jck26]

You can machine this material post heat treatment with few problems. I rough out a tight tolerance investment casting and after HT I finish machine and thread the part. I'm not a big fan of doing heat treatment after I've done threading since it is a close tolerance operation. And if you are using just go and no-go gages to determine if the threads are good you could be at either end point in the pitch diameter without knowing and have the heat treatment push your part to an out of spec condition.

jck26

 

[musashi99]

Thanks, You can single point threads after these processes with out a problem. Tapping is a bit more risky. I have my folks using gages and pitch mics to stay where we need to.