“By controlling the important variables, dimensional changes in heat treatment can be controlled.” Patrick McKenna
Variability is the enemy in our precision machining shops, and reducing variability is a key to sustaining our businesses and improving our capabilities.
When I talk about statistical process control with someone,  I listen closely to see if they are focused on the average (where the process is performing) or the standard deviation ( how the process is performing.)
If they are fixated on the average, I know I need to look at the data myself.  On the other hand if they are talking about standard deviations, I generally take their word on the data…
In the latest  issue of Production Machining Magazine, PMPA Technical Member Patrick McKenna from Nevada Heat Treating Inc., and Daniel Herring, the Herring Group Inc.  teach a nice class on how to reduce process variation in heat treat to minimize the post heat treat variability that all of us face.

Good advice here...

This is important if we are not to waste our production time trying to remove excess material because we left too much stock  for cleanup, or worse, finding the parts have shrunk in some critical dimension, rendering all of the parts ‘scrap.’
This article lists 9 variables NOT in control of the heat treater, and 14 that are under their control (furnace temperature uniformity, load configuration) or shared by the customer ( process selected, batch size, part size).
Not every order we produce is part of a long running job where we can control every input variable, but this piece does a great job of providing sensemaking on what can be a complicated and confusing subject.
I predict that you’ll keep this article in your “great to know” file.