Yea Mike... I think/know that we are both in agreement.
In my opinion, heat treating any material and in specific terms with this thread and 4130 material... goes beyond the mere attribute of embrittlement. Way beyond. But in a short summation, heat treatment can make metals not only harder, but, stronger and tougher. And depending on the material and the 'how and why' you are heat treating it... you can also make metal softer and more ductile.
If you made a part and spent $1000 on it with the ability to yield a certain value of strength... and then had the ability to make the same part but with a 75% increase in strength and fatigue resistance by simply heat treating it...
would you not do it ?? I'll even throw that into the specific respect to airframe materials. I am sure if one were to look at the engineering drawings for any given aircraft part... that it will specify a specific heat treatment.
Really my statement was more to point out that the idea of running 4130x and that one could get away with a thinner wall thickness and have the same strength... isn't necessarily true. When looking at the yield number from my last post, without any heat treatment... all you will end up with is an expensive part with the same, and in most cases, less strength than that of 1018/1020. Now if you were to heat treat it... the numbers would go the other way and yield more favorable results.
I believe that if you don't heat treat 4130x... your wasting valuable time and money. Most of the time it is at your customers expense...
When it comes to putting all of this into a unitized chassis... the 1018/1020 material is better suited for this application... without braking the bank.