I found my machining book from school that I thought that I had lost. I decided to flip through it and see what I could learn, again. One of the items in the heat treat chapter covers a W1 draw die that has threaded holes in the corners. The text notes how...
"If there is a hole, sharp shoulder, or small extension from a larger cross section (Un-equal mass), a crack can develop in these areas (Figure D-37). Holes can be filled with steel wool to avoid problems."
The photo has a caption under it noting that the drawing die (pictured) was made of W1 and was water quenched.
I have never heard of packing a threaded hole with steel wool to prevent cracking in heat treat. Is this a normal operation? I have never had a crack in the D2, A2 & 4140 that I have treated.