Not the most clearly written article but the graph was nice. This one is a little better
Link. "The low temperature phenomenon is called 'tempered embrittlement' and it is irreversible except by reaustenitizing and repeating the entire heat treating cycle."
Apparently the graph was made from
impact testing data. All the articles I read agree that the phenomena occurs in plain carbon steel though maybe to a lesser extent. Nevertheless I avoid the low areas on the graph for impact, not springs, and don't worry about it. Metallurgy is not an easy subject and I don't claim to understand it. (You don't hammer on your springs, do you?)
(For readers, light blue tempering color translates to about 321C. A nice chart
Here. )
