Low carbon recivers were used because that was the extent of the Metalurgy at that time in history.
"Sudden fracture point is commonly known as Brittle Fracture and is a condition that a steel can be put into if not heat treated properly known as Martensite.
Recievers were annealed to remove the Martensite structure..
I believe, historically, medium and high carbon steel was well known and available in the late 1800's early 1900's. I think even today there's the use of medium carbon steels, but I believe there's a reason why high carbon steels are generally not used in receivers and barrels.
I don't believe martensite is a heat treating problem. I think the whole point to the thread is the importance of case hardening, or the formation of martensite on the surface of steel that is unable to harden on its own. The thread is also asking if the hardness, martensite, were removed by annealing if the gun were ok to shoot. So, there seems to be an implication that we don't want to anneal the hard case of a receiver for possibly safety or durability reasons.
