You asked a question, I gave you the best simple answer I could.
Look at their own picture and tell me the receiver is fully heat treated. It isn't, never have been. They are spot treated at the hammer and trigger pin axis holes. Just as you should do if you are building from a non-heat treated flat. The problem is the rest of the receiver is not heat treated. You don't heat treat at 575 degrees. You first heat the metal up to a temperature over 1,000 degrees (for the homebuilder, cherry red is sometimes recommended but the exact temperatures are available depending on the type of steel and carbon content %) and quench it, then you heat the metal up to the lower temperature and place it in sand (for the home heat treatment) and let it cool slowly, I believe this is for tempering or annealing one or the other.
I have posted the instructions before. The people actually in the business of building receivers will use a commercial heat treating oven or kiln to maintain the correct temperatures for heat treating with the receivers most likely in a jig to prevent them from warping during the process. Just because OOW uses the home builders process doesn't mean that is the correct way to do it for a commercial receiver.
It isn't the way the Russians, Bulgarian, Romanians, etc. did it, they fully heat treat their receivers for a reason.
Nodakspud and Armory were able to fully heat treat their receivers, not spot treat. They have a certain number that would warp and those were discarded.
If you want a substandard rifle with a possibly soft receiver that may fail earlier than a fully heat treated one then buy it. I would rather have a fully heat treated one when they are available for the same price range.
Heat treating a flatSurplus rifles AK-47 heat treating guide, (pictures look familiar)
Typically steel is heat treated in a multi-step process. First it is heated to create a solid solution of iron and carbon in a process called austenizing. Austenizing is followed by quenching to produce a martensitic microstructure. The steel is then tempered by heating between the ranges of 150°C-260°C (300°F-500°F) and 370°C-650°C (700°F-1200°F).
Tempering in the range of 260°C-370°C (500°F-700°F) is sometimes avoided to reduce temper brittling. The steel is held at that temperature until the carbon trapped in the martensite diffuses to produce a chemical composition with the potential to create either bainite or pearlite (a crystal structure formed from a mixture of ferrite and cementite).
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