what is the A stands for

The "A" in A36 is the ASTMs designation for ferrous metals (metals that contain iron. A "B" prefix designates a non ferrous metal( metals that do not contain iron).

Any chance you know what would be the best way to heat treat a 2" x 6" x 10" piece of A-36 steel? looking to take 5 pieces that size and fabricate a sort of anvil/hardened surface for hammering anything from mild steel to knife making steels. wanted to harden the top two layers prior to having it welded all together. was told this would be a good steel to do that with but was never told how I could heat treat a piece that big and thick or if it's possible (without sending it out for someone else to temper metal for me) considering I only have a single burner forge that will barely fit a piece of steel this big.