1 in 64 every time you take iron ingots out of a furnace (or electric or arc furnace), stack size of the output is ignored, a full stack has the same 1:64 chance as a single ingot.

1 dari 64 setiap kali Anda mengeluarkan batangan besi dari tungku (atau tungku listrik atau busur), ukuran tumpukan dari output diabaikan, tumpukan penuh memiliki peluang 1:64 yang sama dengan satu batangan tunggal.

Iron ingot from crystal, from iron ore or powder?

It only works when you move the ingot out of the slot into your inventory (i.e. the same trigger that the iron achievement has), so automation won't work.