I'm not a welder (it's a skill I need to learn), but I do have a copy of "Notes on Welding Technique for the use of Welders, Part 2, The repair of armour plate in the field" from 1943.
http://www.iwm.org.uk/collections/item/publication/80002761
It notes that there were four types of armour in use at the time (Machinable quality rolled armour plate, Cast armour, Homo-hard rolled armour plate, Face hardened armour plate), which have different properties.
The advice on pre-heating is echoed as armour plate is prone to cracking through different expansion. Similarly you should avoid stitching or wide beads due to local heating, and avoid any sharp corners to spread the stresses.
It also notes that it is not possible to weld the hardened surface of face hardened armour, but you can weld the back of it. At the time it was rare on allied kit, so it's probably not an issue for you.