Hi,
When crafting an armor or any weapons, you can improve it when you put the max iron in it, so you have a better armor. The thing is that if you have a lvl 5 armor and you are lvl 10 (like me), you have to craft a new armor, but it could be much better if you can put more iron in it. Instead of an "upgrade" for rarity, it's just a lvl upgrade.
Hope you understand lol.
Thank you