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Nordic Carved Armor requires a Smithing level of 50 and the Advanced Armors perk to create. Pieces can be upgraded with a quicksilver ingot at a workbench and also benefit from the Advanced Armors perk, which doubles the improvement.
Perks are special skills required for upgrading weapons, armor, for building a settlement and so on. Perks were important part of gameplay in all Fallout games and Fallout 4 is not an exception.