I've long been aware that nomadic societies such as the Mongols had access to heavy armor, though perhaps rarer then sedentary civilizations. But though I'm aware they possessed and could make complex armor, I've long wondered how exactly they did it. Did they use mobile forges? Were there stationary forges built? Please answer, I'm dying to know!
Forging is a relatively simple process: heat up the workpiece in a fire, and hammer it into shape (of course, significant skill is required to do it well). It can be done with a very basic and portable toolkit, consisting of a hammer (which can be a stone), and anvil (also a stone), and bellows. Beyond this, one needs a hole in the ground (for the fire), and the raw materials: charcoal and iron and/or steel.
Just to show how minimal a basic forge can be:
While these are not nomadic examples, the key part of the toolkit is portable, and nomad-friendly (and in some cases, blacksmiths in sedentary societies are itinerant, travelling from village to village, taking their toolkit with them).
If you have pack animals or wagons/carts, a more complex toolkit will typically include chisels, files, punches, and so on.
What these simple toolkits limit the smith to is fairly small workpieces. This could explain the popularity of lamellar armour (and lamellar helmets) among nomads of the Eurasian steppe. Making large sheets of iron/steel suitable for plate armour benefits greatly from large forges, and industrial tools such as large water-powered hammers (see https://www.reddit.com/r/AskHistorians/comments/bwmt7v/in_medieval_fantasies_blacksmiths_are_depicted/ for some discussion of this by u/WARitter and myself.)
Not so relevant to armour, but casting of bronze, silver, gold to produce various small objects can also be done with a portable toolkit.
A greater difficulty is trying to make iron/steel on the move. Ore needs to be collected, charcoal needs to be made, a furnace built, and then the ore smelted. The classic furnace used for smelting in most regions was the bloomery furnace, which benefits from large size (typically, steel production is more efficieny, compared with a small bloomery which is likely to produce only low-carbon iron). If one was to stay in one region long enough, with available iron ore (e.g., bog iron) and trees (to make charcoal from), this can be done by nomads. However, the grasslands where pastoral nomads commonly live are often quite treeless. Where forested regions border the grassy steppes, they are often much better suited to iron-making; such areas were important sources of iron for the steppe. For example,
discussed iron-making in the Altai, both smelting and smithing.
The iron can be obtained by the nomads either as tribute or trade. Similarly, urban areas near the steppes could provide iron through tribute and/or trade. Finishing metal objects also came through trade and tribute; legal bans on selling weapons to steppe peoples shows that this trade (a) included weapons and (b) the rulers of the states such weapons came from would have preferred that it didn't include weapons.
To return to your point about heavy armour, it's worth noting that "heavy armour" doesn't require iron. One common resource for pastoral nomads is animal skins, and thus rawhide was often the material of choice for armour. While rawhide armour needs to be quite thick to provide reliable protection against arrows (Japanese rawhide armour typically use lamellae (kozane) 3mm or more thick, and these overlapped so that incoming arrows faced a minimum of 6mm), it can provide such reliable protection. Iron appears to provide better protection for a given weight (see https://www.reddit.com/r/AskHistorians/comments/9wh17q/armor_rawhide_vs_leather_vs_hardenedboiled_leather/ where I reply to a question by u/Rough_Dan on the comparison of iron with rawhide - it looks better, but the results come from very different tests).
