It's called Armor.

What do the Rules say? I think the applicable rules are armor rules. The intent behind wrapping chains around yourselves is protect yourselves. That is the job of armor.

Chain fashioned around your self are similar (enough) to Chain mail or ring mail, both are heavy armor. Chain mail requires strength of 13 and imposing disadvantage to stealth. For that you get an AC of 16. Those chains have no HP, and no mechanism for it to be destroyed for simplification reasons.

Adding other objects to the character to act as armor isn't necessarily outside the rules, but isn't something the rules consider. Which means that what happens is left up to the DM. The most appropriate rules for this sort of thing are the armor rules, not the object HP rules. Adding armor in this fashion should affect AC, not add additional HP, and depending on the amount of chain likely have stealth implications.

Object Rules

The barrel is what item HP are meant for, a situations that happens to come up where someone needs to destroy an object. The rules on objects are for objects, not armor. The rules for objects can be found in the SRD ( official ) in Game Mechanics under Objects. Reading them it is clear that the intent is an item caught in the cross fire, or the characters try to destroy an object. It isn't meant for items that are worn to provide temporary hit points.

This path might inspire your DM to employ armor/weapon damage house rules. Your swords might suddenly break or shields shatter. That is the logical extension of applying these HP rules to all things in the world.

Ultimately, it's the DM's Call

To apply armor rules or object rules is the DMs choice. Armor rules are the ones intended for defense, but there is nothing preventing him from applying object rules the way he has already.

Locate Object can find 'any' object as long as there's no lead between the caster and the object.

Even if the object is lead.

The material make up of the object in question is never specified, the only specific limitations is if there's lead between you and it, its over 1,000 feet away, or if you don't know the specific object, then you're only able to locate the nearest object of a particular 'kind'.

Full spell text:

Describe or name an object that is familiar to you. You sense the direction to the object’s location, as long as that object is within 1,000 feet of you. If the object is in motion, you know the direction of its movement.

The spell can locate a specific object known to you, as long as you have seen it up close—within 30 feet—at least once. Alternatively, the spell can locate the nearest object of a particular kind, such as a certain kind of apparel, jewelry, furniture, tool, or weapon.

This spell can’t locate an object if any thickness of lead, even a thin sheet, blocks a direct path between you and the object.