Easier to adjudicate than that though using the rules. The hinges are
smalltiny metal therefore have 15 hp. Hit the AC 810 and if you do 15 hp, the hinge is slag.
Well, the RAW Damage to Objects rules assume they're destroyed and/or rendered useless.
Rules Compendium page 176 among other sources
An object reduced to 0 hit points is destroyed or otherwise rendered useless. At the DM’s discretion, the object might remain more or less whole, but its functionality is ruined—a door knocked from its hinges or a clockwork mechanism broken internally, for instance.
I suppose if you want to extend the rules of taking NPCs to 0 HP (eg you can make them unconscious rather than dead/destroyed) to objects and then negotiate the potential of their new state on their environment (process-like) and its derivative effect (eg door is welded shut and impassable - save ends), you could absolutely run it that way.
I wouldn't see a problem with that. Its just not the classical way to do it in 4e, but it certainly gives folks who are after process a way to handle things. If I was after process, I would make them roll an Arcana (for process purposes) as well to control the flames and get the temperature high enough to produce the results. The default temps of something like Burning Hands (process-wise) would be insufficient to weld/fuse.
EDIT - I typed balance, I meant process.