Just playing around with a design.
I imagine the arm has various heaters built-in to keep the metal warm, so the heat difference doesn't get uncomfortable for the Pyronite; or even damage their crust by sapping heat from it.
The chest thing is based on that one Heatblast action figure where you could see into his chest. The idea here is that the pyronite lost some of the crust around their chest (maybe fighting a crabdozer?), and needs the chamber to maintain a highly oxygenated atmosphere around their exposed inner membrane.