A mail to an electrical expert I know is below:
The soldering iron is designed to work from a nominal 12-volt D.C. supply. Nominal means that there is a safety margin designed in usually around at least 10 percent. In any case the element that causes the tip to heat up is similar to the element in your electric fire or kettle. Suitable for the purpose would be a high resistant alloy of metals used so that the tip gets hot enough to melt solder but does not glow red, that would be too hot and burn the job.
You would need to up the voltage to around 18-volts D.C. to cause serious damage but there is probably a fusible link built in to fuse at that level. If you hear the iron clicking now and then it will have bi-metallic link to control the temp of the tip within 10 degree or less, the more expensive irons are temp controlled by electronics in the iron or its base station.
Hope that helps.
And just in from Justin:
! I understand – I suppose it’s the same reason 12v light bulb doesn’t melt – the current allowed through is determined by the resistance – plus of course the factors Notty mentioned like thermostats etc.