How can I reduce the heat on these pipe soldering pliers?
They appear to be rated at 220W/125VAC, and if that's true you can easily use a dimmer-type device. Most of the ones designed for wall mounting are rated at 600W, but they're not very appropriate without a lot of work to make them enclosed, grounded and safe. 

Most of the plug-in ones are two-wire (ungrounded) and/or rated for less than 220W so again inappropriate. 

This particular one: 




  Impact D1000 1,000W AC Dimmer Control (110-120V)


Is rated at 1kW, so plenty of power, and has a proper grounded plug and cord.