Soldering iron for hakko t12 tips on 12v
Without following the links, your basic problem will be lack of power. The maximum power out of the iron is given by \$ P = \frac {V^2}{R} \$. If you halve the voltage you will only have a maximum of 1/4 of the iron's rated power.

The iron will heat up to the point that the rate of heat loss from the iron is equal to the power input. At 1/4 power I suspect that this may be below the melting point of your solder.