Can a small transformer be used as a welder? I want to solder some resistance leads.

Ordinary transformers are strictly forbidden to work in short-circuit conditions.

And the welding process is always short-circuit state (resistance welding is a direct short-circuit; fused-pole welding, such as electrode hand-arc welding is the contact arc, and then through the ionization of the air to form a short-circuit of a welding method).

This will burn out the transformer.