Heat the steel. Use your torch or soldering iron to warmth the metal object adjacent on the be a part of, starting on the decrease finish of the workpiece. Brass radiator what variety solder and flux? I have an aged brass radiator. It developed a pinhole from the header tank https://elliotrtvvt.blazingblog.com/27344068/5-simple-statements-about-types-of-solder-to-use-on-aluminum-radiator-explained