Brass expands more than steel. When joining materials with differing coefficients of expansion consider two things.
The required joint gap should be at the brazing, not room, temperature.
When the joint cools, the joint is put under compressive stress not tensile. The solder is, in effect, being squeezed. If possible make the female element of joint steel
Not possible?
Use a larger joint gap than usual. The extra volume of alloy will help to accommodate the thermal stress.
Cool slowly.
Re soft iron wire. Fluxes remove oxides (rust). Keep the two apart or you will add a new feature to your assembly!
For more detailed info go to **LINK**
Keith