Stay Brite solders bond with all of the ferrous and nonferrous alloys. Harris 1/8" 8 oz. Stay-Brite Silver Solder Lead Free SB61/2POP. Industry standard solder for HVAC connections. Engineered to provide strong, ductile connections on copper, brass, steel and stainless steel.
Extremely useful solder for joining most metals at relatively low temperature
I encountered stay-brite solder when trying to join stainless steel. Stay brite does an excellent job with stainless and had other advantage that it doesn[t require heating the metal enough to weaken the base metal. It its also useful for stainless ro mild steel and for most ferrous and non-ferrous metals (brass, copper, etc). It does not do a very good job with aluminum. Also note that you need a flux. I have always used Haris's stay-clean liquid flux with this solder. This is s bit spendy though, was it is a few percent silver.