In these tough economic times many commodities are increasing in cost. This is especially true of the precious metals such as silver, gold and platinum. As of this writing (August 2010) silver is trading at $18 per ounce! That is remarkable since just seven short years ago, in 2003, it was nearly down to $4 per ounce!
Now you might be asking yourself, “What does this have to do with brazing?” Well if you are brazing stainless steel, everything. You see many of the recommended alloys for brazing stainless steels have varying percentages of silver in them because this lowers the melting point of the filler metal to around 1100 degrees Fahrenheit and flows very thin to get into the tightest spots. The other advantage is the higher silver alloys provide color matching of the stainless alloy being joined. The main disadvantage is the cost of these special brazing alloys. Some are $90 or more per pound. A suitable substitute must be found. For a strong high tensile strength joint, there are nickel silver alloys containing no silver at all. They are actually copper alloyed with small amounts of nickel. These alloys are only slightly more expensive than the regular bronze brazing alloys and melt at a little lower temperature.
For artistic purposed though, really high strength is not need so long that BOTH sides of the joint are completely wetted by the filler metal. With 304 or any other stainless alloy this can be a nearly impossible because at the temperature that bronze filler metal melts( around 1700 F), stainless steel forms a tenacious skin of chromium and nickel oxides which cannot be removed by commonly available acids like zinc chloride, hydrochloric, or sulfuric acids. They need a powerful and reactive flux containing fluoride. These are commonly available in half pound jars and are called high temperature brazing fluxes. They contain the typical borax along with varying percentages of potassium fluoride. These are toxic chemicals that demand the utmost care and respect in there handling. Unlike many typical brazing operations, such brazing clean mild steel, we will be using a lot of flux. This flux immediately dissolved oxides as they form keeping the metal shiny and the bronze flows right into the joint with ease. The fluoride component also allows for easier flux removal when the brazing is done. Just run under hot water with a wire brush.