Brazing Of Steel

Keith Hale entered the brazing industry in 1969. His previous employers include Johnson Matthey Metals, and Engelhard Ind (Sheffield Smelting). He formed CuP Alloys in 1982 and since retiring in 2012 they retain him as a consultant

It is a common comment that some model engineers, although happy to braze copper, have difficulties when it comes to joining steel. The key to success for any brazing, or indeed soldering operation, is to embrace the basic principle – that of capillary flow. Let us be clear about one thing from the outset, brazing is a metal joining process which depends on capillary flow of molten metal between two closely adjacent surfaces. It has nothing to do with the filler metal. If you are not using capillary flow, you are not brazing.

Brazing is brazing whether you are using a silver solder or a brass rod.

The same principles that apply to joining copper with a silver solder are identical to those joining steel with a brass rod. The principles are independent of the material being used to make that joint. Any reader that fails to acknowledge this might just as well turn the page now!

However, it is fair to say that there are slightly different practicalities to be considered if you are to achieve the necessary capillary flow when brazing steel compared to copper.

The main difference lies in the heating technique when brazing steel.

Steels, and all ferrous materials, be they stainless, mild steel or cast iron, have a higher specific heat than copper. It takes more energy (heat) to raise 1gm of steel by 1°C than it does to achieve the same effect for copper. The values for copper and steel are shown below.

Copper 0.38J/gm-°C Steel 0.51J/gm-°C

This means that for any given torch,

it requires 25% more heat and will take nearly 25% longer to achieve the brazing temperature. So be patient!