Model Engineers' Workshop|December 2019
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!
Dependent on the size of component, the flux being used may become exhausted and stop working. The surface re-oxidizes and the filler will not flow so no joint is made. If you experience this, simply change your flux to one that will accommodate the longer heating time. HT5 is a suitable flux.
Most model engineers will recognize another option and move to increase the heat input. If you are using a propane/air torch, fit a bigger burner or increase the gas pressure by adjusting your regulator to increase the gas flow. More heat is generated simply by burning more gas. Double the flow rate of propane through the burner and you will double the heating effect. If you want more water to flow through a pipe, you would increase the pressure. Right?
However, a common solution taken up by the model engineer is in the belief that hotter flames produce more heat. While this may be true, it is not the whole story. Consider the effect on the skin of a spoonful of hot water, at say 95°C, to that of a steel spark at 1300°C! One involves a visit to the local A&E, the other is a minor irritation.
There is a lot more heating capacity in the water at the lower temperature.
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