After too long I've kicked off my next project, a slide valve steam engine this time. I need to fit two 1/8 inch copper pipes as drains to the cylinder. For the purposes of the question assume the cylinder is 40mm diameter 50mm long and 30mm bore. The pipes will be at the bottom of the cylinder at each end opposite the steam ports.
Any problems silver soldering these pipes into the cast iron? (or any special steps needed, I've heard I should should use copper oxide as a flux for CI)
The worst difficulty with silver soldering cast iron is when you have to heat it unevenly in order to get one part of a casting hot - if you can heat the whole casting at once then there should be no real problems. Just heat it, and cool it, slowly so as to prevent cracking - cast irons do not have high thermal conductivity, and can have a high coefficient of thermal expansion over some temperature ranges.
Heating cast iron above 700 C for any length of time can change the cast iron - the chemistry of this is complex, and depends on the cast iron, and I don't know that much about it, you'd have to look it up if interested - but for silver soldering there is no need to heat it up that much.
The next difficulty is getting good wetting. This depends on the grade and type of cast iron more than anything else. If you can tell me what type of cast iron it is I may be able to offer more detailed advice.
If there is graphite on the surface it may be hard to wet with the molten silver solder. One method for getting rid of the carbon is to use an oxidising flame in an oxy/gas torch, if you have such. If not, first give it a try anyway - IIRC steam engine cylinders are SG cast iron, which wets fairly easily anyway.
I'd suggest preheating the whole casting _slowly_ till it's about
300-400 C, then silver solder away. You can probably get away without preheating, but don't blame me if anything goes wrong (come to think of it, blaming me if something goes wrong after preheating won't do you much good either... :-)
I've never heard of copper oxide brazing flux. There is a silver/copper oxide brazing material used for ceramics, but it's a bit exotic, and not really suitable, for cast iron?!!
As Peter says, the grade of cast iron, and the amount of free carbon in the form of graphite,
has a big influence on whether satisfactory soldering is possible. SG Iron is a lot better than ordinary grey C.I.
We also have tested the possibility of Nickel and Copper plating Cast Iron to get a better
bond, but it is no big help:
Average Chalmers bond strength on cast iron: 53 N/mm2.
Average Chalmers bond strength on Ni plated cast iron: 54 N/mm2.
Copper plated cast iron was worse: 44 N/mm2.
We also tested various preparation and tinning methods on plates of SG Iron.
The best tinning method was shown to be shot-blast with sharp angular iron shot,
followed by pre-fluxing with Zinc/Ammonium Chloride.
The shot blast process removes some of the surface graphite and also produces some surface cracking of the cast iron which can provide some mechanical keying for the solder.
Some improvement is possible with Ferritizing annealing of grey cast iron. The cast iron comprising graphite + pearlite is heated up to approx. 800°C and is held for a while so that the microstructure is given sufficient time for the transformation into -Fe + graphite. If the subsequent cooling-down speed is slow enough, the microstructure forms as -Fe + graphite and will solder more readily.
You can also shift surface graphite with hot caustic soda solution but thorough rinsing is vital or the caustic residue comes back to cause big problems later.
Thanks guys. Given the raw cost of the casting and the beautifully machined bore and the time taken to get there, a 1/4 tap is looking like a solution...
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