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Silver soldering Cast iron

JoeCB

Subscriber
Last Subscription Date
03/19/2020
Anyone had success silver soldering cast iron ? I have a small iron casting with a broken off "ear". Low stress feature on the part. A clean break, looks to be a perfect application for silver solder.

Joe B
 

Axehandle

Registered
I am by no means an expert but i have never had bad results silver soldiering small or precision parts together. Cast iron works fine as long as the joint does not have a lot of surface area, very little gap, is very clean, and well fluxed. You probably don't need to but i would strongly suggest preheating both parts. I use Harris #45 (i believe) silver soldier and stay-silv flux but others should work just fine. Use just enough soldier to fill the gap so it does not bleed over the edge; if done right it will be hard to see the repair, use too much and the repair area will shine like silver and on rough cast would be hard to hide while keeping the original finish. Flux coated brazing rod works very well on cast too. Last keep your heat even across the repair (use the soldier as a temperature indicator) and when done let it cool very very slowly. Should work great.
 

PaulGray

Subscriber
Last Subscription Date
12/24/2019
I’ve also had good luck with the Harris alloys and flux. In my case, joining cast iron to steel. I used the 56 alloy, sta-silv flux and MAPP gas torch. Echoing AH, cleanliness is a must on the prep.
 

cobbadog

Registered
I have never done any silver soldering but could this be done in a similar way to normal soft solder and put some solder on both pieces and 'sweat' them together to minimise the amount of silver solder being used?
Sorry if it is a silly suggestion but if it works then it should be neat.
 

I like oldstuff

Registered
Last Subscription Date
11/09/2015
Sweating them together is a bit difficult. But what will work on a clean not prepped break where you want to put two parts back together is the silver brazing alloy in a paste form. It's the allloy mixed with flux. It's not good for filling gaps though.
Two flat surfaces can be joined with a thin sheet of alloy coated with flux.
Round things like hydraulic fittings are done with a preformed ring and hand applied flux.
 

JoeCB

Subscriber
Last Subscription Date
03/19/2020
Thanks guys for the responses. For cobbadog… the technique for silver soldering is essentially the same as for soft soldering, just different materials ( filler and flux) and MUCH higher temperature ( like red heat )

Joe B
 

PaulGray

Subscriber
Last Subscription Date
12/24/2019
Actually you liberally apply the flux to the areas to be joined and then heat until you see the flux liquefy and spread out (no tinning required). Then the solder is applied and it will spread throughout the area in question. The 45 alloy has a liquidus of ~1370F and a fluidity rating of 6.5, the 56 a liquidus of 1206F and a 8 fluidity rating....
 
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