Silver soldering cast iron

A friend has asked me if I can repair on the cast iron grid from the top of her range cooker. New parts are not available, so few options. The grid is in the form of an outside frame, with internal arms to support pans. The frame is broken in half. The section of the frame is about 12mm x12mm. There have been a few threads on the forum which suggest that cast iron can be silver soldered if it is first heated to red to burn off the carbon from the surface, so I am planning to give this try. The reason for the post is to seek out advice as to whether I am wasting my time trying to effect a repair with solder. If not, would it be better to tin the broken surfaces first, then bring them together and reheat, or to flux them and run in solder after bringing them together and heating.

Advice from anyone who has successfully repaired cast iron would be appreciated.

Mike

Hi Mike i have recently brazed a couple of cast iron iron pieces. The first was a guard for a gear and the second was a base for a model steem engine which was incomplete when cast. The pieces needed heating to quite a high temperature and you will need a hearth to contain the heat. The second was a brazing rod and I had some from when I worked but I don't know what they are as the cardboard tube disappeared ages ago. The third is the flux and I used easy flow which worked well and finally a good torch with enough heat output, I used a bullfinch propane cylinder. I heated the parts and after a cooling time quenched them in a water bath and then gave them a good wire brushing. Then fluxed and packed to make sure they could not move heated to a bright red heat and they flowed lovely and when slowed go cool slowly cleaned up a nice joint. If you want any of the brazing rods I have drop me a message but have a go on some scrap first.

David

Edited By David George 1 on 28/08/2021 14:41:26

I use cast iron rods (nickel) routinely in my day job repairing castings no preheating just a stick welder or after treatment…just grinding back.no failures

Manganese bronze rods are good for cast iron, used to use them on exhaust manifolds.

I just weld it with a Lidl gasless mig

I've done a vice or 2 and bits on my older Drummond

The key seems to be zapping it with enough heat via a series of tack welds

12mm CI isn't much to work with though, whatever you do that's a pretty fiddly job, it probably broke because of tension stresses

Thanks all. I’m sufficiently encouraged to give it a go.

if the silver soldering doesn’t work out I’ll dig out the stick welder and try that with some suitable rods.

Mike

Mike , l have some cast iron rods and flux for gas welding . Think I also have some Sifbronze. You are welcome to small amounts of each if it helps you . Trevor.

Hi Mike,

You and David are on the right track.

When heating the iron initially, use the part of the flame beyond the blue zone. That is the oxidizing part of the flame and will burn off the carbon.

Heating time - 1 minute?

Allow to cool down to room temperature.

Stiff wire brush to clean the pieces.

Then braze normally using that part of the flame just outside the blue zone where it is hottest. When the flux melts concentrate the heat on the area to where you want the alloy to flow and apply the alloy on the opposite side.

Iron requires more heat than copper and you might find that the flux becomes exhausted. In which case, simply change it for one with a longer life.

A bit of practice, then do it. You will be successful.

Keith

I would not recommend silver solder for cast iron. Sifbronze or similar brazing material will be stronger in this application, but you will need a good torch to give enough heat and a high temperature flux. Open up or V out the break, Preheat,clean and then heat to red with flux and it should work. Stick welding rods for cast iron are VERY expensive ! Considering what the item is, IF it breaks in use will there be a hazard ? If yes then it might be better to remake in steel as a simple fabrication, brazed or better,by welding. Good luck, Noel

Stick welding rods are not expensive

a pack may be £50 but you will only use 1/2 one to a repair like Mike wants to do if that so £1 in total to repair a part which if it was available would be £50-100

if you need a rod or 2 mike let me know

Many thanks for all the good advice and generous offers of materials.

In the first instance I will try with the materials I have to hand, but if that doesn’t work I will be in contact with one of more if you to take up the offer!

Best wishes

Mike

Just wanted to say thank you again for all the advice. I used the method suggested by David and Keith using silver solder, and to my pleasant surprise it worked a treat. I tinned the 4 surfaces, clamped the two parts together, then reflowed, adding some more flux and solder. Flowed nicely and gave good strong joints. After cleaning and a coat of matt black heatproof paint the repair is almost invisible

Mike

Hi Mike,

Pleased to hear of your success.

A triumph for the argument of how when using the right materials correctly in a simple process you achieve what others consider impossible.

You have a good, strong, neat joint requiring no dressing for an aesthetically pleasing result.

You have demonstrated just why the brazing process has been around for 5,000 years

Well done. I salute you.

Keith