Cast Iron to Mild Steel

[Chris CrewParticipant @chriscrew66644]

Could anyone tell me if it is possible to braze/silver solder cast iron to mild steel, please? The situation arises due to discrepancies in a pair of castings or my stupidity in blindly following the drawing’s dimensions without first checking the machining allowance, or lack of it, on one of the castings. I have got enough brazing spelter and flux to last a lifetime plus a small stock of Easyflo No2 (cadmium version) but have been advised this fix is not possible. Is this true?

[BazyleParticipant @bazyle]

Yes but be aware of thermal expansion / preheating requirements and check your flux works on CI.

[JasonBModerator @jasonb]

Yes it can be done, heat the CI first to bring the carbon to the surface and then when cool clean with a wire brush. You can then flux and silver solder, unless it is a small casting I would suggest HT5 (Tenacity No5) as it can be heated for longer.

I’d also be tempted to cut the repair piece from CI bar rather than steel as it may be easier to machine with no risk of drills etc wandering into the softer iron

[Tim StevensParticipant @timstevens64731]

Perhaps I can chip in?

The answer really depends on how reliable the finished soldered joint needs to be. If you are merely adding (or putting back) a bit of extra thickness then this may not be a concern, but if the new joint is going to be subject to tensile forces or impact, do consider the results of possible failure. A soldered joint can appear to be ‘stuck firm’ but you cannot test it without risking destruction – this being exactly what you need, if the joint is not strong enough, but not good news, of course.

Cheers, Tim

[bernard towersParticipant @bernardtowers37738]

I have just made a built up piston in cast iron where the gudgeon pin boss was silvered to the piston to make it lighter and so it has held up to 6500 rpm!!!

[David George 1Participant @davidgeorge1]

hi I have repaired a casting which came with metal missing and had no problem inserting a piece and machining it after so it was not visible after painting. I cleaned up the area flat on the mill, machined a piece to fit with a small overhang each side and plus metal on top so it could be finnished to size after brazing.

 

 

 

 

David

 

 

[bernard towersParticipant @bernardtowers37738]

Thats a good save David.

[noel shelleyParticipant @noelshelley55608]

You will need plenty of heat and flux, I would braze, silver solder is to fluid and is unlikely to give a fillet or fill a hole. Good luck, Noel.

[JasonBModerator @jasonb]

Sounds like the OP took too much off a surface and wants to add some thin steel sheet to build it up so having the silver solder flow into the flat mating faces would be ideal

If it’s good enough for Mike Sayers Bently engines then should be OK for other models. You can just see where the silver solder has flowed and left a small neat fillet at the bottom of each of the cylinder “tubes” He did a good write up on the proces sof soldering that block in MEB magazine.

[duncan webster 1Participant @duncanwebster1]

If its more or less cosmetic, can you use epoxy. Araldite in old money, or JBWeld, which seems to have a good reputation.

[Chris CrewParticipant @chriscrew66644]

“Sounds like the OP took too much off a surface and wants to add some thin steel sheet to build it up so having the silver solder flow into the flat mating faces would be ideal”.

The problem is actually on a steam chest cover for a ‘Bridget’ locomotive. A part-built model was kindly donated to the club which I belong and it was decided that it would be completed as an educational project for younger and less experienced members. I was allocated the part machined cylinder castings (they had only been bored and faced) despite being more of a ‘tools & attachments’ man, not a locomotive builder. The steam chest cover castings had been machined by the previous owner and were partly out of truth and already drilled. It was obvious that they were too small to cover the as yet un-machined steam chest cavities and it was assumed this was because they had been incorrectly machined. It was my suggestion that the ‘skirt’ should be machined off and the remaining part of the casting brazed into a mild steel frame to make up the deficiency. It was at this point that I was advised that this would not be possible and the decision was taken to purchase some new steam chest cover castings. However, the cylinder castings themselves are not exactly symmetrical, which is where I probably made an error in not checking the allowance carefully enough before machining the cavity. Now the new cover castings have arrived, even un-machined, it is obvious one is not going to be wide enough to allow satisfactory bolting to the cylinder casting. Hence the need to revert to my initial idea to try and retrieve the situation. (I need only to turn the air blue when I mess up my own property, but I feel very uncomfortable if I have made an error on someone else’s property, so I need to try to put it right).

[RolsterParticipant @rolster]

With the steam chest covers they don’t see much force but its acting like a diaphragm, at a very zoomed in level, as steam pressure dips and rises with each stroke.

More importantly they do see temperature rises and falls at the beginning and end of each run. I would double check the expansion rates of the material proposed to connect to the central cast iron cover part and match it as closely as possible to avoid any cracking in the future.

Ideally if one cover is going to be a problem due to being under sized, forget the casting and replace it with some cast iron stock machined down to suit the pattern and size desired. You want to keep the chest covers ideally the same material as the cylinders, cylinder covers and pistons themselves to avoid any expansion and contraction issues.

BR Roland

[JasonBModerator @jasonb]

Well if expansion rates are felt to be an issue cut a bit of iron off the old covers and solder to the new.

What do the covers look like? is a casting really needed? A slice cut from CI bar may suffice once squared up. I’ve made many a steam chest cover that way not to mention the chests and even cylinders.

[BazyleParticipant @bazyle]

The expansion etc from just steam heat is not a problem. It is the more extreme brazing temperature change that might be, and also with larger pieces than this CI doesn’t like one end being red hot and the other end cold.
However start again with CI eg from an old car disc brake if needing to save money. You don’t want mild steel here as the residual condensate after every run, despite oiling, will make a nasty rust mess in a few years.

[JasonBModerator @jasonb]

I found a photo, it’s only a “box” with a flange around it. rather than faff about with the replacement castings I’d send them back and either cut from solid CI or fabricate it from bronze or brass. easy to silver solder and no rust issues. Bit like this which is the same sort of size

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