Silver soldering cast iron

partsproduction · Oct 5, 2017

For some reason I'd always believed silver solder would not work on cast iron.

Recently I decided to try it (because of something someone here said in a thread BTW), I took some ductile cast iron and cut it straight across with the vertical band saw, then painted both surfaces with black flux.

I already had silver solder sheet (56% silver content) so I cut a piece to fit the cut area after sanding the tarnish off. Then I painted black flux on that too, both sides, stuck it between the two pieces so gravity held the top onto the bottom piece,and heated it to a very low red temperature.

I couldn't believe I'd made my life so difficult because of an untrue concept, that the carbon content would stop it from tinning. The solder line between the halves is very thin, and I took it to the vise, clamped the bottom and adjusted a 14" Crescent wrench to see how much pressure it took to break the bond.

It would not break!

So I proceeded to repair the cast iron dividing head that got broken in shipment.
I guess I'm just too dumb to know I can't do that.

But seriously, I DID think it wouldn't work! Have any of you tried this? I just told an old machinist about this and he said it can't work, he's been in the trade even longer than my 35+ years, and is convinced only that low temperature "silver solder" will work.

The part I fit to the dividing head was ductile cast iron which I keyed to fit a slot in the broken casting after milling the break flat. I allowed just enough room for the silver solder sheet which is .005" thick.

Am I missing something here? BTW, I put this here because it's so common to have to repair old broken cast iron parts. I hope it's OK.
parts

Monarchist · Oct 5, 2017

partsproduction said:
For some reason I'd always believed silver solder would not work on cast iron.

Recently I decided to try it (because of something someone here said in a thread BTW), I took some ductile cast iron and cut it straight across with the vertical band saw, then painted both surfaces with black flux.

I already had silver solder sheet (56% silver content) so I cut a piece to fit the cut area after sanding the tarnish off. Then I painted black flux on that too, both sides, stuck it between the two pieces so gravity held the top onto the bottom piece,and heated it to a very low red temperature.

I couldn't believe I'd made my life so difficult because of an untrue concept, that the carbon content would stop it from tinning. The solder line between the halves is very thin, and I took it to the vise, clamped the bottom and adjusted a 14" Crescent wrench to see how much pressure it took to break the bond.

It would not break!

So I proceeded to repair the cast iron dividing head that got broken in shipment.
I guess I'm just too dumb to know I can't do that.

But seriously, I DID think it wouldn't work! Have any of you tried this? I just told an old machinist about this and he said it can't work, he's been in the trade even longer than my 35+ years, and is convinced only that low temperature "silver solder" will work.

The part I fit to the dividing head was ductile cast iron which I keyed to fit a slot in the broken casting after milling the break flat. I allowed just enough room for the silver solder sheet which is .005" thick.

Am I missing something here? BTW, I put this here because it's so common to have to repair old broken cast iron parts. I hope it's OK.
parts

There is at least one thread - right here on PM - about how WELL it can work. Brown flux, not black IIRC, but it wasn't a current need for my use, so I didn't keep track of it.

JST · Oct 5, 2017

The usual reason I know of for CI being impossible to solder is oil in the pores of the metal.

I found that it will work, if you cook all the oil out first, and burn off the carbon from it. May take several heatings with something like the sliding block from a shaper, which I was adding a thin wear strip to, but it worked. I suppose I could have simply made another, but I didn't have a handy block of CI at the time.

If anything was going to be full of oil, that is it, as it runs covered in oil.

L Vanice · Oct 5, 2017

Nobody ever told me it would not work, so I have been silver brazing gray cast iron for five or six decades, both iron to iron and iron to steel. I have always used white flux and have never owned any black flux. I use cadmium-free silver alloy wire, not sheet. I prep uneven broken surfaces in my blast cabinet using a mix of aluminum oxide and glass beads. Flat surfaces get milled or ground, then blasted. I always get a good bond, as strong as the parent metal.

If I have a broken piece that is difficult to hold together while heating, I will drill small holes through both pieces and tie them together with stainless or plain steel wire through the holes. the wire gets fluxed and brazed along with the joint. After cooling, the wire is cut off and ground down flush. These parts are usually painted after repairing.

Larry

partsproduction · Oct 5, 2017

covered in oil.

I suppose that's true, it goes deeper than superficial. The part I soldered on was not a part running with oil. The cast iron I soldered to it was commercially available ductile cast iron, likely free of oil.

Would more than one cook off be needed? One to turn internal oil in pores to oil coke and the next to burn that out?

More experimentation is in order. However, if there is a fresh break and it's possible to key the two surfaces together beforehand and then solder it the key itself would have to have a holding effect, and such a key was just my answer to keeping the parts aligned during soldering.

Two parts can be pressed together and a hole drilled down the break line which would act as a key too when a piece of mild steel rod is added. That would serve both purposes, alignment and added strength.

billzweig · Oct 5, 2017

Some types of cast iron are more difficult to solder than other. In general one should avoid silver solders containing phosphorus (usually the high % copper solders). Solders with some zinc and 40-50% silver work the best (for me, anyway). And yes, oil in the iron can make silver soldering or brazing very difficult even after good burn out.

Jeremy · Oct 5, 2017

This brings up a couple questions. I tried silver soldering a cast iron casting for a vintage Trahern pump that I have. I was led to believe that I could flux the joint really well, tip the whole assembly on its side and just lay a piece of the silver solder across the joint. Then heat to red head with a rosebud and it would pull the solder into the joint. While it looked good, I knocked the part off a box, about a six inch drop, on to concrete a couple months ago and it snapped at the joint. The silver solder hadn't really penetrated at all. So...is flattening the solder and sandwiching it in the joint the way to go?

Fluxed up (inside joint and all around the outside, piece of solder laid on top)

Result from non-solder side of joint. What you see is from a previous attempt at a repair (the blob of silver solder).

silver-solder_2nd%20attempt_cleaned%20up.jpg

I used Harris 4531 Safety Silv 45 Silver Brazing Alloy and Superior Silver Brazing Flux #601.

true temper · Oct 5, 2017

Very interesting, keep talking guys I want to learn more.
Tom Walz talkes about cleaning steel with oven cleaner, do you think that would work?

When you burn the oil out of the pores how do you get rid of the carbon left behind?

partsproduction · Oct 5, 2017

I'll listen too, because my two experiments may have been a fluke. I bought the silver solder sheet in ebay years ago and have used it often when joining pieces of steel, for some reason it works better for me than tinning both sides and then joining them, and I suspect that there is just less chance for oxidation when there is a close silver solder sandwich.
Muriatic acid also helps when applied before flux, probably after heating to burn the oil out, but I haven't tried that with cast iron yet.
The nice thing about silver solder in a very thin line is that paint bridges it so it's less likely to show through with use, unlike braze. Brazed joints seem to show themselves over time.*
A good test would be several constant section rods either round or rectangular silver soldered and compared with calibrated bending loads against the same shaped rods that have no joints.
Normally cast iron fails when loads exceed design expectations (like when dropped or shipped) so even if the joints only reached 70% of solid cast iron that would often be acceptable for some uses.

* I understand that silver soldering is considered a brazing operation but my reference is to V'd out large area brazing here.

JST · Oct 5, 2017

true temper said:
.....
When you burn the oil out of the pores how do you get rid of the carbon left behind?

It took several times of heating very hot to get the block to where the solder would stick.

This was not a case of a break, but a surface that had been saturated in oil. There was no relatively clean area the way a break has a previously untouched surface.

johansen · Oct 6, 2017

it took quite a bit of scrubbing the cast iron half nuts from my southbend before soft solder would stick to them.

i was scrubbing the half nuts with a brass wire brush while applying zinc chloride flux while heating them just hot enough to melt the solder. while i know its both faster for the flux and braze to attack the metal at higher temperatures, its also faster for the surface to oxidize, so i have no doubt an equal amount of time would have been required had i tried to do the same operation at red heat with a silver braze.

some time ago i pulled some mystery cast iron parts (i suspect from a boiler or furnace) out of a ravine, on the order of >60 years old, water had penetrated at least 1mm or .050" into the cast iron and this was evident after breaking it apart.

so i've heard that oil can penetrate that far into the cast iron, and seeing the discoloration myself from water penetration i believe it.

regardless how many thermal cycles it takes to bake out the carbon, if you've got hydrogen gas escaping from trillions of points per square inch of surface, then your solder or braze is not going to stick to the metal.

billzweig · Oct 6, 2017

Oil can penetrate quite deep in cast iron. That why cast iron is used for bushings.
Another thing to remember is that silver solder must have an accurate gap over the whole joint to work properly, usually about 0.001. Too big a gap will not wick properly and too small will not allow the silver solder to penetrate. When repairing a cast iron break the two surfaces are often in too close a contact. One should arrange, file or machine those surfaces to leave a clean 0.001 gap all along. When making new parts it is easier to control the gap.
When repairing oily parts, or where in the break there are gaps, best is to bevel the joint, heat the part in a muffle furnace for some time and then to tig weld it (using mild steel rod) while hot. I did fix a number of broken machine tools parts this way. The material around the weld becomes dead hard because of the carbon, but it can be ground smooth with an abrasive disc.

Greg Menke · Oct 6, 2017

A few years ago I had very good luck with low phosphorus coated brazing rod on cast iron- in this case a set of fairly thin section cast iron gear guards for a Dalton lathe. They were under a lot of tension as installed and broke in 2 places as soon as they were disturbed, and a 3rd break occurred when the part was dropped.

I slightly vee'ed out the joints and the braze took right away- perhaps in part because they were entirely free of oil etc and the thin sections were easy to heat. I took the opportunity to adjust the fitup so the reinstalled guard would not be in tension. The repaired joints stood up to reasonable handling though the whole guard assembly was sufficiently fragile that I didn't want to break it further with more stress tests.

Jeremy · Oct 6, 2017

billzweig, that may have been my problem. Freshly broken joint that fit back together perfectly. I'll do a little filing and see if that works better.

magneticanomaly · Oct 7, 2017

I've had good success oxyacetylene braze-welding cast iron, good success stick-electrode welding it with 55% nickel rod, good success oxyacetylene welding it with cast iron rod and with old coathangers, less but not zero success silver-soldering.

Gray cast iron is iron with graphite flakes dispersed in it. Fractures will tend to occur through the graphite. So a fracture surface will have less iron and more graphite exposed. I do not know how easy it is to "burn off' the graphite, nor how much iron oxide will cover the surface after that process is complete. But whereas silver solder certainly will not stick to graphite, nor to iron oxide, good flux WILL dissolve some iron oxide..

I've had better success silver-brazing machined or filed CI surfaces than fractured ones, which makes sense since more iron and less graphite will be exposed on a cut surface than on a fracture.

Rudd · Oct 8, 2017

magneticanomaly said:
I do not know how easy it is to "burn off' the graphite,

I've been reading on this off and on for a while, I read somewhere that a certain acid used as a pickle would get rid of the graphite, but cannot recall what acid it was. I tried heating and then sandblasting on some smaller pieces and was not able to break them apart. These were all new durabar pieces, not breaks. I'd like to get a pressure tight seal between two pieces that will stand 100 psi steam.

Hopefuldave · Oct 10, 2017

Rudd said:
I've been reading on this off and on for a while, I read somewhere that a certain acid used as a pickle would get rid of the graphite, but cannot recall what acid it was. I tried heating and then sandblasting on some smaller pieces and was not able to break them apart. These were all new durabar pieces, not breaks. I'd like to get a pressure tight seal between two pieces that will stand 100 psi steam.

Could it have been Oxalic acid? It's my go-to for removing burnt on carbon from pots and pans (yeah, my cooking's that bad/dangerous...) Caustic Soda (Lye) was always SOP for de-coking 2-stroke exhausts when I were a lad...

Dave H. (the other one)

Tony Quiring · Oct 10, 2017

Jeremy said:
This brings up a couple questions. I tried silver soldering a cast iron casting for a vintage Trahern pump that I have. I was led to believe that I could flux the joint really well, tip the whole assembly on its side and just lay a piece of the silver solder across the joint. Then heat to red head with a rosebud and it would pull the solder into the joint. While it looked good, I knocked the part off a box, about a six inch drop, on to concrete a couple months ago and it snapped at the joint. The silver solder hadn't really penetrated at all. So...is flattening the solder and sandwiching it in the joint the way to go?

Fluxed up (inside joint and all around the outside, piece of solder laid on top)

Result from non-solder side of joint. What you see is from a previous attempt at a repair (the blob of silver solder).

I used Harris 4531 Safety Silv 45 Silver Brazing Alloy and Superior Silver Brazing Flux #601.

This looks like a cold or non joint.

Likely due to contaminants or lack of heat or both.

The surface needs to be coated with solder as it is a bond but this is not solder color so no bonding.

If not hot enough to melt solder it will not stick and if contaminated beyond what the flux can handle it won't stick either

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

billzweig · Oct 10, 2017

Rudd said:
I've been reading on this off and on for a while, I read somewhere that a certain acid used as a pickle would get rid of the graphite, but cannot recall what acid it was. I tried heating and then sandblasting on some smaller pieces and was not able to break them apart. These were all new durabar pieces, not breaks. I'd like to get a pressure tight seal between two pieces that will stand 100 psi steam.

Graphite is very inert and reacts with few substances. Graphite will oxidize at high temperatures, over 500C, so heating the pats to be brazed or welded helps (as well burns any oil absorbed).
Chlorosulfonic acid is just about the only chemical, but will probably react with the iron as well and could create an even bigger mess (I have never tried it). But what is really needed for brazing or soldering is to remove the semi loose and surface embedded graphite particles - those can often be seen in a clean break - and a good brushing with a rotary steel brush will do it (I do it often).
Chemicals that remove carbon (not graphite) deposit from metal (like pots) are not really dissolving the carbon, but just lifting it from the surface.

Restorer · Oct 11, 2017

The primary reason for failure to bond is "Oil in the Pours" and insufficient Flux applied.

Years ago I repaired a lathe tailstock casting that was broken. To remove the oil, it was suspended in the "Paint Line Stripper Tank" of my employer. The tank contained a boiling solution of Oakite 32 and water. Boiled it for 8 hours. The next day slowly preheated the casting to dull read, with a clean wire brush removed any loose particles and residue. While still hot applied a generous amount of flux just where the silver solder was to go. Then the casing parts were clamped together, reheated from one side and Silver Solder applied to the other. Solder flowed, clamp taken up some more, completing the operation.

The major advantage of Silver Solder over Braze is that it does not work harden with vibration or mechanical deflection. The disadvantage is its cost!

Regards,
Restorer

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