Brazing of free cutting steel?

[johan]

For my project I have to replace a few castings. So I have the choice of:
- build up constructions by welding or brazing
- mill and turn from solid blocks
- combination of both to reduce swarf and/or complicated constructions and setups

Depending on the part either solution is possible. But if I have to mill/turn I would prefer to start from free cutting steel like the 11SMn(Pb)30 that I can relatively easily buy. When this has to used in a construction, welding seems to be not a good solution. So the question is can you braze it with standard amateur equipment being a propane torch and the normal silver solders? I have my doubt because of the sulphur.

This is not for parts of the boiler or so. It is for part of the engine, motion, bearing holders etc. Not exactly cosmetic parts but also not heavily stressed ones, it is a small loc after all.

[ettingtonliam]

Well, the experts may well tell you not to, but most of my steel stock is free cutting, and I've silver soldered small fabrications without any apparent ill effect. There again, my standards might not be of the highest, but generally what I do works.

[johan]

The only standards I have are:
- will I be able to pull it off?
- will it work good enough without failing its function?

Other than that? Oh yeah... "it won't take too much time will it?"

[chris vine]

I have silver soldered plenty of free cutting ms. To start with I had a lot of trouble, but then I changed flux and it became a piece of cake..

[johan]

Care to share what kind/brand of flux and silver solder you used?

[kipford]

For Silver solder free cutting mild steel I use CUP Alloys EF Flux, had no problems to date.
Dave

[chris vine]

Sure, I will have to go and have a look. But I am sure Dave is giving you good info with the CUP EF flux.

Chris.

[jasonb]

As there is often some bulk to steel fabrications you want a high temperature flux that will last longer and not become exhausted before the parts are upto temperature. Tenacity No5 or CuP's HT5 are the best option to use on steel unless it is a very small item.

The round boss on this is leaded steel



As are the spokes on this

[johan]

Thanks for all the information. This will help deciding the materials to be used.

[Roger]

Just a few comments. Any Steel quickly gets a thick layer of Black Oxide that's a real pain to clean off. I keep the Silver Solder from spreading by using thin wire, formed to fit the part closely to the joint. I also paint around the joint with Nail Varnish for that purpose. A thin wash of flux over ALL of the part helps keep the Black Oxide at bay.

If you do the above, you will only have to get the assembly just glowing a dull Red and the Silver Solder will flow into the joints. If you apply the Silver Solder while it's hot, you'll have to get it a lot hotter and keep it there while you apply the Silver Solder all around. The lower the temperature and the shorter the time it's hot, the less Black Oxide you will get and the easier it will be to clean up.

Don't leave Steel for any length of time in Citric Acid, because it will eat it away!

[jasonb]

I'm the opposite and seldom place solder onto the job preferring to feed in the stick but do use it occasionally when I've a short bit of solder to use up. I rarely mask the bare metal and find 20-30 mins in brick acid gets it clean. Take this one for example before and after a pickle and the black scale just drops off along with the HT5 flux. Doe sdepend a bit on what you are making.





Citric will take a fair time to attack steel, the brick acid I use will start to attack it a lot quicker but this can also be used to advantge. I often make steel fabrication to replicate castings and a bit of surface pitting can help give a part that "cast look" and make it less like a barstock engine.

If you have access to a small bead blaster then that is also a good way to do the final clean upon fabrications and will take an acid cleaned parts like this ( that is a copper deposit from the acid not rust colouring the surface)





To clean metal like this. The square base of the second part is Sif Brazed on which needs more heat, the rest silver soldered

[Roger]

In the end it all depends on where you want to spend your time. You can mask, protect and put the exact quatity of Silver Solder where it's needed and have very little to clean up, or do less preparation and spent the time cleaning up the overspill and oxide afterwards. Both methods work fine. However, the smaller the parts, the more I'd err towards the former, because it's easy to end up with way too much Silver Solder on the joint which is a pain to remove.

[johan]

All good points and nice photos.

When using soft solder (like 2% silver-tin) I prefer to put small parts of solder on the piece. But with silver solder I have the habit of keeping it as a rod. Mostly because the flux has the tendency to push the silver solder bit away. I'm not overly concerned about the look of the piece itself. Haven't find very thin silver solder wire yet. Thinnest I found was 1mm.

For steel I use HCl to get rid of the scale of hot rolled steel and rust. Works well with a hard plastic brush. But stinks and you have to keep it out of the workshop otherwise everything is covered in a thin layer of rust.

[jasonb]

Wire is available down to 0.5mm dia. Another thing you can do is Hammer a larger rod flat and then cut the resulting thin end into strips with tin snips.

Yes it is harder to keep small pieces in place, winding a ring of wire around a part can help. If joining near finished parts together that may possibly be threaded or drilled then a resist is more useful than if you are just fabricating a "casting" that will be finish machined after soldering.

On larger jobs that may take a while to get upto heat I often find that preplaced solder will melt before the rest is hot enough and just form a ball on the surface which can roll off. Also if the two parts being joined are of greately differing section the smaller one will tend to get hot first and the solder wick onto that and away from the other part. And on something like that frame structure I showed above which was done in one heat stood vertically there is no easy way to preplace solder along those corner joints. These are the reasons I like to feed the rod in 99% of the time.

The brick cleaning acid I use is Hydrocloric and as you say Ideal for removing mill scale from hot rolled steel as well as pickling parts.

Scale is another thing that comes into it. The small parts Roger has been showing over the last couple of years are what 1/10th scale? where any fillet on an original casting will be smaller than what I tend to work on which could be anything from 1:1 where I'm replicating an old out of production model or fractional horse power engine to typically 1/3rd to 1/5th scale on say a hit & miss engine where a good size fillet from the solder is desirable. I will also tend to go over the fabrication with a dremel or die grinder to rough it up a bit to more closely replicate a casting so a bit of variance in fillet etc all adds to the effect.