# Soldering mild steel



## Captain Dan (Feb 7, 2008)

Hi,
I have a laser cut small bridge kit that is made out of a mild steel approximately 1/16" (1.6mm) thick. The parts are too small for welding. I would like to solder it together. (Hopefully picture uploaded)

My question is about flux and heat. My thought is to use a propane torch, soft solder and regular soldering paste. Is this a good idea or should I use something else on steel. Obviously, on brass, this would be a no brainer.

Thanks,
Dan


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## Captain Dan (Feb 7, 2008)

Let me try the picture again


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## Jack - Freshwater Models (Feb 17, 2008)

Dan, 

Any tin/lead based solder will work on steel. Liquid or paste flux will also work as long sas it is matched to the solder. Pretty much the same as soldering brass. 

Jack


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## Captain Dan (Feb 7, 2008)

Thanks for the info. I will give it a try.
Dan


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## armorsmith (Jun 1, 2008)

Dan, 

If you have the equipment, you might try brass brazing. It is stronger and may work better on the smaller parts. Brass has a much higher melting point, making it better. Solder melts so low, you may be unsoldering the last joint you made trying to make the next. 

Good luck!! 

Bob C.


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## xo18thfa (Jan 2, 2008)

50% Tin/ 50% Lead solder works fine on steel. I like the brown paste flux best. Steel must be very clean to solder. Brush or sand to bare metal, then wash with soapy water and dry off. Heat slowly, just enough to do the job.


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## jimtyp (Jan 2, 2008)

Brazing takes a lot of heat. Might be too much for the thin mild steel:  Brazing Intro  

I've soldered brass to 1/8" mild steel. Bob is correct about un-soldering the last joint you made. I use heat sinks on any joints near where I just soldered.


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## armorsmith (Jun 1, 2008)

I guess I should have been more specific. Braze the larger joints (BTW - can be brass or silver), and solder the details. 

Bob C.


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## David Leech (Dec 9, 2008)

Dan,
I used laser cut mild steel to make up a new cab for my Canadian Pacific Hudson.
What I found was, it wanted to rust as soon as I finished with the soldering.
One minute it looked great, and then after soldering I left it to cool down and I don't think that I looked at it until the next morning and it was a nice rusty color all over.
It wasn't in a particularly damp room or anything, so I guess the cleaning, and fluxing, and heat just made it vulnerable to rusting.
Other than that it worked great.
Just a good clean up and rust primer and it worked perfectly.
All the best,
David Leech, Delta, Canada


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## armorsmith (Jun 1, 2008)

David, 

Steel, when manufactured, will have a 'mill finish', which will contain some scale, oxidization, and oils. That is why you have to clean it well before soldering. This cleaning process makes the surface of the steel susceptible to rusting. When you add the heat from the soldering process, you multiply this exponentially. It does not take high humidity to make it surface rust that bright orange-ish color. 

If you really want your finish to hold well, I would recommend using an 'air eraser' (model makers grit blasting outfit) to clean the metal surface before painting. I picked one up at Harbor Freight for about US $20. Follow this with a decent primer and top coat and the paint will last a LONG time. 

Bob C.


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## Havoc (Jan 2, 2008)

- you could first tin the parts, it is then a lot simpler to solder. You can find this at the plubmers section in the diy. First a good clean, then put the stuff on it, heat and when it flows, wipe with a damp cloth. You get a nice shiny tin surface that takes solder very well. 
- I like the leadfree 2% silver solder for most things. It is a bit harder otherwise it works like ordinary solder. 
- hard soldering is an option, but I wouldn't use "brazing", it isn't as easy as "silver soldering". Brazing uses brass as the filler, silver soldering an alloy containing 10-40% silver. Melting point depends on the silver content. It will need more heat than soldering but it is stronger. You can find silver solders from 450°C to 890°C so different temps for different parts are possible. Make sure you get the correct flux. 
- rust after soldering is mostly due to the flux and/or using a gas torch. You made the surface very clean as well. The flux is corrosive (often some kind of acid) and a gas torch contains water vapour. 
- you can weld thin steel but it isn't easy. TIG would be your best option.


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## SE18 (Feb 21, 2008)

I did my first steel solder last night using silver solder (forgot what percentage) and flux. I sweat soldered it with large propane torch. It had to be heated up a lot to work. I'm still a novice at soldering steel but will try some more. It was 4 fender washers, including a larger one for the flange (I'm making train wheels). 

I've got a question. If I hold the work to be soldered in my vice or in my shop press, will the heat from the large propane torch anneal the vice or shop press? I'll await your verdict before doing any more work!


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## Havoc (Jan 2, 2008)

Whatever type solder you used, I would not do it. You already find it a lot of work to get it hot. With a few kg steel added to it as a heat sink it will only get much harder. 

Better put together a small hearth. Get a few fire bricks and lay a few at the bottom, set a few on edge at right angle to each other, maybe one on top so your work is surrounded at 2-3 sides. That way you'll need less heat to get it hot. Then use parallel clamps or a few pieces of flat (with holes) and bolts to keep everyting together.


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## cephius (Jan 10, 2008)

I have an Acetylene B-Tank and torch setup used to sweat copper pipe. Copper has a higher thermal conductivity than steel. I'm sure I could solder two parts of steel together with this rig.

My only concern would be the strength of the joint. Remember the first rule of a good solder joint is to start with a strong mechanical joint.


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## armorsmith (Jun 1, 2008)

Cephius, 

You are correct, in that this sheet teel may be silver soldered with the rig you are referring to. For any brazing operation - silver or brass - I recommend a small Oxy-Acetylene torch. Your smaller rig takes much longer to gain the necessary metal temperature required for the proper wicking of the filler material. Please notice that regardless of the filler material, silver alloy or brass, it is a brazing operation due to the much higher temperatures to make the joint. With the proper equipment and materials, I have brass brazed 18 ga steel it a 'T' with almost no brass showing. This takes a steady hand and light application of fluxes and filler materials. 

I would recommend looking for a mid range (cost wise) Oxy-Acetylene jewelers rig. It will have small enough tips for the torch to be able to do any kind of work you might want to do in G scale. If you get adventureous, you might even try your hand at gas welding. The only difference between TIG and Gas welding is the heat source. The technique for both is the same. 

Havoc, 

TIG is a good option if you have the equipment. Most hobbiests don't have that level of equipment on hand, or the access to it. 

Bob C.


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## Captain Dan (Feb 7, 2008)

I want to thank those that gave advice about soldering steel. This afternoon I had a go at building the steel bridge kit. After getting the parts to fit correctly, I just used a stainless steel wire brush to clean the steel where I was going to solder. I used a standard acid core 60/40 solder (lead/tin) and used "Stay Clean" flux. (the type that comes with Stay Bright solder kits) The soldering could not have been easier. I used a propane torch. The solder drew up into the joints like a champ! I had several parts to solder, but I guess because of the steel's heat transfer properties, the solder more than about "1 1/2" inch from the flame did not re-melt allowing me to solder several parts in one operation. I was surprised how little work the steel required to clean in preparation for the soldering. However, the clean up after soldering was very time consuming, much more so than brass.

However, after a lot of clean up, I painted with Rust-oleum primer to prevent rust. (Thanks for that advice!)

I have tried to upload a picture, but I cannot.

Thanks to all,

Dan


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## Totalwrecker (Feb 26, 2009)

Posted By SE18 on 09 Nov 2009 07:34 AM 


I've got a question. If I hold the work to be soldered in my vice or in my shop press, will the heat from the large propane torch anneal the vice or shop press? I'll await your verdict before doing any more work! 

Not unless you get your vise red hot, it should be fine. I'd need to see what your press looks like but if the plates are thick, it should be ok too. You'd have to get it hotter than your work to aneal. Brazing and hard silver solders are different. The Hard silvers run around 1300 degrees and up. 60 -90% silver (jewelry grades), silver bearing are much cooler...

John


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## SE18 (Feb 21, 2008)

thanks for the feedback. This is sort of an unrelated side topic to "steel". 

I've always wondered how molten steel is poured into steel rail cars. What prevents the steel cars that carry molten steel from melting. Same with the steel pouring buckets. This has always bugged me.


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## Totalwrecker (Feb 26, 2009)

Not sure about the steel industry, but having gone through the local smelters rubble I can tell they lined their crucibles with fire brick. I've got bricks with half melted ends.... 
It could also be done with different alloys that raise the melt temp.


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## SE18 (Feb 21, 2008)

thanks; guess they'd need a special smelter for making the steel with the higher melt temp as well, and so on.


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