silicon bronze adhesion question

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I was asked to build a small desk for a client.I wanted to do something along the lines of a rubbed bronze look on the steel (which I have something for now) but wanted to use silicon bronze brazing rod for a nice look on the welds. I know we have had some discussion on the use of it but I have not had great success in getting it to stick well. It could very well be my brazing skills, but I was wondering specifically if something like an acid etch would improve adhesion of the braze. Any thoughts? Tips?Regards,RobGreat Basin WeldingInstagramBlue weldersRed weldersMy luscious Table DIY TIG Torch cooler
Reply:Don't think of it as brazing.  Approach it more like regular  TIG welding, just the puddle flows much easier and you don't need quite as much heat.  You should maintain a short arc length and concentrate the heat just as if you were TIG welding.
Reply:Robert,Grinding leaves a surface layer of smeared carbon platelets that will interfere with the bronze wetting out and forming a good solid state bond with the parent metal.Try using a tool that actually cuts metal a way rather than grinding / abrading it off for your faying surface prep.   I often times use a grinder to rough in bigger preps when making a cast iron repair but I always go back over it with a carbide burr and remove the ground surface layer before starting to braze.    Another old school trick is to take a highly oxidizing flame and sear the faying surfaces to burn off this smeared carbon layer.   But I think the surface left by a carbide burr (or file) is a much better option.Some times you can use the fact that the molten bronze doesn't like to cross a carbon boundry to your advantage.   For instance, if you take a heavy graphite pencil (like a carpenters pencil) and draw two lines about a half inch apart on a clean piece of plate you can run a bronze brazed bead down between them that ends up with nice straight highly defined edges.  I've used that technique to put some bronze lettering on steel before.  It has some other artistic / design applications as well.  If you draw the out line of something with a heavy graphite pencil line you can get the bronze to flow right up to it and stop.  Hope this helps.
Reply:Thanks guys, I will try some of these out today.  I love the pencil idea.I have done a few sticks of SB but in destructive testing it doesn't hold worth a crap. The weld looked good but not enough penetration found in testing. I will run a few tests to see where it gets me.Regards,RobGreat Basin WeldingInstagramBlue weldersRed weldersMy luscious Table DIY TIG Torch cooler
Reply:Robert,If it failed because the bronze pulled a way from the steel (or cast iron) parent metal it's most likely because you had that smeared layer of carbon left from grinding.   I'll bet if you tried the test again but used a carbide burr to cut a couple thousandths off of the faying surfaces first you'll get much better results.When "bronze welding" with an oxy-fuel torch you don't have "penetration" like it's normally understood with the arc welding processes.  Since the parent metal never melts what you actually get is called a solid state bond that happens at the molecular level right on the surface of the parent metal.  Basically what happens is the molten bronze get's down in the spaces between the surface molecules and when it solidifies forms a mechanical "lock" with them.  Any how that's my understanding of it.Last edited by HT2-4956; 06-23-2014 at 12:11 PM.
Reply:Understood, and thanks. I can see that makes sense. Ill burr off a layer or two then beat it to see what I have.You think muriatic acid for removal of the scale would have the same results? (more of a point of curiosity than anything.)Regards,RobGreat Basin WeldingInstagramBlue weldersRed weldersMy luscious Table DIY TIG Torch cooler
Reply:Robert,Volume 2 of the AWS handbook series has some good information on this subject.  It's under the heading "Braze Welding" at the very end of the general chapter on Brazing.
Reply:Excellent. Thanks.Regards,RobGreat Basin WeldingInstagramBlue weldersRed weldersMy luscious Table DIY TIG Torch cooler
Reply:Originally Posted by 4956Robert,Grinding leaves a surface layer of smeared carbon platelets that will interfere with the bronze wetting out and forming a good solid state bond with the parent metal.Try using a tool that actually cuts metal a way rather than grinding / abrading it off for your faying surface prep.   I often times use a grinder to rough in bigger preps when making a cast iron repair but I always go back over it with a carbide burr and remove the ground surface layer before starting to braze.    Another old school trick is to take a highly oxidizing flame and sear the faying surfaces to burn off this smeared carbon layer.   But I think the surface left by a carbide burr (or file) is a much better option.Some times you can use the fact that the molten bronze doesn't like to cross a carbon boundry to your advantage.   For instance, if you take a heavy graphite pencil (like a carpenters pencil) and draw two lines about a half inch apart on a clean piece of plate you can run a bronze brazed bead down between them that ends up with nice straight highly defined edges.  I've used that technique to put some bronze lettering on steel before.  It has some other artistic / design applications as well.  If you draw the out line of something with a heavy graphite pencil line you can get the bronze to flow right up to it and stop.  Hope this helps.
Reply:You do need to remove all mill scale from the weld area. The bronze will not fuse through mill scale.Also, since this is really brazing, not welding, your joint design may have to be different than for regular steel welding. Fillet and lap joints should be OK, but butt welds will have little strength and should be avoided.You didn't mention which welding process you are using but in my experience, TIG is better than O/A, and MIG is better still.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Thanks, yeah. I'm using tig.Always clean for tig.Regards,RobGreat Basin WeldingInstagramBlue weldersRed weldersMy luscious Table DIY TIG Torch cooler