Brazing Steel

This was originally a question from a forum.  It was a good question, and I thought some of you may benefit from the information.

Forum question:

 I am brazing machined 01 tool steel parts together, and/or tool steel parts to 302/316 stainless Steel parts. I have harris safety silver 56, with a melting point around 1200°, and I use lots of black paste flux while soldering.  A typical situation would be soldering a 1/4″ 302 stainless ball with a 1/8″ hole drilled into it to a depth of 5/32 onto the end of a length of 01 drill rod (1/8″ diameter rod at lets say 2 inches long). I do this by filling the hole in the ball with flux, applying a few bits of solder to the hole, placing the ball onto the end of the rod, which is secured to a vise, or some sort of rigging. I cover the entire ball and top of the rod with flux as well. while keeping pressure on the ball (steam buildup) I will heat the ball/rod to the soldering temperature and solder the two parts together. I make sure not to overheat the ball, and the copious amount of flux keeps the parts fairly clean.

 

Here is the question:

Have I made the rod brittle just by heating it to 1200°, and should I now allow the hot metal assembly to cool to room temperature slowly or is it safe to quench the assembly in water to speed up my work flow?  In use, these parts are put under a great deal of stress, so obviously strength is the goal.  I would rather the rod bend than snap.  I have done some totally unscientific “testing” but the results have been… well, useless.

-jeremy 

 

 Response:

Your liquidus point on the 56% is 1205 F.  You should be getting it about 50° hotter than that to provide enough energy to complete the phase transformation. (Additional time at 1205F will also work but is typically harder to measure.) 

You do effect the steel.  You can temper the steel if you wish.

We have found that joints are stronger if they are allowed to air cool.  However this is usually an issue based on the difference in coefficients of expansion. Obviously, this is a huge problem with carbide to steel. In your case I would think that the coefficients of expansion are close enough that it should be very little if any problem.

 

The 56% Silver alloy you’re using is a very popular alloy because it is very easy to use and it is a pretty good color match with steel.  However, it is a relatively weak braze alloy.

 

Braze Alloys

BAg is the American Welding Society (AWS) prefix for silver-based braze alloys.

1. Bag-7 Braze Alloy

The 56% is a Bag-7

2.  Bag-3 Braze Alloy

There is a 50% with Cadmium that is a Bag-3 that is much stronger but cadmium is a proven carcinogen and, if you use it, you will probably be inhaling the fumes.

3.  Bag- 24 Braze Alloy

Bag-24 is a 50% cadmium free alloy. Most industry used to use Bag-3.  Much of it is switched to bag 24 which is about 30 to 40% weaker than the Bag-3.

4.  Bag-22 Braze Alloy

Your strongest braze alloy is going to be a 49% silver with manganese BAg – 22.  It has a liquidus point of 1290 and it is gummier and slightly harder to use but it is much stronger than any of the other cadmium free alloys.

 

If you are really concerned with strength I would use the Bag-22 braze alloy.

We typically sell the 56% Silver Braze Alloy for hobbyists, eyeglasses etc. It works well and is easy to use.  For tool manufacturing we typically sell the 50% cadmium free or the 49% with manganese.

For the tools we manufacture we use the 49% with manganese and purified flux because nothing is better.

 

Brazing Flux

If you are using standard black flux you probably have something like 10 to 20% inert material in it. There is a new standard EN – 1045 for flux purity and effectiveness.  If you use a purified brazing flux it will be all active materials and you will eliminate voids from inert materials in your braze joints.

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One Response to “Brazing Steel”

  1. TomWalz says:

    Your method sounds good and I am not sure what the question is.

    Following are some considerations that come to mind.

    Proper Cleanliness of all parts.

    Make sure there is a method and practice employed so that there are no trapped gasses.

    Use Mega flux for zero inert material and minimal trapped gasses.

    Heat through the ball to minimize crystalline structure change and thus change in hardness in the rod.

    Perhaps cool the rod with air, flux or even ice if post braze tempering is problem.

    The important consideration is the braze joint strength. 0.003″ to 0.005″ is a good trade off between maximum tensile and maximum impact strength and thermal stress relief.

    Do not be afraid to construct a sprue as in metal casting.

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