tool steel heating question -probably very basic! 1

[jspake]

hi guys, i am new to these forums, so please bear with me. i am NOT an engineer, i am a metalsmith. i have a question about some of my procedures, and i was hoping someone could let me know if i am behaving foolishly, and if so, how? i'll try to make it as straightforward as i can.

i am brazing (silver soldering) machined 01 tool steel parts together, and/or tool steel parts to 302/316 stainless parts. the solder i have is harris safety silv 56, with a melting point around 1200°, and i use lots of the stay silv 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 (on a hunk of metal) or is it safe to quench the assembly in water to speed up my workflow? frankly i have done both, but i don't know enough about the reality of what is actually happening to the internal structure to know which way will give me a stronger part. 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.

thanks for taking the time to read this. just by looking at the thread titles i can tell that i am already way out of my league. i appreciate anyone who would take the time to set me straight. thanks!

-jeremy

 

[redpicker]

How hard is the drill rod to begin with?

Your brazing temperature of 1200F will soften the O1 tool steel (assuming it had been hardened prior to brazing). You will not make the rod brittle due to the heating or quenching, but quenching the assembly after brazing may cause unwanted distortion. You may be affecting the corrosion resistance of the stainless ball by heating to 1200F (quenching won't effect the stainless much at all), but depending on your application, this may not be important.

rp

 

[tomwalz]

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% 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. CBag-7
The 56% is a Bag-7
2. Bag-3
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
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
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% 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.

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 flux it will be all active materials and you will eliminate voids from inert materials in your braze joints.

Tom

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[jspake]

thank you so much for the information, suggestions and feedback!

the hardness of the rod we are using is rockwell B89-B110, according to the listing on mcmaster. maybe i will try a temper cycle to see if we get any different results.

i have only ever used the safety silv 56, as its what we have just always used. i have tried some low temp cadmium free solder, but have not quite been able to get the hang of using it, so i have stuck to the safety silv. i think i'll give your suggestions a try. thanks for the help, guys!

-jeremy

 

[Wrenchbender]

I wouldn't worry too much about the type of brazing wire that you are using. There should only be subtle differences in the strength of the assembly because the layer is so thin. (Technical term for what's going on under an applied load is 'triaxial constraint'. That's the same reason why you clamp two pieces of wood in order to squeeze out all the excess glue: thinner is stronger. Similarly soft-solder joints on copper plumbing hold up pretty well even though the material is weaker than copper.)

Have you made the rod brittle by heating it to 1200? I don't think so. If you're heating with a torch, you are prob over 1200 but don't stay there longer than necessary. (1330 is when things start to happen.) At 1200, if anything, the O-1 steel may have become softer since some of the carbon at the surface may have burned out ('decarburized'). For the stainless steel ball, as mentioned above, it may no longer be stainless.

Is it safe to quench to speed up the workflow? Metallurgically, it probably doesn't matter since these parts are so small they will probably 'air quench' before you get water on them. Sure, cool them immediately if someone wants to touch them barehanded.

 

[tomwalz]

Wrenchbender,

You are correct from a physics standpoint. From a chemical standpoint there is tin in the 56% and Manganese in the 49%.

Generally optimal strength for these alloys is assumed to be at a braze joint thickness of about 0.0005". I would think there would be maybe 0.001" clearance at a minimum to allow an easy fit. With a gap that size I have seen the chemistry make a difference.

Not sure if that applies in this case but alloy chemistry is often underappreciated in its ability to solve brazing problems.

Tom

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[MechEngr07]

What I would worry about most of all is decreasing the hardness of the O1 tool steel during the brazing process. As hardened, O1 is around 63/65 Rc, at a tempering temperature of 600 deg F the hardness drops to 54/55 RC. By the time you reach a temperating temperature of 1000 deg F the hardness has decreased to only 33/35 Rc.

 

[jspake]

correct me if i'm wrong, but i am under the impression that i should be more concerned with the "toughness" of the O1 rod than the actual hardness? i would certainly rather have the rod bend than snap. is a decreased hardness more apt to make the rod bend, or break? if break, than what is the way around this? am i wrong in believing that as long as i am not decarburizing the steel, i am not dramatically changing the toughness by heating it? this is all great food for though, i appreciate everyone's thoughts on this. thanks again!

 

[KevinMH]

If McMaster is listing the hardness of the O1 you're purchasing as Rockwell B 89 to 110, you're purchasing annealed, not hardened O1. As long as you stay below 1330 F during your brazing operation the worst that should happen is a little decarburization and a possibly a little softening of the O1. Decarburization will depend on time at temperature and exposure to oxygen. Softening will depend on time at temperature.

In either case, what should occur is a slight increase in toughness, and possibly a slight decrease to bending forces, where the rod will bend sooner.

If you get the rod above 1330 and cool quickly, you may form martensite and the rod will be harder and more likely to snap rather than bend.

 

[tomwalz]

Toughness is sort of the opposite of hardness. Not entirely true but not a bd place to start thinking about the issue.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.