etchant to confirm if silver brazing is on ferrous metal surfaces

[dtimberlake]

We've been induction heating some 15 lb parts to join them with silver solder for years.
There have been issues from time-to-time over the years, but for some time now the joints have been consistently strong.

Yesterday 34 assemblies were "shear tested", and 27 failed at surprisingly low force levels.
Our investigations are just beginning.

I'd like to confirm if there is a complete coating of silver solder on both of the fracture faces.
Is there an etchant that will differentiate between the steel/iron and silver solder?

I've used Naval Jelly to etch Steel to pretty successfully evaluate welds and reveal obliterated serial numbers etc.


Thanks,

Dan T

 

[Compositepro]

It appears to me that you have trapped air/voids in the bond line just as it occurs with adhesive bonding. Brazing uses capillary action to draw the brazing metal into the bond, but there must be a path for displaced gas to exit. In this case you have sealed the bond line at the edges before the gas has been removed from the center area of the bond.

 

[btrueblood]

Looks like heavy oxidation over both surfaces, was a flux used?

 

[EdStainless]

Back to the basics.
If they say that they didn't change anything that means that they aren't paying attention.
Surface flat and clean.
The correct amount of flux and braze.

 

[dtimberlake]

The silver braze is .010" thick sheet cut to size, dipped in flux, and then placed between the components prior to heating.
No reliance on capillary action drawing the silver into the joint.
There are three Ø3/16" drilled visible in the steel hammer faces.
I figured those holes were an attempt at venting.

thanks all,

Dan T

 

[3DDave]

If it was mine to do, I would braze/melt/apply silver solder to each face on its own and then heat the two parts with a small amount of flux and slide them into position or at least close the gap like a wedge.

The method as described looks like a great way to trap all the oxides and other contaminants with no place for them to go.

Part of the value of capillary action is that as the metal enters the joint it shoves the flux and trapped contamination ahead of the adhesion point and ejects that material.

 

[EdStainless]

The steel parts need to be bright and clean to start with.
For a minor change you can try painting the flux (correct amount) on to the steel parts.
And then stack and heat.
How are you heating?

 

[mfgenggear]

generally it was improper procedure. it should be a fixed process , the gap is important and it to be at the oven elevated temperture.
gaps in the braze or lack of penetration is all due procedure and proper gap.

 

[dtimberlake]

The shop just brazed 170 assemblies . Only 2 failed.

Recall previously 27 of 34 failed, and at very low test pressure.

More news at 11:00.

thanks all.

Dan T

 

[btrueblood]

Agree with mfgengr, your process is out of control. You have no idea why the prior run failed, nor why 99%-ish worked in the current run.

 

[dtimberlake]

The brazing dudes feel they may have isolated the problem to a particular batch of commercial silver solder sheets.

The Tier One alloy supplier is now involved. They will be evaluating a couple of sheared hammers, pads, and the suspect batch of silver solder in their lab.

More news at 9:00.

 

[TugboatEng]

You haven't told us the alloy that you are using. Brazing iron, steel, and stainless alloys are typically going to require silver contents of 45-56%. Silver allows are available down to 3%. Also, there are boron modified fluxes that help when brazing steel and stainless steel alloys. These typically have a brown color instead of white. You haven't mentioned any materials that you are using. There are also cadmium bearing and cadmium free alloys. There will be a significant change in process temperature between the two. If you go cadmium free you'll probably benefit greatly from using the boron modified flux due to the higher working temperatures.

 

[mfgenggear]

Op
As stated , review the process planing, and materials, temperture and time , the materials use, the certifications , both for the bad batch, and the batch theat past hydro test.
radiographic of the joint after braze will detect improper braze penetration. By the way silver solder and braze are not the same. Brazevis a stronger bond with different materials. And there is low temperture braze and high temperture braze. It will depend on the heat treat and hardness of the parent materials.
Also an inspector can visual inspect the lap joint. But xray is required for the internal correct penetration.
Joints have to have clean areas with no contamination. And has to be handled with white gloves.

 

[dtimberlake]

On Nov 11 OP (that's me!) wrote -
I'd like to confirm if there is a complete coating of silver solder on both of the fracture faces.
Is there an etchant that will differentiate between the steel/iron and silver solder?

I guess I should have said silver brazing filler metal.

-------------

Small points -
- Finished assemblies are subjected to a mechanical test that puts the brazed joint in shear. Not a hydro test.
- The image in my first post identified the ASTM material of each part.
The hammer is a steel forging to ASTM A29 . Either 1141 or 1151 steel.
The wear resistant cast pad is ASTM A532 Class II Type B. 15 % Cr-Mo.
- We use the silver braze supplier's own recommended flux. It is described as " a boron modified water based active fluoride/borate-type flux in paste form."

- The OEM for the 40% Silver, No Cadmium product we use describes this product like this - ".. an intermediate temperature silver brazing filler metal"
- Similarly, the 50% silver, 16% Cadmium product is described as a "silver brazing filler metal"
I'll stop saying silver solder .

thanks all,

Dan T

 

[WKTaylor]

DOD-HDBK-249 METALS AND ALLOYS, RAPID ON-SITE IDENTIFICATION OF (RECOMMENDED PROCEDURE FOR CHEMICAL SPOT TESTING AND ASSOCIATED PHYSICAL TESTS TO VERIFY METAL ALLOY CLASSIFICATION) (METRIC)

7.3.1 Spot testing for identification of silver brazing alloys is infeasible. Where possible, suitable samples to permit analytical chemistry tests shall be secured. If etching is done to detect residual sil-braze metal in joints, this shall be accomplished by swabbing the joint with 50 percent silver nitrate reagent (see 5.1). Instant black coloration will occur only on the sil-braze deposit. No standard is required.