Question:
We are investigating brazing a large cylindrical component to a series of tubes and supporting them in a graphite fixture. Both are manufactured from Inconel 625 alloy. We will use a BNi-3 braze alloy. Can you provide us with any tips on how to proceed?

Answer:
Here are some ideas to help:

1. Filler Metal

A. BNi-3 (Nicrobraze 130) is a good heat-resistant, nickel-boron-silicon brazing filler metal especially suited for low-temperature brazing of highly stressed parts.
(i) Composition: 2.75–3.5% B, 4.00–5.0% Si, 0.5% Fe, 0.06% C, 0.02% P, 0.02% S, 0.05% Al, 0.05% Ti, 0.05% Zr, balance Ni.
(ii) This braze alloy is designed to flow well where joints have close tolerances or large surface areas.

B. The brazing range for this alloy is 1850-2150°F.
(i) The solidus is 1800°F.
(ii) The liquidus is 1900°F.

C. Brazing of Inconel 625 should take place in vacuum.
(i) The braze alloy can be used in a 100% (pure) hydrogen atmosphere but not with graphite fixtures.
(ii) An inert-gas atmosphere may be acceptable, but oxygen content is a concern (see 3 below).

D. Time at brazing temperature must be carefully controlled.
(i) To avoid erosion in the area of the thin tubes
(ii) To avoid negating the effects of the (flash) nickel plating.

E. The form of the braze-alloy filler metal is important for proper application and contact.
(i) BNi-3 comes in powder, sintered or cast rod, preforms, plastic-bonded sheet or wire, tape and metallic foil.
(ii) In paste form, the braze alloy will likely come in a syringe controlled by air pressure. The hypodermic needle used on the end of the applicator should be cut at a 60° angle, and the long section should be on the upside of the applicator. In this way, as the paste comes out of the needle it will be forced downward into the joint to assure sufficient contact area between filler-metal paste and the part.
(iii) The binders in the paste can be volatilized in the furnace, and by the time the part reaches 600°F, 98% or more of the binder will be gone, with all of it gone by 1000°F.

2. Component Geometry

A. The mass of the castings in comparison to the drawn tubes will require careful evaluation with respect to heat-up time and time at temperature.

3. Base Metal

A. Inconel 625 is a nickel-chromium based alloy with excellent joining characteristics.
(i) Composition: 58.0% Ni (minimum), 20–23% Cr, 5.0% Fe (maximum), 8–10% Mo, 3.15–4.15% Nb +Ta, 0.10% C (maximum), 0.05% Mn (maximum), 0.05% Si (maximum), 0.015% P (maximum), 0.015% S (maximum), 0.40% Al (maximum), 0.40% Ti (maximum) and 1.0% Co (maximum).

B. Due to the material’s aluminum and titanium content, it is highly sensitive to the partial pressure of oxygen in the atmosphere.

4. Plating

A. Electrolytic nickel plating is strongly recommended and required if the Al+Ti+Ta content is greater than 0.40%.

B. Brazing temperature, time, time at temperature and cleanliness of the surface all play a role in the effectiveness of the plating.

C. The electroless nickel most commonly used is a nickel alloy with 11% phosphorous, which has a melting point of around 1600°F. If the phosphorous percentage is too low, the coating will not work as a barrier at 1900°F.

Next time, we will finish these tips.