I just found this. It is about carbide but the basics are the same.


Brazing Tungsten Carbide for the First Time

The surface condition of tungsten carbide can make the difference
between parts that are joined with a strength of 100,000 psi. and parts
that fall off by themselves. You need to make sure the surface is
clean and ready for brazing. The best way to determine this is to use
a tungsten carbide supplier that can tell you about the surface
condition of your parts. Often good tungsten carbide comes ready to
braze from the manufacturer. If the parts do not adhere well see the
chapter on cleanliness and surface treatment.

Do not get the tungsten carbide oily or greasy. Make sure the steel is
clean also. Do not heat either part without a protective flux coating.

Flux
Braze alloy joins to metal. Metal oxidizes faster if it is hot. If
you heat metal without flux you will form an oxide layer that is
similar to rust. Brazing over oxide is like painting over rust.
Rarely doe anyone get in trouble using too much flux.

Braze alloy
There are many choices. The safest to use is a pretinned tip or
trimetal also called plymetal or sandwich. It comes as ribbon. It is
flat and has unique properties that relieve stress caused by
overheating. It is a tremendous aid when heating large parts or
working with a torch. It is more expensive than wire. If you are
starting and doing just a few parts this is safer and probably easier.
0.15" (15 thousandths) is about as thick a ribbon as you will need.

Prepare a clean piece of steel. Make sure there are no oils or
greases. Alcohol and detergent or soap may both be necessary. Easy
Off Oven Cleaner works very well but is dangerous. Remember steel
comes oiled to keep it from rusting. An oil layer you cannot see or
feel can ruin a braze joint.

Coat the steel with a layer of Black Flux. (Black flux has more Boron
than White Flux and works longer at higher temperatures.)

Put down your piece of braze alloy cut to shape and coat with a light
layer of flux.

Put the clean tungsten carbide on top of the flux layer and coat the
outside with a thick layer of flux.

Hold the part in place with a ceramic rod while brazing.

Heat the whole part as evenly as possible until the whole joint is at
the proper temperature. This is usually at least a dark cherry red.
Remember that the part has to be hot enough all the way through. This
can be a problem even on the parts as small as saw tips.

Once the parts are hot enough you will see the alloy flow out of the
joint. You may also feel the part move just a bit as the alloy turns
liquid. Once the part is the right color (dark cherry red to cherry
red) and the alloy is coming out just a bit (and it may feel mushy),
then wiggle the part just a little bit to allow any flux or fumes to
escape. If you work the part too hard you can force all the braze
alloy out of the joint and ruin it.

The flux cleans off with warm water. You can use mild brushing if you
wish to speed the process.