1) Do
NOT hard braze a heat treated part. Brazing will utterly destroy any HT you did.
2) Solder it with silver or tin solder, the kind used for rifle lugs and sights.
3) As Mete said in a very concise way, one of the most common mistakes is to think that the torch melts the solder. This will lead invariably to a poor job, as the torch will melt the solder long before the work is hot enough to accept the solder itself. The solder must wet the work, and this happens only if the work is hot enough to melt it. Another big mistake is heating the work like hell and then use the torch to drip solder on it. On hydraulic tubes it may work, on a knife it produces an unsightly mess. Besides, you'll heat the work too much.
Just heat the work for a while, then briefly remove the torch and touch a piece of solder to the work: if it melts, you can solder it, otherwise heat it more, frequently repeating the process.
This is a good way to get proper temp, but requires a good hand and is better suited to soldering where the solder won't be seen, as from behind the guard. You need a tight fit and you must not exagerate with the solder or it will drip through and along the blade making a godawful mess of your knife (don't ask how I know!:grumpy
3) To solder a guard from the front side, the best way is to take the solder rod or wire, and hammer it flat and thin, perhaps 0.01" thick. Cut two little bars out of this, long as the blade is wide, and about 1/16" wide and lay them 45° between guard and blade (the guard must be kept against the blade in one way or another. The knife must be tip up in a vise. Heat the tang and the lower part of the guard with the torch, being careful not to let the flame go over the upper edge of the guard. Let the pieces soak the heat in SLOWLY (use a low flame) and watch carefully for the solder to melt. As soon as the solder melts, remove the flame.
The solder will be sucked into the junction by capillary action, and leave a very clean weld. You can spray with water or let air cool.
The thinner and tighter the junction, the less solder you'll need, the stronger the bond will be, the cleaner will be.
Do not rush things either fitting the guard or doing the soldering.
Silver solder is hotter-melting than tin weld. Both work very well.
Careful and slow are the keywords.
You may need as much time you needed to make the blade just to fit the guard, or even longer, at least initially, but it's well worth it.
