Gunsmith silver solder is a 56% silver braze alloy that is very easy to use and has a good color match with steel.  It is often used for hobbyists, and gunsmiths ro mount scopes and sights on rifles.

This braze alloy contains 56% silver and the rest is copper and zinc. It is very nickel compatible. It will alloy with the nickel to create a very strong joint.  

You can purchase it in either ribbon form or wire.  The ribbon is 0.005” thick and 0.500 (one-half inch) wide and is easily cut it with scissors. One ounce is over 6 feet. The braze alloy in wire form is .031 diameter.  1 troy oz is about 20 ft. 

It starts melting at 1150 F and your liquidus point (full melt) 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.)

 Test on scrap first.

1. Clean the surface with a detergent or a caustic. Dish soap works well as does oven cleaner.
2. Cut a very small piece of the ribbon to size. Dip this ribbon in flux.
3. Place this fluxed ribbon between the two parts to be mated  
4. Hold the parts in place and flux heavily around the area to be heat affected.   The heat of brazing can discolor the nickel by causing oxide formation. Flux will help prevent this.  

 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 gunsmithing I would think that the coefficients of expansion are close enough that it should be very little if any problem.