1. The surface treatment on the carbide did not stick to the carbide.   There is the gray film on top of braze alloy.
2. There are a great number of bubbles on the braze alloy which are probably trapped flux.  It could also be trapped oil, or grease which would explain why the braze alloy did not stick to the steel.
3. When brazing the tip should be brought up to full temperature.  Then the tip is moved away from the steel maybe a quarter of an inch to allow the flux and flux gases to escape.  Then the tip is allowed to suck back into the joint.  The tip should not be pushed back into the joint since you need .003” to .005 between the carbide and the steel.  Join thickness does not look like a contributing cause to the failure here. 
4. The braze alloy did not stick to the steel in all areas.  This is usually indicative of a contaminant such as oil and / or grease.  We recommend cleaning the notch with a soap or caustic cleaner before brazing. 

See our Articles on Brazing for other factors that may have contributed to the failed tips.