You didn't say what your quenchant is. All the standard quenchants should not catch fire if the blade and tongs are fully immersed.
Some basics:
Liquid Oil will not burn. It is the vapors of the oil that burn. Oil vapors will not burn unless they have sufficient oxygen. That is why the vapor jacket in the oil around the quenched blade does not burn.
When you quench the blade, it boils the oil and creates a hot vapor above the surface. This vapor, plus the oxygen in the air can ignite if there is a spark or hot enough surface like the tang or tongs.
Ways to prevent flare-up are:
1) Only heat the blade as hot as it needs to be. Obviously, overheating the blade before quench is not a good thing.
2) Don't overheat the tongs. Besides being bad for the tongs, it creates an ignition point above the surface of the oil to ignite the vapors. If you have to hold a blade for a long time for some reason, switch tongs to a cooler pair just before the quench.
3) Use a commercial quench oil. These have retardants to slow down or prevent flare-up.
4) Fully plunge the entire blade and hot part of the tongs under the quenchant surface. This is why a proper quench tank needs to be much deeper than the blade length. For the average knifemaker, an 18" deep tank is good, and 24" deep better.
5) Use enough quenchant. Besides doing the cooling better, it provides a larger thermal mass to draw the heat from te blade and tongs, thus avoiding as much vapor above the surface.
I used to do fire safety demos. We had a high energy spark coil with a spark plug mounted on the end of a wire. The spark plug was fully submerged in a glass jar of gasoline. When we hit the ignition button the sparks were visible, but the gas did not catch fire or explode.
We would pull the spark plug out, take the jar of gas away and cap it, and hit the ignition button. The spark plug and gas on the wire would burst in flame (because they now had oxygen).
We then plunged the burning assembly in a tall pipe filled with CO2. The flames went out immediately because there was no oxygen.
The final thing was a pipe filled with one drop of gasoline and then flooded with oxygen from a breathing tank. We had a spark plug screwed into the pipe near the bottom. A piece of tissue paper was placed over the end of the pipe with a rubber band holding it in place. When we hit the ignition, the pipe went off like a cannon ... from just one drop of gasoline and lots of oxygen. In fact, gasoline vapor has four times the explosive force of TNT. They make bombs that use the atomized gasoline (or similar flammable) and air. These are called FAEs (Fuel-Air-Explosives).
