Quick and dirty heat treat. Heat the parts you want hardened to red hot. If you really want to check it's hot enough a magnet will no longer stick to it when its hot enough.
Quench in oil. Used motor oil or even some waste cooking oil will work for this. Water quenches too fast and uneven when it creates vapor bubbles on the surface. Now the pliers will be hard to super hard, depending on how high the carbon content is. Don't drop them, especially if they are high carbon.
Now take some fine sand paper or scotch bright and shine the heat treat section so it's nice and bright and shiny. You need to get all the oxides off from the quench so you can see the next part.
After shining them up reheat slowly and evenly and watch the color change of the cleaned surface. For a pair of pliers you want to take them to a deep straw or even an orange brown color. Assuming they are a medium carbon steel (probably 4140 or similar) that will get a you a nice medium hardness (HRc 50 give or take a few points). If you know the are a high carbon steel go a little hotter and let them turn a little purple/blue. You can re-quench or let air cool once you have acheieve the color you want. Shine them up one last time and put them back to work.
If they are cheap low carbon steel then all the above is a waste as low carbon steel will not get much harder than its normalized state.
That said I have two or three pairs of pliers and several tongs that are simply my hot work pliers. I buy better one that I don't use on hot work.
-rambling
