This site has helped me in so many ways that I wanted to give back something that I learned many years ago.
Have you ever tried to heat-treat a piece of steel, scratched it with a file and found that it didn't work out?
You can use tempilaq paint or pencils but they are sometimes hard to read and you will need different tempilaqs for different alloys. A sure fire method to ensure that you hit the decalescent point, regardless of alloy, is to touch the part with a magnet. As you heat the steel, it will go through a cell change to an austenitic structure, which is non-magnetic. The correct temperature for that particular alloy or steel type is when the steel has absolutely no magnetic attraction. Be quick and don't heat the magnet and quench it fast.
If you have a tool that is suppose to be hard and tough, but isn't, there is an old trick that I found in a 70 year old Audel's manual, and works quite well. Heat the part to bright red with your acetylene torch, turn down the oxygen and spray the carbon soot all over the desired area as thick as possible. Turn the oxygen back on and heat keep it heated for 10 minutes and then follow the magnet instructions above. This will provide a case hardened material. If you leave it heated longer, the carbon will migrate inward and the case hard will turn to through hardening potential. 30 minutes will migrate the carbon approx 1/8" inward (look this up). Another method that works very well if your heat source is from another source is Kasenit. It does the same job; case hard or through hard, depending on the soak time. Case hardening does not require a temper but through hardened material usually needs a temper to avoid brittleness.
You can use the Kasenit with a oxy/act torch to do small to medium size parts. heat them to a dull red and dip into the Kasenit and heat back up to cherry red and quench.