My friend made a throwing knife (not sure what type of steel, but we spark tested it and it looked like it was high carbon.) He brought it over and we were going to temper it at about 520 degrees farenheit. Is that too low? I'm afraid of tempering it at the wrong heat because i would feel terrible if he threw it and it broke. Also I have been wanting to make a ninja star for a while now, what type of steel should I use?

Welcome to the world of edged weapons. Your seemingly simple questions are really quite complicated. The short version is that you cannot find a recipe if you do not know the ingredients. Think about it please and I mean no disrespect. Yoiu ask aboiut tempereing but have not mentioned hardening. Consider wanting to frost a cake without baking it first! Heat treat is all about first hardening the steel then reducing the hardness to prevent wot you mentioned, brittleness. And like a recipe there are a lot of different things that affect wot we do with steel that has enough carbon to heat treat.Spark testing is a good thing to try ,works better if you have a ot of experience doing it, and keep in mind the ambient light each time. Also bes tto have some samples of known metals you can compare to. For mystery metals I think about the best method is to take a sample of the materil,,and ht it and test. I know this is alot more than you wished for in an anser. but life is like that,,lol I share your fear about making something that may fail in use. By the way there is alot of info on this site. In the bp section under pages there is a chart on found steels and wot they mmay actually by. and in the forums there is a lot about heat treat. including quenches, temps, tempering etc...May even wish to know about normalizing. Have fun!

Throwing knifes are usually tempered quite soft. If they are to hard they will break or crack upon impact. Getting into the purple range should do OK. I am sure someone with more experience will chime it too. I do my tempering in a small toaster oven and check the colors as I go along. I step up the temps and usually do 2 hours each at the lower temps and nice long overnight soak at the final temp and let it cool down in the oven before taking it out. As Rich stated normalize before heat treating and tempering to prevent issues when quenching during the hear treat.