Most sawmill blades that have carbide teeth brazed on them are not made out of suitable steel for knifemaking, as it won't get very hard. I have a friend that works at a big sawmill that got a new saw once, and he brought me 20 30" blades that were brand new that fit the old saw and they no longer had a use for. None would harden. I later found out that they are made of 4140, that is very tough, but not made to harden. Some are made out of .2% carbon (8620, I think).
Brand new steel is cheap and easy to get, and you know exactly what you are working with, so you know exactly how to work it to get predictable results. Admiral steel, Jantz Supply, K&G Supply, Texas Knifemaker's Supply, and many others handle quality blade steel at cheap prices.
If you want to make knives from recycled steel (I prefer to do it this way, too), Nicholsen files are 1095, saw blades that don't have carbide teeth are usually something like L-6, older vehicle springs are usually 5160, hay rake tines are usually 1084 or 1095, and most plow points are 1080/1084. It is best to learn to heat treat your blades, or to work with a steel that you can send out for heat treatment to get the best results from your work.
Sorry for such a long winded post, and I hope it helps.
Todd