I make knives. If you use circle saw blades that are NOT carbide tipped you are probably fine, if you do not get it overr about 450f while cutting and grinding. Higher than that your going to be removing some of the hardness.

Large older saw blades where usually L6 or 15n20, both good steels. Newer smaller blades are suspect and "mystery steel" You can tell some about the alloy of steel by the sparks it sends off while grinding.

The power hack saw blades mentioned are a whole different ball game. Usually something like M4, which is a carbon and tungsten rich steel. When heat treated it forms small tungsten carbides in the steel matrix. This makes it very wear resistant and it takes much higher heat to cause it to loose its temper. It is often used for lathe and mill tooling.(as opposed tto purer carbide tooling). It is very difficult to grind (wear resistant) when compared to many tool steels. It also has a very abrupt elastic limit. In thin sections like power hacksaw blades it will flex some, but, then abruptly snap similar to D2 which forms vanadium carbides as apposed to tungsten carbides. Most good files are 1095 or W2 which are mainly just plain high carbon steels. Good springs and many farm implement tooling like disks and non hard faced harrow teeth are 5160. All of which can make good knives if heat treated properly. Found steel can be fun and good practice material.

But, many found steels are mystery steels. It can be fun to mess with them and experiment. But, really the piece of steel is cheap when you start looking at things like grinder belts, good burl wood for handles corby fasteners. Then there is the time spend grinding, hand sanding, heat treating and fitting it all together. Even if I pay $20 or more for a quality piece of known steel to me it is cheap in the long run. On knives for sale I only use known steels. That is because every steel has a best heat treat method to achieve the best results. A common saying among many knife makers is "if the Lord himself send down the perfect steel it would be nothing without the proper heat treat."

I have watched Forged in Fire. On common high carbon steels an experienced smith can do a very good job of hardening by eyeballing the color of the steel before quench. But, that show never shows a temper cycle. A proper temper cycle takes time for the best result. Untempered high carbon steel is very brittle. It would never survive a chopping test. Just dropping it could cause a hardened untempered piece to shatter.