In making your own tools, the hacksaw blade is a new thought for me. Personally, I have brought old files back into the working environment for a long time. Just look at all of the various shapes and sizes of files and let your mind wander. I have made many special grinds of various files. Best thing about it is that the steel is already hardened. Just don't overheat the steel while shaping and you will have a sharp tool for many cuts.
I was told some years ago to avoid using files or similar for scrapers, as file steel is very brittle and can therefore shatter under load. I was told that you needed to take the temper out to make them safer. Wouldn't hacksaw blades present similar hazards?
Be careful with files; they should not be used unless you first heat them to cherry red and re-harden and re-temper to the proper hardness! They are too hard to be used as is. A bad catch can cause the file to shatter. The sharp shattered pieces have the ability to maim or even kill.
There is a good grade of high carbon steel in files. As long as you heat the entire file to cherry red when you forge your tool, the steel is OK to use. The hardness of the tool has to be balanced with the brittleness. A Rockwell hardness of 62 -64 seems to be about as hard as most feel that it is safe to go. The high quality "high speed" tool steels that are used today are not harder; they just don't soften when their edges are slightly overheated.
I know that many have used file reground as scrapers and have "got away with it!", but you could be just an accident away from loosing an eye. Is it worth it?
Hacksaw blade usually are hardened only at the teeth. The back of the blade needs to be flexible and tough to keep from breaking. I grind off the teeth and make narrow parting tools and they work wonderfully. I used to use files until I knew better. Now I anneal them, grind off the teeth ( a belt sander will work well for this) and then I reharden the tip after I rough shape it. This works really well. I am currently working on a way to harden the first 3 or 4 inches using easily available tools that the average woodworker may have. It's easy with a large propane torch or Oxyacetylene torch but I'm finding it challenging with a small propane tank. Drill rod is easy to find at your local bearing supply houses and it works really well for small skews and small hollowing tools.
Many hacksaw blades (and bandsaw blades) are bi-metal, with only a thin margin where the teeth are being a hardened steel. If you use this metal for a parting tool, make sure to keep the sharpened point in this portion of the blade, else it will never get, let alone stay sharp. What works better is to find an old hand saw (even if you pay a visit to your hardware store and buy a new one), and cut strips from saw blade. The whole blade should be fair game, which will make a lot of parting tools, and these are usually somewhere on the order of 3/64" - 1/16" thick. I bought some small diamond cut-off wheels for my Dremel tool, and use these to cut the handsaw blade. I make a thin rectangular shape (using my belt sander), then drill two holes in the back half, and rivet (or use small machine screws) the blade between two pieces of wood for a handle. The handsaws are usually made of good steel, rather than a bimetal composite. And one old saw will make a lifetime's worth of thin parting tools.
All heating to a cherry red does is remove the hardening that was previously done. (Providing that you let it cool slowly.) There are many grades of steel that are used in file making. All are hardened and tempered differently. Either oil hardening or water hardening high carbon steel are the simplest to work with. Some of the best grades of high speed steel require very exact temperatures and times to get the best results. These are best left to the "Tool makers".
After forming your now soft tool, first try hardening with about 4 or 5 gallons of oil. First heat to cherry red than quench and stir. Do this outside with a cover handy. Be prepared, as the oil may catch fire. Next heat only to straw colour and quench again. If this doesn't harden your steel, try the same thing over but this time use water. Your results will not be as good as a tool maker will get with modern heat treating furnaces.
Another approach is to make your tools from high grade tool steel and have a tool maker harden them for you. This is probably the best way to go!
Yet still another way is to use drill rod that is already hard and carefully grind a tool without overheating and destroying the hardness of the steel. Round drill rod makes very good shear scrapers that are easy to roll to the correct cutting angle. First grind a flat on the top for the first 3/4" or so.
Remember using files as scrapers without first heat treating them is dangerous and shouldn't be done!
Sounds correct, although the temperature seems a little low. Best to go with the straw colour glow - this should work for most carbon steels.