John,
Most HC railroad spikes only sit around .3% carbon.
As far as you heat treat steps go, it is unnecessary to harden the blade, then re-anneal, then re-normailze, then re-harden. You won't really be gaining anything from that except extra practice
. When making a blade, after you finish your rough forging, normalize the blade one or two times, then anneal it. You want to anneal it to make it easier to work as you grind and file it. After you finish with the rough grind, then normalize the blade. I usually do about 3 normalization cycles. You will probably want to do at least 2, 3 for good measure, and anything after that probably won't make much of a difference. After the last normalization cycle, there is no need to anneal, just go for your final heat and then hardening quench. With the HC railroad spikes, in order to gain maximum hardness use the fastest quench you can, cold water will give you some hardness, but a brine quench will give you a little more. It still won't be up there with a good high carbon steel, but every little bit counts. Following the hardening quench that is when you temper the blade. 535 F is far too hot for a tempering temperature for such a low carbon steel and is really too hot for most blades and steels. Tempering temperatures vary depending on steel of course, blade length, blade uses, and hardening methods. For example, a sword must be tempered to a higher temperature than a small knife, this is becuase the sword will see greater stress than the knife and must therefore be softer than the knife in order to absorb the additional stresses. If you differentially heat treat a blade (harden only the edge) you may not have to temper at all. For a through hardened spike knife like you have made, you will not have to temper very much due to the lower carbon content, since the blade will not be as hard to begin with, you will not have to reduce the hardness as much inorder to impart the necessary toughness. I would say that about 300F would be about as high as you would want to go. You can always start lower and work your way up. Something you can try is what is known as an edge flex test. Sharpen the blade, then take a brass rod, clamp it in a vice and place the edge on the rod. Apply pressure on the edge to try and flex it, if the edge folds, you know that you tempered too much, if the edge chips, you know that the blade needs to be tempered at a higher temp, if the edge flexes and returns to true, you know you have a good temper. Now the edge may not be hard enough to flex and return to true in its hardened state and may just fold due to the low carbon content, and if thats the case, you can temper the whole blade at a low temperature and call it good, or leave the edge at is maximum hardness, and temper just the spine with a torch (a small little propane torch will to the trick). Clean the blade off so you can see the heat oxidization colors occur, place the blade's edge in water to prevent it from heating, then heat the spine until you see the colors run. Continue heating until the colors move down towards the edge, but not to the edge. That will give you the best of both worlds, hardness and toughness. And is really the best choice for a spike knife.
Finally the last bit of advice I can give is to experiment with differenet methods and tempering temperatures until you find one that works best for that particular steel. This of course means destruction testing your blades, which you may not be willing to do at this stage, but its definately something that you should start doing if you plan to be making knives on a more serious level.
Hope that helps, feel free to ask for some clarification or expansion if needed and let us know how it turns out.
Graham Fredeen