R90Steve wrote:. . . By watching the spot change color the gunsmith knows when the spot will be annealed after cooling. After the machining process he again heats the spot to a shade of red (I think) and after quenching the spot is again hardened.
I'm not likely to be adding anything to this thread, but I'm not going to let that stop me. I've been interested in hardening and tempering high carbon steel for quite a while--more or less in the vein of what a blacksmith would do.
Generally speaking, a blacksmith will heat up high carbon steel to a glowing cherry red in subdued light. At this point the carbon in the steel is in solution with the iron. He will then quench the steel in water or oil which cools the steel so quickly that the carbon doesn't have time to precipitate out of the steel and form carbon nodules within the steel. At this point the steel acts much like glass. A sudden shock can cause it to shatter. Steel this hard isn't useful for many applications. It muse be tempered to a softer condition with a remaining hardness as needed for the application.
Tempering steel is done at a much lower heat than the original hardening process. Sometimes tempering the steel is called "drawing" it. What the blacksmith will do is reheat the steel to a certain temperature. Going all the way to a glowing condition will effectively take all the hardness out of it. That is the steel is annealed, very bendable, and not very breakable.
Heating the steel to a lower temperature will remove some of the hardness from the steel, essentially splitting the difference between dead hard and dead soft. What temperature the steel is tempered at will vary the steel somewhere between the two end conditions.
The blacksmith will judge the hardness he's looking for and will set it by watching the color of the steel change. That is done by grinding or otherwise cleaning the steel so that the silver of bare steel is seen. The colors come from oxidation of the steel and the final hardness is a function of the color achieved. We've all ground a drill bit on a grinding wheel too long and watched it turn blue on the tip. We know that the tip of the bit has been tempered too much and won't cut steel now. We know that we have to carefully grind away the too-tempered end of the drill bit to get to the harder untempered steel. Now we do it right, cooling the bit frequently, and are rewarded with a sharp drill bit that will cut steel.
Going from cold to hot (hard to soft) we heat up the steel and can see a rainbow of colored oxide form on the once silver steel. Initially you will see a pale yellow form. At this point the steel is still very hard. Then it will turn brown as the heat is increased. The steel becomes softer yet. As we heat the steel more and more more colors appear running toward blue and violet. The steel becomes increasingly softer, but less brittle.
Depending on the application the blacksmith will stop the process where he needs it to be.
You've noticed that clock springs are usually a deep blue. The springs have been tempered to the ideal tradeoff between brittleness and elasticity to act and last as a spring. Some springs are silver, which means that the outer oxide has been removed for some reason--perhaps cosmetic.
You can have some fun playing with this stuff. If you have an old hacksaw blade that would be a fun one to try. (Don't use a bi-metal blade though because only the cutting edge is high carbon steel.). With your handy dandy oxy acetylene torch, or perhaps mapp gas torch heat the blade till the end is glowing red. Then quench it in water. Then put it in a vice and hit it with a hammer (guarding your eyes). The blade will snap like a piece of glass. Take one of the hard pieces and try to file it. It won't file. The hardened blade is hard as or harder than the file.
Then you can draw the temper with a propane torch. Clean the hardened piece of blade on a grinder or sander, or perhaps even a wire wheel. Get it all silver. Now play the propane torch on one end and you will see the oxide colors form that I was talking about. Further testing with a file or a scratch awl will reveal that the bluer the color runs the softer the blade has become.
R90Steve wrote:. I think knowing the correct shade of the correct color, requires knowing the charicteristics of the particular metal being worked on and a drive flange would be a cheap thing to experment on.
Yeah, since we don't know exactly what steel the flange is made of we're really not quite sure what we're doing hardening and tempering wise. High speed tool steels are particularly hard to figure out. I could never harden and temper that kind of steel even if I had a good idea what I'm doing. And if the tool steel is air hardening, I could never get it soft without lowering the temperature veeeeeeeeeery slowly.
One I broke off a tap in a part I really needed. There was no way the broken part of the tap was going to come out. No problem, thought I, since I knew something about blacksmithing. I decided I would anneal the tap and heated it with my oxyacetylene torch. I never did get that tap to soften. Ultimately I heated the part itself up to red hot and then drove the broken end of the tap through with a punch. I then redrilled the hole with a larger size and tapped it for a larger bolt.
Ken
