Just a simple question for the knowledgeable people.
If you heat a piece of steel up to a black heat, well above any tempering-colors, but well below any redness in the steel.
Would that also be annealed?
I don`t know everything about this stuff, but i just thought, if you temper at too high a temperature, and ruin the hardening, you are generally left with soft steel. From what i know, soft steel is annealed.
So, my question is: Do you really need to heat steel up to a red heat, to anneal it??
Uh Oh, this looks like an opportunity for Kevin to go on one of his long winded techno-babbles
. Annealing in the traditional sense is recovery and recrystalization for the purposes of undoing any effects of cold deformation, cold metal workers such as copper smiths, thin smiths or silver smiths have a much greater appreciation for annealing than we do. If we were just dealing with a single metal, like pure iron things a still rather complicated. If you just want to stress relieve you can go to a lower temperature, traditional stress relieving operations are often sub critical, and involve heating to a range in which there is recovery of strain effects but not actually reforming of the grains themselves. This will make a stress free and softer metal but will not result in the softest condition, you could grind it and perhaps bend it but machining and other cutting operations could be a bit trying still.
The next level is heating to above non- magnetic and actually making all new grains with a whole new crystalline orientation. This tends to really wipe out any leftover effects it the metal, and takes all the deformed grains and sets them back to a more natural shape or size (to go really techy- it sets anisotropic effects back to isotropic). These are all the things that can be done with just a single pure metal like iron; add carbon and things get much more interesting
With the addition of carbon you get steel and then you also have slew of phases/structures to work with. Heat a piece of hardened steel to above 350F and the trapped carbon will begin to get unstuck and gather in preferred locations to make very super fine tempering carbides, this is what results in the initial small drops in Rockwell hardness when you temper. If you increase the temperature to above 600F those carbides continue to grow until they become larger globs of carbide, quite visible under the microscope (they also have a tendency to go from a dark color to a lighter color under the scope), they grow by borrowing the carbon that provides the hardness in the steel. At temperatures higher than 900F they become large spheres in a field of carbon depleted ferrite, ferrite that is also undergoing the recovery and eventual recrystalization previously discussed. If one stops before reaching critical and recrystaliztion then the steel is one form of spheroidal annealed, and can be quite machineable. Here is an image of spheroidal carbide in a piece of my 1095:
If you keep heating until you reach critical and non magnetic, those carbide globs will then swiftly begin to get smaller as they are dissolved in the new grains, from here you go to another type of anneal (lamellar). Slow cooling (wood ash, vermiculite, shut off forge) will cause the carbon and iron (cementite and ferrite) to separate out into a banded structure known as pearlite, and is the most common form of annealed steel that bladesmiths work with. It is not as machineable as spheroidite but it does grind well and is workable. Here is an image of pearlite surrounded by heavy carbide that has collected in the grain boundaries of some of my 1095:
Simple anneals such as lamellar work fine for anything that has .8% carbon or less, but when you get more than that the extra carbon becomes very aggravating for machining. How many folks reading this have squeaked and dulled drill bits in 1095 that they could easily file and bend? Extra carbon like to pool up in clusters or sheets and those carbide sheets will far exceed the hardness of anything short of carbide or diamond bits. Think if it like cutting a tree with a chainsaw that has a rock grown into it, dead soft wood with a nasty surprise! Lamellar anneals tend to allow for more of these sheets to be problematic, and that is why spheroidal annealing is the most popular way to go with higher carbon steels and those with carbide forming elements. Hard little spheres suspended in soft iron offer very little resistance to a cutting tool. But a long band of carbide will just strip teeth.
When you run into this squeaking of tools you cant just heat it to blue or dark dull red, as this will only allow more carbide to laugh at you, you will then need to go above critical dissolve those sheets, and then reheat and turn them into little spheres.
If I have just made some folks world a lot more complicated than they ever wanted it to be, then this post has done it job
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If your steel is less than .8% C and very simple just get it hot, let it cool slow and be happy. If it has more alloying, or more carbon, a little more planning may have to be done.
I think i need to heat up some steel soon. Been too long since i`ve done it now, so i`m getting forgetful.
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), I got lucky with that choice. I just developed an obsession with what is going on inside the steel, and started accumulating equipment. I think that may be the difference between a job and a hobby, I am very aware of every large expenditure for the knifemaking end of things and am always justifying it with how soon it will pay for itself. With my other interests I don’t even notice the bill that is accumulating and don’t really think about whether it will ever justify the expense.