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advanced annealing methods

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Before we can sand, grind or file our work, we should get it into a soft state. As explained in the heat treating section, there are various internal physical forms that steel can take. I also explained normalizing. In many cases annealing is just a variation of this. But not always, so I will cover various methods of annealing, from simple to complex to address the needs of steel workers.

First we try the simple method of bringing our stress free steel to the curie temperature. Then allowing (even encouraging) the steel to cool as slowly as possible. This is usually done by placing the item in a 'hot box'. I use vermiculite, others have used with good success, ashes or other various things. Some smiths will just allow the steel to remain in the gas forge after turning it off to cool there, with the residual heat in the forge slowing the cooling rate. What matters is that it holds in heat to keep the steel hot for as long as we can. Many times this is all we need to do to get our steel soft enough to finish shaping.

Sometimes this does not work, because the steel cools fast enough to at least partially harden on its own. This is most always the case with air hardening steels. In this example we have to use other methods to soften the steel. I will cover two of these, which mostly apply to higher alloy steels, but I have had this happen with L-6 and even its cousin 15N20 more than once.

There is a method that works well. This is referred to as a “Sub-critical anneal”. Here we take the steel up to the curie point as normal, but after air cooling, we bring it back up to a temperature about 100F to 200F below the curie point, then allow to cool in the hot box. What we have done here as I am sure you have guessed, is to temper the steel at so high a point , that it is at a very low hardness. In many cases this is enough to be able to work our steel.

Sometimes we have a steel (such as stainless Steels) that will not get soft enough to grind well even with 36 grit. This next procedure is called “Spheroidal Annealing”. This process is best done with a temperature controlled forge. After heating to the curie temperature, we use the forge to slowly cool the steel, at a controlled rate. There are different rates for various steel, but in most cases, a cooling rate of 40-50 degrees F per hour until the steel temperature gets below 1000 degrees F is enough. What happens in this process is rather than the carbon returning to free flowing in the steel or collecting in the face of the steel cube, It will collect into spherical balls. And the grains grow larger as well. This makes the steel in its softest state possible. Many blade steels come from the mill in this state, but most do not. I admit this is not easy, nor always possible in a coal forge. Even with a gas forge it is costly to run the forge for 12 hours or more.

These are posted apart from the heat treating because I did not want to complicate the hardening section. I hope these three methods I have show will answer a few questions we all have had through our experience with working metal.

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  • 7 months later...

Great post, Steve. I think it's appropriate to add that the traditional, slow cooling anneal from well above critical really isn't appropriate for any steel with more than about 0.85% carbon (hypereutectoid steels), because it encourages formation of grain boundary carbides that promote brittleness, interfere with stock removal operations (grinding/filing, etc.), and can be very difficult to fix prior to hardening. With those steels -- which include O1 and 1095, to name two very common ones -- you probably should be looking at a spheroidizing treatment of some kind instead of a full anneal.

I would add that there are multiple ways to achieve a spheroidized condition, so if you're interested in this it's worth doing some research. The sub-critical anneal that Steve describes will give you a spheroidal condition. Kevin Cashen and Howard Clark have both written some helpful stuff about this, which can be found via Google with a little searching.

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  • 8 years later...

Interesting read.  Not having much practical experience. But having refered to the Machinist Handbook, on numerous occaisions years ago. What is written by the OP falls right in line with what is in the book.   I had accidently annealed a project, simply by getting disgusted with it, and leaving it in the dying forge (solid fuel)  where it cooled very slowly.

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