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Annealing advice.


Carlos Lara

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On the sword forum I frequent, I advised someone to try annealing a blade by heating to orange and then letting it air cool. I know this isn't the best means of annealing, but I've done it before, and the result seemed workable enough to me. I've looked into it a bit more, and probably in the future I'll try letting the blade cool down in the forge, seems like a nice and easy way of doing it. But on the sword forum, another poster recommended heating to orange, then letting the blade cool down in the dark until black, then quenching in ice water. Would that work as well as cooling down in the forge? What's the science behind that? Seems to me to be identical to air cooling (just a faster cool), but I'm a beginner! Couldn't find anything online about this. Thanks!

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Hi Carlos.  I'll recommend picking up a copy or Dr Larrin Thomas' book  Knife Engineering: Steel, Heat Treating and Geometry
He's got a PhD in metallurgy and does a great job explaining all the processes involved in heat treating (which covers annealing, normalizing, quenching, tempering etc) and gives recipes for many different typical bade steels.  
A big part of an accurate answer to your questions depends on the specific steels and alloys you are using.  It might be OK for some steels, but definitely dangerous and risking cracking on others.
 

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as always

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billyO

 

 

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Step one is to not describe things by color alone.  What you call orange may be very different from what I call orange, and ambient lighting plays a huge role in perceived color.  It is far superior to talk about critical temperature (the temp at which phase change occurs, which varies by alloy) and how much above or below.  

 

Heating above critical and allowing to cool in still air is not annealing, it is normalizing.  Generally speaking, unless you have a controlled oven, normalizing is going to be a better approach than any makeshift attempt at annealing.  

 

Annealing is good for getting as soft as possible by not allowing stresses to occur due to temperature change after relieving all stresses upon heating.  These are generally stresses from thermal expansion/contraction, as well as phase changes and recrystallization.  The risk is primarily grain growth, but carbide development can also be a serious issue in some alloys.  

 

Normalizing relieves stresses like annealing, but, being a faster cooling process, it will cause more stresses upon cooling.  These stresses are pretty minimal and you avoid the risk of grain growth and bad carbide development.  

 

Grain refinement comes from recrystallization, which (generally) requires stresses to have occurred, then heating above the critical temperature and cooling back down.  The key is to not have too many stresses before heating (so you don't develop cracks before you relieve the stresses) and not spending too much time too hot and thus get grain growth, as that is the opposite of grain refinement.  

 

Cooling rate below about 900F on normalization cycles is fairly irrelevant as far as grain growth and carbide development go, but there will be some extra stresses induced.  These stresses will leave the steel slightly harder, but will aid in more grain refinement in the next thermal cycle (above critical).  

 

If you are looking for the best way to get as soft as possible (lowest amount of stress) in a simple forge set-up with simple steels (air hardening alloys and such can be quite a bit trickier), then I suggest a normalize cycle (or two or three), followed by one or two very high tempering cycles (about 1250F, just make sure you don't go over critical).  These temper cycles do not need to be very long to have huge benefits in blade geometries.  

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