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Hi all, I've got some questions about normalizing before a quench.

 

In "The Complete Bladesmith", Jim Hrisoulas cautions against leaving steel for too long above critical temperature when quenching, and against quenching multiple times, because the metal is subject to rapid grain growth above that point.

 

I've seen other sources talk about normalizing the steel before the final quench (often repeatedly) to relieve stress and refine the grain. As I understand it, normalizing (a basic 10XX carbon steel) involves heating the steel above the critical point and then letting it air cool.

 

How does one grow the grain and the other shrink the grain? Does normalizing need to happen at a lower temperature? Or is the grain size determined by the rate of cooling?

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I can't answer you, but I can add a rephrased question:
Is this where AC1-AC3 comes into play? Above AC3(complete austenite) grows grain, and in between promotes growth of new, smaller grains?
(and is there a AC2?)

Edited by Steffen Dahlberg

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Hi all, I've got some questions about normalizing before a quench.

 

In "The Complete Bladesmith", Jim Hrisoulas cautions against leaving steel for too long above critical temperature when quenching, and against quenching multiple times, because the metal is subject to rapid grain growth above that point.

 

I've seen other sources talk about normalizing the steel before the final quench (often repeatedly) to relieve stress and refine the grain. As I understand it, normalizing (a basic 10XX carbon steel) involves heating the steel above the critical point and then letting it air cool.

 

How does one grow the grain and the other shrink the grain? Does normalizing need to happen at a lower temperature? Or is the grain size determined by the rate of cooling?

 

Short answer:

Grain growth happens at elevated temperatures. The hotter you get the quicker it happens. Stresses occur when cooling, both from thermal expansion and phase change. More rapid cooling leads to higher stresses. Grain refinement occurs when new grains are formed (think grown, but it is actually more like splitting), which again is at elevated temperatures. Multiple quenches are dangerous because each quench is an opportunity to crack due to the higher stresses. This also leads to the quickest grain refinement, for the same reason. There is more information on this forum about it and if you include my name in the search criteria (use Google site search, not the built in forum search tool) then you will find some more in depth posts.

 

 

I can't answer you, but I can add a rephrased question:

Is this where AC1-AC3 comes into play? Above AC3(complete austenite) grows grain, and in between promotes growth of new, smaller grains?

(and is there a AC2?)

 

Check out this version of a Fe-FeC phase diagram; it has A2 listed. For hypoeutectoid you want to be above A3, but for hypereutectoid you want to be above ACM. That is of course if you want all carbide dissolved, which may not always be the case. A1, A2, A3, and ACM just denote where a phase change occur involving austenite. Well, A2 is just the Currie point.

Iron-Carbon diagram.jpg

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You are also going to get into the discussion on what is normalization and what is thermal cycling. Assuming that you are dealing with one of the alloys that knifesmiths deal with. In normalization you take the steel up to about 1600° only long enough to heat the blade evenly and then air cool. This allows the carbides to dissolve and distribute the carbon and relieve stress by allowing the iron crystals in the steel to reform. It also helps for iron crystals of a more uniform size.

 

Thermal cycling is done to refine the grain size. You heat the blade, typically three times, to, ideally, progressively lower temperatures. This allows the new and smaller iron crystals to form at the boundaries of the old crystals. Some makers, not having a regulated oven to do this in, have to approximate the process in their forges.

 

You do have to do either process right or you will be fighting against yourself. That's where Jim Hrisoulas cautions not to leave the steel at high temperature for long periods. Heat is the main factor with grain growth but time is also something that effects it. Also don't get too uptight about grain growth. While it can be a problem most steels are going to have alloying elements that will put a drag on grain growth, such as aluminum, which is use for deoxidation, and vanadium which is added for the purpose of controlling grain size.

 

Doug

Edited by Doug Lester

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And of course things get more complicated with exotic steels, such as those that air harden, or those that have carbides that you want to dissolve (which means a soak at higher temps). This is why it is best to start with something like 1080. It is about as straight forward as possible and then you can expand from there.

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You are also going to get into the discussion on what is normalization and what is thermal cycling. Assuming that you are dealing with one of the alloys that knifesmiths deal with. In normalization you take the steel up to about 1600° only long enough to heat the blade evenly and then air cool. This allows the carbides to dissolve and distribute the carbon and relieve stress by allowing the iron crystals in the steel to reform. It also helps for iron crystals of a more uniform size.

 

Thermal cycling is done to refine the grain size. You heat the blade, typically three times, to, ideally, progressively lower temperatures. This allows the new and smaller iron crystals to form at the boundaries of the old crystals. Some makers, not having a regulated oven to do this in, have to approximate the process in their forges.

 

Thanks Doug, I wasn't aware there was a difference between normalizing and thermal cycling.

 

I've been reading this thread, and here's my understanding:

  • Stress in the steel seeds new grains. The mechanism is complex and I don't fully understand it. Thermal cycling provides a stress/heat environment that allows these grains to form. Multiple quenches from above critical temperature should do the same thing, but more dramatically and at greater risk of cracking the blade.
  • Grain growth is mostly a function of temperature, and is exponential. Soaking at the critical temperature isn't too detrimental, but getting hotter makes the grains grow bigger faster. Working in a forge without close temperature controls is where you run into problems.

Does that sound right?

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Sounds like you're getting it. One thing to keep in mind with this vocabulary is that all normalizes are thermal cycles, but not all thermal cycles are are normalizes. A heat and quench as also a thermal cycle.

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Changing the iron phases in the steel will seed new grains on the edges of the old grain boundaries. That's whether you are going from ferrite to austinite or austinite back to ferrite. Allowing these crystals to reform relieves stress in that atomic bonds between the iron atoms in the crystals. However, there are conditions with some of the more complex steels that you can increase, at least temporarily, the internal stresses in steel by thermal cycling the steel. That's why you must know what you are working with and why simpler steels like 1080 and 1084 are easier for smiths to deal with.

 

Doug

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