Ash annealing and grain growth

T

Tenebr0s

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Jun 3, 2012
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I always try to forge at a reducing heat and/or normalize -- and then I anneal the blade in rice straw ashes.
This got me thinking: does this kind of annealing cause grain growth and thus undo the grain reduction of the previous thermal cycle? Or, is it just that I have to make sure to anneal at no higher a temperature than my last heat in the normalizing cycle?

Can this temperature be sub-critical? I would think not for a full annealing, but in this case, my final forging/normalizing temps are in the dark red.
 
That word "anneal" can be used so many ways.
You are correct in your thinking: if you go very much higher that your lowest thermal cycling temps for your post-forging grain reduction/stress relief, then, yes, you can lose all you had hoped to gain.
It appears as if you have a good grasp of what you are trying to accomplish.
Here - rather than shoot for a "full anneal" by going high and covering in ashes - which for the most part is fairly useless, as the steel will cool too quickly anyway, do it a tad different.
(It doesn't sound like you have a heat treating oven?)
Turn off all the lights in your forging area.
After your post forging thermal cycles, I would do one cycle right at austenizing temp and quench in hopes of creating some martensite and pinning the grain size.
Then, stick the blade back in the forge, in and out, in and out, watching closely for color to come back in the blade.
When you see a dull red, which is going to be approaching 900-1000 degrees, remove the blade and let it cool to a full black.
Wait another 30 seconds.
Do it again.
Do it again.
Do it again.
What you are doing is gradually spherodizing the carbon and giving it a sub-critical anneal, which will allow you to machine the steel as much as you like, without doing a full anneal.
Just make sure to give the steel a good soak to dissolve all the carbon evenly when you austenize for hardening.
 
What steel are you using? Karl is correct but also if you are trying to anneal a hypereutectoid steel, the whole heat it and stick it in ashes for cooling thing will precipitate lamellar pearlite if you heat above austentizing temps you have to quench to below the pearlite nose before any slow cooling or you will have pearlite and the problems that come along with if

-Page
 
Thank you both for the replies. Sorry, I should have mentioned I'm using Hitachi white steel, so basically a high-carbon, low-alloy steel. I don't have a heat treat oven right now, so I can only go by color/magnet test. I've considered doing a pre-quench for grain size reduction, but I'm a little worried about over-stressing the steel. I've done it once or twice just because I failed with the quench, so went for it again. Seems fine to me, but the smiths here (the ones I know anyway) tend to be of the opinion that you shouldn't generally quench this steel more than once - maybe it's the high carbon percentage (1.2 - 1.4 %)?

Always wonder about the rice ash anneal. It seems like it's closer to a normalizing, since, as you say, it's not that slow a cooling.
 
Grain growth is a function of time and temperature. It is not being said that being for a short time above your normalisation temp, undo's the grain reduction. If you stay there for a longer time you may undo it. Soft annealing is mostly done in a sub critical temp. According to your steel it may be around 700 degrees C. I soft anneal O2 from 700 to 20 degrees in 4 hours time. This annealed situation is the best structure prior to hardening.

Prior to the annealing you can do a few normalization cycles by austenitizing the blade, and than quickly flash quenching in and out the oil without hardening the blade. Normalization reduces the grain due to phase transformations. Each transformation from pearlite to austenite and from austenite to pearlite causes a reduction. Slowly cooling doesn't.

Indeed I wouldn't quench a 1,2 - 1,4 %C steel too often, there are chances of cracking, microcracks etc.
 
White paper is a hyper-eutectoid steel. It should be spheriodized to anneal it. Page and Karl have the right procedure. Spheroidite is best made by a very slow cooling from 1250-1300F to below 800F over many hours. The alternative is repeated sub-critical annealing as below.

forge
normalize and cycle the grain down
heat to about 1450F and quench ( oil is fine for this quench)
At this point you should have a fine grain structure and a martensite/pearlite mix
heat to 1250F and air cool to black, quench in water
repeat last step a couple more times.
The steel will now be spheroidite, and will file/grind/sand/drill/machine easily.

When shaping is done and the blade is ready for HT it will be in a fine grained structure that will easily austenitize and make the quench work well.

The only reason I can see to recommend that Hitachi paper steels only be quenched once is that the quench is often in water. Repeated water quenches just multiply the number of times it could go wrong. If the steel is well annealed before the final quench, it should be fine. If a final quench needs to be repeated for some reason, re-anneal the blade before the second attempt.

Now, about slow cooling in ashes/vermiculite.
If hyper-eutectoid steel is slow cooled from above critical, the structure formed may be quite hard ( relatively). The pearlite can precipitate in layers called lamellar pearlite that resist drilling and filing almost as well as martensite does. Lamellar pearlite can skate a file just like a freshly quenched blade. That is why such steels should be water quenched from 800F to ambient, and not allowed to slow cool between 800 and 200F.
 
Thanks again everybody. Stacy, I'm going to take some time with your post and my metallurgy textbook. I really appreciate you taking the time to give such a detailed response.

I do my best to learn everything my teachers over here do, and I always value their methods. They certainly make great knives. But I also try to understand why things are done the way they are. If there's a better way, I want to know it. It's interesting if ash-annealing is a sup-optimal (or bad!) method, since it's a standard practice here. That could largely just be because of tradition, I don't know.

I look forward to learning as much as I can, trying various methods, and finding what works best.
 
TenebrOs,
I would not suggest that you question or criticize the methods taught you in a Japanese shop. The masters there are rather rigid on the techniques that have been honed for centuries. They work well, and the difference between them and metallurgically more correct methods are small. In some cases the master may consider anyone who questions their methods a fool, and toss him out of the shop. Others may be more into modern methods and have studied metallurgy. These folks will be more willing to discuss the methods used, and why one way or another works best for Hitachi steel. Much of the info from the West is based on steels with considerable alloy content. The Hitachi steels are very close to 1.2% carbon and 98.8% iron. Such steel virtually does not exist in the US, so the methods used here are slightly different than in a Japanese shop. You can do things to Hitachi white paper steel that would ruin a steel with more alloying.
 
I have returned !! I decided to test all this stuff so I went " under the knife" . Interesting experience as it's the first hospitalmstay-opreation-rehab in 70 years !
I'll be back in the groove quickly on the forum.
It's an old American thing to want to know WHY .Respect the old ways but learn the why too.
 
I only question this stuff in my private time : )
When a master teaches me, I'm just grateful.
I also may question, but I'm not writing anything off.
 
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