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Author Topic: thoughts on grain size  (Read 5296 times)
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Ed Fowler
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« on: May 07, 2014, 10:23:22 AM »

Some thoughts on grain size from Wikipecia:

 http://en.wikipedia.org/wiki/Grain_b..._strengthening

What you might call scientific, I have not sent it to Rex, he is buried in work this year. I invite you to give it a read and consider it.    

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« Reply #1 on: May 07, 2014, 10:56:16 AM »

I can't see any article...
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Ed Fowler
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« Reply #2 on: May 07, 2014, 01:18:11 PM »

try this:

http://en.wikipedia.org/wiki/Grain_boundary_strengthening
« Last Edit: May 07, 2014, 01:31:06 PM by Ed Fowler » Logged

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mreich
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« Reply #3 on: May 07, 2014, 07:13:45 PM »

Thanks for posting this, Ed!

That's some interesting reading. I especially liked the part that describes the actual size of the grain in large and small grains. No wonder we need high powered microscopes to see anything remotely like a size 10 grain.

I didn't realize the size of grain had so much to do with the strength of the steel. Man, that's awesome knowledge!

Piss on anyone who says multiple hardenings are worthless. We know even better now!
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Ed Fowler
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« Reply #4 on: May 07, 2014, 08:12:16 PM »

The fine grain coupled with the low temp forging is what sets the table. We knew we had it nailed when we tested a blade with one side showing a different pattern on the etch. When it required different forces as measured with the torque wrench to flex the blade one way then the other it took a few days for what we had just seen to soak in, then we were elated!

Wootz does not like to stretch but it does not mind compression. This is what gives us the greater toughness.

Thanks for the words of encouragement.
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ChrisAnders
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« Reply #5 on: August 24, 2014, 05:49:20 PM »

One thing worth noting that article points out is the point where decreasing grain size starts to decrease yield strength.  Now, the sizes achieved in knives is no where near this, but it's something to note.  I was doing some research today that showed a point where toughness starts to rapidly loose the improvements from decreasing grain size.  It didn't show a reversal, but there is a point, around ASTM size 10 or 11, that the toughness levels off and doesn't show significant changes beyond that.  I am extrapolating the graph here. 

Another thing not mentioned in the article is that decreasing grain size has a drastic effect on the required quenching speed to harden the blade.  This applies mostly to plain carbon steels or very low alloy steels such as the 10xx series and steels such as 15N20, W1, W2, etc.  This could be part of the reason some people see no further gain in going beyond 2 quenches in steels like 1095.  Past that point, the grain might be fine enough that even water cannot adequately harden the steel.  It does not seem to be a problem for low alloy chromium steels like 5160 or 52100, though I'd expect more issues from the latter.  The chromium appears to allow for full hardening in anything like reasonable knife sizes (less than 3/8" thick).  I say appears because I've only seen tests done on 5150, not 52100.
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Ed Fowler
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« Reply #6 on: August 24, 2014, 07:43:41 PM »

If the grain size is a uniform 10 (for example) the yield strength will be low. This is why a matrix can make a significant contribution, when a micrograph reveals 10 and finer you have a matrix which can be very tough. It was the matrix that we had achieved that contributed to Rex's interest.
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« Reply #7 on: August 24, 2014, 08:15:41 PM »

I've been thinking about that.  An array of sizes is not really avoidable.  There will always be smaller and larger grains.  An array of sizes, particularly large grains mixed with significantly smaller ones, leads to grain growth.  The larger ones absorb the smaller ones and grow at their expense.

What do you mean the yield strength will be low?
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Ed Fowler
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« Reply #8 on: August 25, 2014, 07:59:24 AM »

Obsidian has a very fine and uniform grain size it breaks like glass. Same with hardened blades.

I was working for an outfit that was building bridges for interstate highways, I worked on the cement mix crew - we had to weigh individual wheelbarrows with 4 different sizes of gravel and sand, the mix was very specific if one was heavy the man on the scale would remove a little until it met the weight requirement, if too light he would add a little.

I asked why? It was explained: large bridges have to be flexible, they move with the air and traffic the crosses them. The matrix of the cement is what allows it to flex. If we used too much of any aggregate it would break.

I have always remembered his thoughts. The bridge is still in use, after almost 60 years on interstate 6 & 40 Idaho Springs Colo. It is weathered but as good as new.

Next time you see some cement that is failing, note the sizes of the aggregate and you will probably understand why. The cement curb at our post-office is nothing but broken cement and 1 inch rock. Had they used the right mix it would be solid.

We got lucky when we developed the matrix in our 52100 and quality 5160. It all depends on everything from the chemistry to the final blade.  Other than the above explanation I don't feel I need to know, I just enjoy the performance qualities.
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« Reply #9 on: August 25, 2014, 09:06:31 AM »

I am more familiar with concrete than I should be.  Other than that, you lost me.
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Ed Fowler
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« Reply #10 on: August 25, 2014, 09:20:03 AM »

OK I will try again. Blades break when the fracture runs through grains, it is called a tear when it goes around grains. The object is develop a matrix and temper and naturally more to force it to tear instead of break. Rate of reduction, grain size and grain flow also contribute. Does this make sense?
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« Reply #11 on: August 25, 2014, 10:22:59 AM »

I will add another attempt at explanation:
Fill a bucket with1 inch marbles, is it full?
Then add some 1/2 inch marbles and shake it, is it full?
Keep adding smaller marbles and shake it, is it full?
Then add find sand, shake it - is it full?
Then add talcum powder, is it full?
Then add water, is it full?
This is an old demonstration the kind of applies.

The 1 inch marbles have obvious fracture lines, easily to run a wedge down through and separate them.
As the matrix becomes more complex the fracture line has a tougher course to run providing it cannot fracture objects in the matrix.
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« Reply #12 on: August 25, 2014, 06:10:27 PM »

OK I will try again. Blades break when the fracture runs through grains, it is called a tear when it goes around grains. The object is develop a matrix and temper and naturally more to force it to tear instead of break. Rate of reduction, grain size and grain flow also contribute. Does this make sense?

That kinda sounds backwards.  Tearing sounds like it goes through the grains, while breaking sounds like it would go around them.  I've seen pictures of grain boundary fractures; they don't look like they tore at all. 

The concrete and marble analogy kinda makes sense, but there aren't any voids in steel like the marbles one and, as above, the grains in steel will always have a variety of sizes, at least in knife blades.  There are ways to get single grains, but they are rare and specialized.

Can you elaborate on what you mean by matrix?
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« Reply #13 on: August 26, 2014, 07:58:26 AM »

Matrix: That which contains and gives shape or form to anything. This definition is from the dictionary.
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« Reply #14 on: August 27, 2014, 03:39:16 AM »

That is what I would call unconstitutionally vague, at least for purposes of our discussion here.

On another note, how different were the torques from one side to the other on the wootz blade referenced in the beginning of the thread?
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