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Research on ultra fine grain steels

Posted by CliffStamp 
Re: Research on ultra fine grain steels
March 10, 2015 05:24PM
I tried to draw a few circumference around a few possible grain (4-5um in size) but keep seeing interior lines intersect with those circumferences. I polished & etched a finished blade. Hard to get the ffg bevel flat, thus only see a strip. 15,20 & 25 seconds didn't show any new info except more fine precipitated cementites.

If you don't mind, please trace a few grain boundaries that look to be visible to you. Thanks, Cliff.
Quote
CliffStamp
The gain boundries look to be visible in the second picture, 4-5 microns in size.
Re: Research on ultra fine grain steels
March 10, 2015 06:08PM
There are no distinct grain boundries visible but partials, have a look at this :



Now with the very low temper you can see the boundries clearly, however as soon as the more complicated parts of tempering set in (austenite to bainite, martensite to ferrite, cementite precipitation, secondary hardening) then they gradually disappear.

However you can still see bits of them especially if you look at the first one and then look for those patterns.
Re: Research on ultra fine grain steels
March 10, 2015 06:17PM
My blade tempered at 135C, so there shouldn't be much conversion taken place to mask/hide the grain boundaries. I am thinking, maybe I temper this blade to 225C, perhaps Fe3C precip may dotted & coarsen more along the grain boundary. What do you think?

Quote
CliffStamp
There are no distinct grain boundries visible but partials, have a look at this :



Now with the very low temper you can see the boundries clearly, however as soon as the more complicated parts of tempering set in (austenite to bainite, martensite to ferrite, cementite precipitation, secondary hardening) then they gradually disappear.

However you can still see bits of them especially if you look at the first one and then look for those patterns.
Re: Research on ultra fine grain steels
March 10, 2015 06:24PM
I would work on etch time and focus. While cementite will precipitate at grain boundries, once you go above 200C multiple other reactions happen as well.
me2
Re: Research on ultra fine grain steels
March 11, 2015 07:24PM
Has anyone found anything saying that an array of grain sizes benefits things?

My main problem with many maker's claims of ultra-fine (10 or smaller) grain size is I'm not sure the claimed sizes are being reached. Ed Fowler claims a size of 14 and smaller (hence the question above), but my attempts to ask how many blades have been checked have been met with crickets chirping. Was it only once, a couple of times, every 10th blade, that was tested and showed consistent results? As we have seen, heating speed is critical to the grain size reduction. Ed heats with a torch, which I'm convinced plays as big a role as anything else he does.

My other issue is I'm not sure that such fine grain sizes will show their benefits in knives. I couldn't see it in the difference between D2 and FF D2. Now, they were tested in a fairly narrow region of blade angles and uses, so that has to be considered, as well as the fact that the blades were relatively small and the impact toughness benefits would likely not be seen just during cutting.

Bluntcut, were these blades you are micrographing heat treated at the point just above A1 temperature, with minimum carbon and maximum carbide in mind? If so, I wouldn't be surprised to see some larger carbides in the structure. In other words, that's kinda what you were aiming for, so don't be disappointed to see it. I've toyed with the same kind of hardening for 1095, but haven't had time to try it. 1400-1425, soak 5 minutes from plate quenched (fine pearlite) 1095 (minimal soak time and carbon diffusion distance required).
Re: Research on ultra fine grain steels
March 11, 2015 08:35PM
Chris, thanks for taking time to humor various claims and virtual/vapor x,y,z benefits on voodoo ht.

Yes, blade in pictured was hardened with aust temp 1425F w/ AQ 66rc, after 275F tempered it down to 64rc. I've another blade aust at 1420F, 67@aq and 65@275F. Soak time was 10minutes. After I become proficient with the microscope, I prob will ht 6 coupons to calibrate my oven over Curie temp. and then 10 blanks with aust temp barely below & above Curie temp w/ 3 diff soak time (1420/1414/1408F; soak H/M/L minutes). Will try 1420F first make sure AQ hardness > 65. The 10th blade will get 1400F aust (if 1420F > 65rc).

I understand, your assertion on low aust temp causes carbide size growth. I've seen that with D2. But I didn't see much difference in carbide size between my 1425F & 1470F & 1500F blades. As I mentioned earlier, I believed I over inflated my carbide size during 1 of my prep step. I will put a correction for my ht plan above.

I am going toward low aust temp, aiming for 99.999% lath martensite; 2% or less RA; yield carbide in 0.2-0.5um in size. RA prob most likely be close to grain boundary, so mart conversion then carbide precipitate are undersirable. Yep, there are a lot of want or wish(ful thinking) but tinkering is a lot of fun.

I look at fine grain for more attributes than just physical dimension. Like what boundary consist of; mis-align angle; facet and interface with matrix & carbide, so on. Good example - FF D2 supposedly super fine grain didn't show big advantage (or at all or worse) than plain jane D2, where (I am guessing) FF D2 grain boundary loaded with precip carbide and highly mis-aligned, which translated to poly stress risers. LOL - maybe I shouldn't talk/write like a dunce newb and just read more, right? Even though my ht D2 still have large carbides, I think my D2 blades I tested are quite tough because I aim for clean boundaries. OK, you can stop laughing now grinning smiley

As for EdF - yeah seeing & deterministically reproducible astm grain 15 would be uber cool. 11 days to ht procedure would be 10 days too long for my impatience ht.

Quote
me2
Has anyone found anything saying that an array of grain sizes benefits things?

My main problem with many maker's claims of ultra-fine (10 or smaller) grain size is I'm not sure the claimed sizes are being reached. Ed Fowler claims a size of 14 and smaller (hence the question above), but my attempts to ask how many blades have been checked have been met with crickets chirping. Was it only once, a couple of times, every 10th blade, that was tested and showed consistent results? As we have seen, heating speed is critical to the grain size reduction. Ed heats with a torch, which I'm convinced plays as big a role as anything else he does.

My other issue is I'm not sure that such fine grain sizes will show their benefits in knives. I couldn't see it in the difference between D2 and FF D2. Now, they were tested in a fairly narrow region of blade angles and uses, so that has to be considered, as well as the fact that the blades were relatively small and the impact toughness benefits would likely not be seen just during cutting.

Bluntcut, were these blades you are micrographing heat treated at the point just above A1 temperature, with minimum carbon and maximum carbide in mind? If so, I wouldn't be surprised to see some larger carbides in the structure. In other words, that's kinda what you were aiming for, so don't be disappointed to see it. I've toyed with the same kind of hardening for 1095, but haven't had time to try it. 1400-1425, soak 5 minutes from plate quenched (fine pearlite) 1095 (minimal soak time and carbon diffusion distance required).



Edited 1 time(s). Last edit at 03/11/2015 08:37PM by bluntcut.
Re: Research on ultra fine grain steels
March 12, 2015 12:45AM
Quote
me2

My main problem with many maker's claims of ultra-fine (10 or smaller) grain size is I'm not sure the claimed sizes are being reached. Ed Fowler claims a size of 14 and smaller (hence the question above), but my attempts to ask how many blades have been checked have been met with crickets chirping. Was it only once, a couple of times, every 10th blade, that was tested and showed consistent results? As we have seen, heating speed is critical to the grain size reduction. Ed heats with a torch, which I'm convinced plays as big a role as anything else he does.
.

questions were asked on KD about a similar HT scheme and the maker's answer was, "You can't duplicate this. I heated the blade with a torch to a color i wanted then quenched." so i don't know what to think.
on the whole subject of grain size, has any made an ultra-fine test mule that could be tested side by side with a test mule made of fine grain steel? i am not volunteering. using suggestions from Roman Landes, i have finally found a recipe for O1/1.2519/O2 that works with my kiln and quench oil.
scott
Re: Research on ultra fine grain steels
March 12, 2015 05:32AM
Quote
bluntcut

Good example - FF D2 supposedly super fine grain didn't show big advantage (or at all or worse) than plain jane D2, where (I am guessing) FF D2 grain boundary loaded with precip carbide and highly mis-aligned, which translated to poly stress risers.

I am not sure that is even a sensible conjecture because the flash quenching rate, extremely rapid, is one of the main benefits of isolated heating methods which will minimize precipitation in the quench and they are obviously tempering quite low so there will be little diffusion in tempering.

I think there are a few main considerations :

1) friction forged D2 has much less primary aggregate and thus the carbide volume is severely reduced compared to standard hardened D2, this has to have a reduction in abrasive wear resistance and compressive strength, though it will be off set by the increase in martensite hardness

2) how precise are the testing methods

In consideration of the second point, as a rough starting point, the tensile stress (using known approximations) from 66 to 60 HRC will change from 350 to 310 kpsi, so say 15% increase in tensile strength and similar would be expected in compression strength. The compression strength increase will be offset somewhat by the loss of primary aggregate. The first question then is are the methods actually capable of telling apart two steels where the properties vary by ~15%. This is the most critical question to ask in any empirical comparison.

I have run comparisons on steels as different as 3Cr13 and 10V on slicing cardboard where aside from the massive difference in primary aggregate, the hardness was ~10 HRC points in the difference. Yet even with extreme controls on angle, grit, cutting mechanics, if I didn't random sample the cardboard cutting then it alone would introduce scatter larger than the difference between those two steels.

Could I tell apart two steels which had a ~15% difference? Yes I believe I could however I would want at least five comparisons to even make an estimate of which one was which and I would repeat that on at least two other media for consistency before I would be even reasonably sure. That difference is just so very close to the kind of precision you can get by hand which at best, even with multiple runs on large scale trials will be 5-10% .
Re: Research on ultra fine grain steels
March 14, 2015 10:01PM
OK, this probably my last hi-jack post blah about my SQ grain. After SQ and hardness (no temper). I broke off an attach coupon to a paring knife to see grain *as is* surface (no etch). 400x color negative, because of uneven surface the microscope only able to captured a thin strip in focus.


With eyeballs & a 15x Peak Loupe, I can't discern individual grain. I have 7 coupons need to polish & etch.

btw. My oven TC is almost spot on with few degrees flux. 1420F =65rc; partially failed to harden at 1415F (some 58r spots, most 40s rc). Completely failed to harden at 1410F & 1405F (~40rc).
Re: Research on ultra fine grain steels
March 18, 2015 11:20AM
I wasn't blowing smoke when said about carbide reconfiguration regressed my ht a little bit. Better fine carbide & grain are here to stay...
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Re: Research on ultra fine grain steels
March 18, 2015 12:43PM
As you may not know, if you don't have a membership on BF you can't see anything attached there. I can't see the images you reference in that thread for example.
Re: Research on ultra fine grain steels
March 18, 2015 01:30PM
Quote
CliffStamp
As you may not know, if you don't have a membership on BF you can't see anything attached there. I can't see the images you reference in that thread for example.

Sorry, I inadvertently attached the latest pic. Here it is.

Re: Research on ultra fine grain steels
July 19, 2015 08:50PM
BC has been putting up some videos recently on his knives which are UFG, they may be of interest :




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