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Author Topic: thoughts on grain size  (Read 5297 times)
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Ed Fowler
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« Reply #15 on: August 27, 2014, 08:32:34 AM »

The torque readings going one way were 45 - 50 ft lbs, going the other way were 65 - 70 foot pounds. It took us a while to realize what we had witnessed, then it was a day for celebration.
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John Silveira
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« Reply #16 on: August 27, 2014, 10:38:12 AM »

nothing has been mentioned about the Subgrains ? 

i suppose it's not an aspect of grain refinement and matrix that we necessarily need to involve ourselves with and just accept it as part of the structure of the grains themselves - the important aspect is basically grain sizes and matrix ? 
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ChrisAnders
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« Reply #17 on: August 27, 2014, 01:14:15 PM »

Subgrains are most important in low carbon and microalloyed structural steels.  I have not seen them referenced in quenched and tempered steels. 

With regard to the wootz blade, do I understand correctly that the wootz structure was evident on one side and not the other?
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Ed Fowler
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« Reply #18 on: August 27, 2014, 01:39:28 PM »

Yes, the two sides were different, you could see it in the etched blade. The difference was confirmed by the torque values. Evidently one side got a little hotter than the other somewhere down the line. What we wish to achieve occurs in a very narrow thermal band.
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ChrisAnders
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« Reply #19 on: August 27, 2014, 06:56:46 PM »

Was this with repeated bends?  Have you been able to replicate this again?  So, which side was on the inside of the blade during the lower torque bend?
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Ed Fowler
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« Reply #20 on: August 27, 2014, 07:55:55 PM »

It did 10 90 degree flexes. the torque values were consistent. The hardened area of the blade did not tear.

We have not done it again. If some time in the future we get a similar etch patterns we will test it.

Butch tested another blade at a hammer in in front of a bunch of bladesmiths.  This blade cut exceptionally well, until no one wanted to cut any more, then Butch put a torque wrench on her and she completed 35 90 degree flexes, consistent torque value at 65 foot pounds.

The Wootz side, it did not mind compressing, resisted stretch.
« Last Edit: August 27, 2014, 08:05:10 PM by Ed Fowler » Logged

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ChrisAnders
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« Reply #21 on: August 30, 2014, 07:30:05 PM »

So what are the benefits of a finer grain size?  I've not seen them in one place, and figured this would be the thread to do it.  I'll list the ones I know, as well as some possible detriments, but it all depends on what you want to do if it hurts or hinders.

1) greater impact toughness - this levels off and appears to possibly hit a maximum around an ASTM size of 11-12.

2) greater yield strength - this has been discussed earlier in this thread.  There is a maximum point, and beyond that, the strength starts going back down, but even an ASTM size of 15 isn't nearly that fine.  In steels with less than 0.8% carbon, coarser grained steels can be stronger, but that only applies to steels not quenched and tempered and is certainly not universal.  I'm also not sure that finer grains mean noticeably increased strength in quenched and tempered steels tempered at less than about 450 F.  The strength of the martensite pretty much overshadows everything else at that point.

3) less warpage during quenching - this has been known for a long time, since before 1939. 

4) decreased quench cracking - this is a real problem, and finer grains help prevent microscopic quench cracks.  Even stuff you'd never think of will crack if you give it a chance.  I cracked some round rod made of 4140 once, so higher carbon and more complicated shapes need all the help they can get.

5) decreased ductility - this might be a detriment, but it depends on what you want to do.  Steels being processed that require high ductility, where strength is secondary, will have better properties with larger grains

6) decreased hardenability - this was also discussed above, but for completeness I'll put it here.  Steels with finer and finer grains require faster and faster quenches to reach full hardness.  This really only applies to the 10xx and a handful of other steels, as even the small amount of Chromium in 5160 will allow essentially full hardness in the center of a 1" round bar.

OK, that's all I got.  Anything else I missed?
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davidm
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« Reply #22 on: August 30, 2014, 09:25:02 PM »

I asked on another thread with no response.. I think since this thread is about steel, grain, etc..
Can someone give an explanation to the following excerpts /claims, is this a superior product or process?  And why promote the forged knife, made on a "custom" or individual basis, if the same or better level of performance is available in the production knife market? 

 "Although hardened INFI knives are 58-60 Rc we have yet been able to chip an edge. The edge can be dented or misaligned but its high level of malleability at such high hardness has never been duplicated by any other steel that we are aware of or have tested."

"Tougher, by an enormous margin, than any other steel we've ever tested. It has unparalleled edge holding under high impact and in cutting tests, and shock resistance that begs you to "bring it on". INFI has an ease of re-sharpening that you have to see to believe and higher levels of lateral strength at high hardness than have ever been achieved by any other steel."

http://www.bussecombat.com/about-our-steel-infi/





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ChrisAnders
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« Reply #23 on: August 31, 2014, 06:56:41 AM »

Infi has a low carbon level, something like 0.55-0.6%.  Impact toughness is highly dependent on carbon content, the lower the carbon the higher the impact toughness.  However, part of that is marketing and words like "enormous margin" are highly subjective. 
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Ed Fowler
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« Reply #24 on: August 31, 2014, 04:11:31 PM »

Chris you put me back into research mode, been reading about grain size most of the day. I will respond to your post. At this point I believe that it is tough to generalize the influence of grain size to all steels. So far I have studied read about grain direction and grain size. One author reported an endurance limit of 78.000 on specimens cut with the grain and one of about 58,000 on specimens cut across the grain, for a steel of 139,000 tensile.

Grain size is  which is extremely important in respect to the ability of a notched specimen to resist impact, seems of relatively minor importance in fatigue.
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ChrisAnders
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« Reply #25 on: August 31, 2014, 07:45:03 PM »

I think you may be getting grain direction and grain size mixed up there Ed. 

Grain direction comes from the rough shaping operations of the cast billets.  It is caused by inclusion in the steel.  These are typically oxides, nitrides, and sulfides.  The sulfides are typically used as examples.  These inclusions are drawn out in the direction of rolling or forging and give the steel different properties in different directions.  This is called grain direction, a term I have abandoned because of it's potential for confusion.  I use mechanical fibering now.  The term originated before the actual grains were known, almost certainly coming from man's familiarity with the different properties of wood based on the direction of the grain.  This leads to a great deal of confusion.  The effects of mechanical fibering are evident in the example you gave.  The fatigue limit is 26% less in the transverse direction (58,000) vs. the longitudinal direction (78,000).  I'd definitely call that significant.  Whenever possible, steel is used with the longitudinal direction in line with the greatest stress.  If this is not possible, then the weaker mechanical properties are taken into account.  The longitudinal direction is parallel to the direction the billet was rolled or parallel to the direction of greatest deformation for large forgings.  The transverse direction is across the direction of rolling or forging at 90 degrees.  A third direction is also possible.  This is the through thickness direction, running through the thickness of the plate.  This is the weakest direction. 

Grain size is the average size of the steel grains, which are the little bits of steel that you see under the microscope and the multiple quenches refine to smaller and smaller size.  These are the things Rex is measuring for ASTM grain size.  These grains are independent of the mechanical fibering (grain direction) discussed above.  The grains change each time the steel is heat treated.  The mechanical fibering is virtually unaffected by heat treatment.  As to your example, I do not know how grain size effects fatigue limits.  My guess is that finer grain size increases the fatigue limit, though I can't say how much. 

The specific effects of grain size are steel and use specific.  However, the Hall-Petch relationship in the Wikipedia link in the beginning of this thread applies to all metals, and can be used to predict the strength increase from grain refinement.  It may, in modified form, apply to all materials that have grains, though I've not looked into it. 
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« Reply #26 on: August 31, 2014, 09:03:19 PM »

That post was just a comment on what I was reading while exploring grain size. I thought it was interesting and while I had the reference figured I would post it up. 
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ChrisAnders
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« Reply #27 on: September 01, 2014, 07:04:41 AM »

It was quite interesting.  I have some references here that I'll check.  I've never looked into it before.
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« Reply #28 on: September 01, 2014, 06:21:01 PM »

Chris:
Your description of fine grain pretty well covers it.

I believe I found where the part about decreased harden ability in fine grain comes from. I feel it is from long soak times and  found a chapter titled decremental hardening, where in steels are only partially brought to critical temp and quenched. The reason for this was because fine grain will grow with long soak times.

Multiple quench as we do it does not require long soak times, and we can then temper our blades to achieve the ductility we want.

I also found where US Steel had a patten on "multiple quench", they went to four quenches. I have found that with the steel we are working with 3 quenches is where we get the best "bang for our buck". The part about their patten is on page 1264 of the 10th edition of the US Steel handbook. The part that was not included in the write up was called the "properly designed cycles".  I feel our team figured "properly designed" cycles out.

If you take the time to read from different arenas covered in the handbook you will find that there is really nothing new, just putting knowledge from one area to another.

Again the decreased ductility can be avoided by a high rate of reduction at the right temperature etc.
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« Reply #29 on: September 01, 2014, 06:50:06 PM »

I'm not sure what you mean on the decreased hardenability from increased soak times.  This would possibly cause the grains to grow, though the times would have to be extremely long.  I've see pictures of O1 soaked at austenizing temperature (1450-1475) for nearly an entire shift (5-6 hours).  No grain growth was evident.  To be sure it had issues, but grain growth was not one of them.  Undissolved carbides, which O1 has from Tungsten and some other elements, prevent grain growth by acting as speed bumps to the grain boundaries.  But I digress.

Bigger grains mean better hardenability.  Finer grains mean less hardenability.  The grain boundaries are great places for all kinds of stuff to happen.  One of the things that happens is that is where pearlite starts to form when steels are cooled from austenizing temperatures.  The more grain boundaries (due to finer grains), the easier it is to form pearlite during quenching instead of the martensite we all want.  I've seen 1095 quenched in a highly recommended fast commercial quenchant just for steels like it, and it still had a few little spots of pearlite in it.  Now, they were not detrimental, and the as quenched hardness was still around HRc 66, but they were there. 

For any given property, there is more than one way to get there.  Many times, one way will completely overshadow the others.  The decrease in ductility from very fine grains can be more than overcome by proper tempering.  Also, realize that pearlitic steels, as in the spine of one of your blades, are still very ductile when compared to hardened and tempered steels.  Making very fine grained pearlite will decrease ductility, but it might be like taking a cup of water from a bathtub, its just not that big a deal.

Can you explain the mechanism behind higher ductility from high reduction rates by forging?  I think this might also be one of the things that, while it contributes, it's not the major factor and is overshadowed by the annealing steps prior to hardening.  The annealing steps are done to the entire blade right?  Then the edge is hardened?

Again, I don't believe that the fine grain sizes found in HEPK style blades will lower the hardenability in any practical way, unless one chooses to use a 10xx or similar plain carbon steel with essentially no alloying other than carbon, iron, and standard manganese (0.25% to 0.75%).  Hardenability is critical in industrial application, and they generally use much thicker sections than knife makers.  With that in mind, even the hardenability drop from ultrafine grain in something like a 2" diameter bearing of 52100 could be detrimental, provided through hardening is a requirement. 
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