Please understand I'm not trying to be academic or dismissive about this testing. This is good stuff!
Grain structure is a very good indication of a quality heat treat. It is, however, somewhat indirect. What I'd REALLY like to know is harder to get to... and that is this question: "How much does grain refinement improve toughness".
You might answer that question by stating "5160 is very tough already, so why bother?". That misses the point that I happen to be intersted in understanding. (We might want to triple quench, say, O1, or 3V, or A2, etc. A2 is fairly tough, but not like 5160. Would triple quench help A2?)
What I'd really like to see is how refining grain structure yields measureable differences in, say, a Charpy V or C notch test, or some other well understood, repeatable test of impact toughness. But I know that Charpy machines are harder to find and than Rockwell testers among knife makers(!).
To me, one of the primary reasons to use a carbon steel blade is because you get an inherently tougher blade, JUST by choosing to do a good job heat treating 5160 versus, say 440C. I want tough, but I want really tough and very hard also.
So, to dismiss the issue of grain refinement's potential increase in toughness misses this idea... and I'm going to make up some data to illustrate (and assume Charpy tests can be easily acquired, big assumption):
Let's hypothesize that 5160 single quenched to Rc58 registers, 100 ft-lbs in a Charpy C-notch test.
Let's hypothesize that 5160 triple quenched to Rc58 tests out to 120 ft-lbs on the Charpy C-notch test. A ~ 20% gain is nothing to sniff at.
That is a benefit of going to the trouble of doing the triple quench. And a measureable one. I can measure grain size, but what does that translate into, repeatably and measurably?
As a knife buyer [this is basically my opinion now] I would PREFER to have a triple quenched 5160 at the higher hardness of Rc61 (instead of Rc58) if I knew triple quench would yield a blade of 100 ft-lbs at Rc61, same toughess as single quench at Rc58, because the harder edge will:
1. resist edge impaction, denting, and bending better
2. cut things like rope and game longer since edge won't roll as easily or, perhaps, abrade as rapidly
3. and it'll be harder to sharpen [but personally, I don't care because I use diamond stones, and they make small, not-so-expensive diamond stones that can be taken to the field also... we are talking about an expensive custom knife in the first place, so a $15-$20 stone is pretty small amount.]
I'm a knife customer, so a maker that can offer me 100 ft-lbs of 5160 tough at Rc61 is the maker I'll choose over the guy who offers 100 ft-lbs of 5160 tough at Rc58. Just personal opinion now.
OTHER customers might want an Rc58 blade triple quenched at 120 ft-lbs, because they like their knives to be easier to sharpen. That's ok too. It's still a triple quench benefit.
So, I would like to know about toughness improvements from triple quench first, refinement of grain structure second, and to me as a knife buyer with opinions, ease of sharpening is WAY down the list.
Ok, that was viewpoint #1, above... show me the Charpy, then the grain.
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Viewpoint #2 is also valid, and ideally, we could analyze also (although it would be more difficult to reproduce cutting results, read on...). Here it is in short form.
If refining of grain structure through, say, triple quenching also yields some kind of benefit in abrasion resistance of simple carbon steels and low chrome carbon steels (52100), then another benefit is that the knife will cut, say, same amount of rope at a lower hardness versus a single quenched blade with larger grain. Lower hardness usually means tougher down to some point, and while softer usually means you give up some strength, and it also means easier to sharpen to some degree... e.g. if one only has more ordinary stones or a rock out in the boonies. This is a valid benefit to grain structure improvements, just not the one I'm interested in.
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So in summary, I'd appreciate if rlinger could examine grain somehow... mete can you help with methodology? Can a 10x loupe help? Acid Etch and what to look for on the samples (Bill?).
Does anyone know who could test these samples for toughness? Or would you need a differently sized sample to set up the notch tests?
Sorry, was trying to make sure I was clear, it got longggggg. It's harder to be concise and clear.
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