What does good grain look like?

T

The Gopher

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Mar 4, 2011
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One thing I always hear guys talk about is if steel xyz has "good grain". While I understand the concept, what I don't quite understand is what good grain looks like and what bad grain looks like. I recently broke a blade trying to straighten it after heat treat. The blade is O1, I now know that O1 is a more difficult choice for a simple setup and have switched to 1084, but I'm trying to learn from what I've done. Below are the best pictures I could get of the broken blade. is this grain "good" or "bad"? thanks fellas.





 
Yes, that looks good. Good grain is fine grain. It should look frosty and like broken cement. If it looks granular, like sugar, the grain has grown a bit larger. If it looks geometric, like stacked small blocks, then the grain is huge.

Break and old file for a comparison of what very fine grain looks like.
 
Most likely your O1 grain size is fine/small. It's a clean/cleavage break, which doesn't reveal much topological details to read/guess grain size or microstructure. I concur with Stacy's statement on macro view of broken surface. Here is TiN (coated) drillbit fractured surface camera macro+15x Peak loupe (one way convex len, unlike common symmetrical convex lens loupe which negate/reduce/cancel camera macro)

ud1K1Ez.jpg


Clean/cleavage break implies intergranular fracture, which is much easier/weaker than fiberous/irregular/trenches/etc surface. Smallest feature you can detect/see, likely to be a cluster of grain. Trenches/lines are fracture pathways, degree of meandering = corresponding to degree of strength & toughness.

The finer the grain the harder to discern grain boundary(gb) at as-quenched, temper blur/hide gb even more. I think, best read/detect would be using BSED(back scatter electron diffraction) at fracture surface (instead of polished + etched surface). I have 3 diff polished+etched samples, where BSED at 50K & 100K (magnification) failed to detect gb.
 
Here is a link to a very good read about grain size and how normalizing affects it. It has good pictures showing the grain of a good file with factory heat treat, compared to overheated steel grain size, normalized 1 time grain size, 2 times grain size, and 3 times grain size.

http://paleoplanet69529.yuku.com/forums/129/Reference-Forum#.V0YD5iETn6c

Just to help clear something up (please, correct me if I am wrong), but "normalizing" is taking the steel up to above the critical temperature in order to "blow up" or rather enlarge all the "grain" in the steel so that they all become uniform in size and position, as opposed to the very dissimilar sizes and positions within the steel after all the high heats and pounding on the steel over and over when forging. It's also done with stock removal sometimes because the state of the steel when it comes to you may require it, or the condition just may not be known etc. So 1600 degrees F is pretty much a typical and good temp for accomplishing this, getting everything uniform and equal.

"Thermal cycling" is what come after the first high normalizing heat, where you re-heat the steel successively at lower temps below the first high heat, such as the next heat being at critical temp, usually ~1500F cool to black, then say ~1450 cool the black, and then ~1300 cool to black (these number will very dependign on steel being used and the individual bladesmith/knifemaker), and the go for the final quench at the critial temp. Some guys will incorporate a quench in one or more or the descending heats. This needs to be done after normalizing because it leaves the grain very large and coarse. So these descending heats are what cause the grain to shrink and become nice and fine. :)

I only bring this up because it is discussed a b it in the thread I posted a link to above.

Also, here is a very good video illustrating what thermal cycling does to the grain. :thumbup:

[video=youtube;RQDSz66EJlU]https://www.youtube.com/watch?v=RQDSz66EJlU[/video]

~Paul
My YT Channel Lsubslimed
... (It's been a few years since my last upload)
 
A text book normalizing is not just above critical temperate but also above Acm, which means all the carbides have been dissolved except tungsten or vanadium carbides maybe. The steel is then air cooled and usually firms a mixture of pearlite, bainite, and maybe some martensite, depending on the steel. The critical point is it's all austenite, except maybe the high temperature carbides noted above. Hearing to critical and air cool is not technically normalizing, as the temperature isn't quite high enough, except in something like 1080, where there aren't any carbides above critical unless special steps are taken.
 
The link to the paleoplanet thread had fabulous photos, if a picture is worth a thousand words, then I just read a dissertation on steel grain structure!

Sent from my SM-G900V using Tapatalk
 
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