Can you look at this grain?

Josh Rider

Josh Rider

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I haven't done any destruction tests on my knives so I figured now's as good a time as any.
This is 01 tool steel hardened and tempered to 59-60 estimated.
It was tough to break.
Does this look ok?

 
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yes and not. Very strange fracture face.
The upper part looks like fine, but the major part looks coarse.
The fracture inspection should be done in the hardened state, does it apply here?
 
Yes it was hardened in my kiln and quenched in canola. (I don't usually use 01 so I haven't purchased aaa.)
Tempered at 450 twice for 2 hours.

The piece was 3/16" thick.
Blade on the left spine on the right in the picture.
 
did you saw ot grind into the blade before braking? as said part of the knife looks great but im not sure what is up with the bulk of the area
 
The blade was ground. Then I put it into the vice and bent it until it with a pipe until it snapped.
 
I could grind it flat and take another picture if that would help.

The grey concrete looking area is where it sheared off at an angle.
 
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To me, it looks as though it was a slow break rather than a fast(impact) snap. That would give it time for dislocations and tear fractures. Try putting it in a vice with about 2" sticking up, drape a towel over it for safety and whack it good with a heavy hammer. You should end up with a cleaner break.
 
It was a slow break. Ok cool, I'll do that when I get off work today. Thanks for all the help fellas.
 
For a cleaner break, should it not be broken BEFORE tempering? Does tempering have any affect (effect?) on grain size? For sure pre-tempering will provide a cleaner break.

Ken H>
 
First,
When checking the grain, break as-quenched with no temper.

Second,
That sample shows the very fine grain of the steel, and the tearing along the grain boundaries as it sheared away. If it had been broken as-quenched, it all would have likely looked fine grain and the break would have been straighter.

Think of a brick wall. If you hit it with a car, some of the bricks will break in half at the site of the impact. These will show an even fine break. This is the frosty part of your blade break.
As the force moves out along the wall, it looks for the path of least resistance. It starts to shear the bricks apart along the mortar joints. It will make a big stair step break. This is what you are seeing in the coarser part of the break on your blade. The break runs down the grain boundaries and makes big steps. If you lok close under a loupe, you will see that the grain is still fine, just the break is a lattice.
 
Breaks made by bending often look weird. If you think about it, the outside was in tension, the inside was in compression, there was a neutral plane, and that plane moves as the break propagates, creating weirdness. Tensile strength (on the outside of the break) is lower than compressive yield (inside of the break) so forces are unbalanced and changing. The end result is a fracture that can look pretty weird.

You need to notch it and hit it with a hammer. Untempered breaks cleaner and is probably more informative.
 
Makes sense! I didn't know that grain size changed that dramatically after temper. I found an old thread showing grain during stages of normalization compared to a factory heat treat. Very cool stuff. I'm trying to tune my kiln so I'll be breaking more steel. I'll be sure to break after quench with the steel cooled down.
 
Grain size doesn't change unless it is heated past critical and recrystallized. Untempered martensite just fractures easier and more uniformly.
 
Grain size doesn't change after tempering. What you see as a result of the tempering does change.
 
Rick Marchand said:
Grain size doesn't change unless it is heated past critical and recrystallized. Untempered martensite just fractures easier and more uniformly.
Click to expand...

That's true, but I think tempered martensite looks rougher than untempered because the additional ductility makes for a rougher surface. Since a lot of the judgment is based upon how rough it looks, I think a tempered blade can look more coarse than the same blade untempered. *shrug*
 
I concur with Nathan. The tempered break will stretch a tad more as it goes a bit farther before reaching the catastrophic failure limit. As it reaches this limit, it runs down the grain boundaries, stretches a tad, and runs down some more. This creates the "rough" look. The grain size is identical in the rough as well as the shiny parts of the break, and the same in tempered or untempered.

Compare it to breaking a piece of wrought iron. The actual structure of the WI is solid, but the break looks fibrous due to the stretching as it repeatedly fails.

In examining grain by breaking, the observer is merely examining the "look" of the grain. The actual grain size can really only be examined after sanding to a very flat and smooth surface and then etching to see the individual grains. I love the folks who say the broke the blade and measured the grain size with a micrometer by counting how many grains were in a mm.
 
This has been a good informative thread - Thanks to all for sharing info. I've been told a good way to compare grain size is to break an old file and look compare that grain to your test break. Each should have that "grey" look with grain size so small it's not really noticeable.

Ken H>
 
I did some pictures of broken hardened O1, too, to compare if it's usefull to normalize/thermal cycle O1 steel as it came from the supplier regarding grainsize.

I cycled one piece as suggested by "NickWheeler" in a different Thread.

"1600, then down to black
1550, then down to black
1525, "
1500, "
1475, "
1450, "
1425, "
1400, " X's three. "

The two pieces were hardened at 1472F in pre heated canola. The cycled piece of O1 came out with 66HRC, the one left as supplied with 64HRC. The two pieces haven't been tempered.
I broke them with a hammer as they were put in a vice. Then i took two pictures of each piece with different magnifications.

The piece with thermal cycling.
fein2_zpshxbahhn8.png


fein3_zps2yjta0fs.png


The piece without thermal cycling.
grob2_zpsooudaywp.png


grob3_zps1ypr6mud.png
 
Three normalizing cycles are sufficient for grain refinement. Much past that and you enter an area of diminishing returns... and in some cases, issues with hardenability. I would suggest to start a bit higher to make sure everything gets into solution and the grain size is even. Maybe...

1650F cool to magnetic
1550F cool to magnetic
1500F quench in oil
1300F still magnetic, 3 times.

After cold work, I would stress relieve at 1250F, check for straightness, then perform the final hardening at the low end of the austenitizing range for the steel you are using. Of course, if you intend to push the limits of hardness, take it further. I like to play it safe, maintain consistency and utilize well thought out geometry.
 
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