crack inspection

M

mitch8

Joined
Oct 11, 2010
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what do you guys use for crack inspection? i was thinking about getting some dye penetrant but i thought i should see what the pros are doing. is there a cheap way of effectively checking for cracks?

thanks again, mitch
 
Silly as it sounds, I just look at the blade. You can see most all cracks pretty good.
 
what grit do i need to take it to in order to see cracks? or does it not matter as long as you sand in the opposite direction of the crack?
 
Like Stacy said.:thumbup:

Good lighting helps obviously, but you can finish to a clean 500 to 800 grit. That along with short dip cycles in ~1:4 ferric chloride (or white vinegar) followed by polishing will emphasize all kinds of fun stuff in your steel's surfaces. Dark lines on bright steel.

Worthy goal for smiths to understand and eliminate up front IMHO.

Best to you.
 
Busse does dye pen on some of their blades. I prefer magnetic particle, but have access to it at work.
 
Unless you have some serious problems in your forging or HT, cracks are not a normal problem. The ones you do encounter usually announce themselves quite well. I have had tiny cracks in water quenched blades, but these were clear to see. Like Andy said, a dip in FC and a re-sanding at 400 grit will clearly expose and surface flaws or cracks.
 
when you say that ferric chloride will reveal "all kinds of stuff" will it also show grain growth or decarb? is there a way to see these things with the naked eye?
 
Grain growth can't be seen with the naked eye on a blade. The grain size can be looked at by breaking the blade in half and seeing how fine the grain looks in the break. This is just a visual guess, but will help when compared to a broken file as a standard for fine grain.

Decarb will show up with a FC etch, because the amount of carbon affects the darkness. Deliberately decarb a piece of bar stock while heat treating it. After it cools off, temper it at 400F for an hour. Grind one side down well. Etch for 5 minutes in 3:1 FC, and compare the look between the two sides. That will tell you what clean martensite looks like compared tom decarbed steel.

Other things you might see with a FC etch are - deep pits, porosity, deep scartches from grinding, hammer marks that are too deep, slag inclusions in the steel, scale driven into the surface durring forging, and alloy banding.
 
bladsmth said:
Grain growth can't be seen with the naked eye on a blade. The grain size can be looked at by breaking the blade in half and seeing how fine the grain looks in the break. This is just a visual guess, but will help when compared to a broken file as a standard for fine grain.

Decarb will show up with a FC etch, because the amount of carbon affects the darkness. Deliberately decarb a piece of bar stock while heat treating it. After it cools off, temper it at 400F for an hour. Grind one side down well. Etch for 5 minutes in 3:1 FC, and compare the look between the two sides. That will tell you what clean martensite looks like compared tom decarbed steel.

Other things you might see with a FC etch are - deep pits, porosity, deep scartches from grinding, hammer marks that are too deep, slag inclusions in the steel, scale driven into the surface durring forging, and alloy banding.
Click to expand...

About the grain growth etch theory, I will say this from personal experience. Before I fine tuned my shot built HT oven I wasn't aware that my blades were overheating by 150 or so degrees. I HT'd a couple of chef knives in 01 with no profiling on them, then ground them down to .007, thoroughly cleaned and etched. Both blades came out all sorts of splotchy. I knew it wasn't from decarb b/c there couldn't be any left after grinding a 3/32" blade down to .007". My conclusion was that the FC showed me the grain growth, and after breaking a blade I was right that the grains were outrageously large. I know that we want to discount it because of who teaches it, but in my personal experience, it certainly revealed what was inside the blade.

And why wouldn't this method work? After all, doesn't an etch reveal the difference between a hardened edge and a soft spine? Isn't it what we use to reveal a hamon? Why discount it so quickly when one suggests that it reveals the grain growth? Sure, the standard is to break the blade to see the grain on the cross section. But is that all that different from grinding the blade even further to reveal what happens at the core of the blade? If you've never done it, I highly suggest that you purposefully severely overheat a blade to cause grain growth, then grind past the decarb and etch. I'm guessing that you'll findings will be like mine and you will see the splotchy nature of the grain growth.

I admit to not being a metallurgist and will humbly admit that I could be wrong. OK, sorry to derail the thread, thanks for letting me ramble. By the way, I just noticed this was my 2000th post :)
 
Without seeing it and some other info - What you were probably seeing was bundles of alloy, not grains. The concentrated areas of alloy should etch different than on the plainer areas.

I don't think there would be any difference in etch in large grain vs small .... because the metal structure in either should be the same. To put it in a simple example, a 2X2" piece of maple should stain the same as a 12X12" piece.
 
but those miss-colored alloy bunches (if that's what they are) are still bad for the knife, correct? and how do you avoid alloy bundles during heat treat? i'm using o1
 
It is easy to over think this. As to banding being bad - it's probably more like a symptom rather than the disease. Some really great performing and deeply loved blades out there have all kinds of cosmetic issues. No worries as long as you keep moving forward. Work hard to get control of your temps and follow maker's HT recommendations as a start. I've had fine performance with a 1450 to 1475F soak for about 7 min. Full quenched in medium speed oil (Brownell's Tough-Quench here for Crucible O-1). Temper at 375f x 2hr and test edge. Adjust temp and go again for an hour or so. Test/repeat till balanced. Take notes (free, easy and so underrated).

Surface banding in modern O-1 is possibly just alloy segregation caused by poor temp control somewhere after it left the mill. See above and realize that somebody will come along shortly, read this and recommend you pick up some 1084 and try that. You'll be miles ahead to go read the HT stickies above and find out why if you don't already know. Now go get busy:D
 
Alloy banding can be best avoided by putting all the alloy into solution and then cycling down the temps. Start high enough and hold long enough to get it in solution and evenly distributed. Then cycle downward.

If trying to remove or avoid this in O-1,there are a few things that can be done. First ,start each heat with a pre-heat at 1200°F, and heart slowly and evenly.
Austenitize at 1650°F and a 10 minute hold.
Then repeat at decreasing cycles of 1500° & 1350°.
At this point the grain should be small and the alloying evenly distributed along the grain boundaries. The steel is now ready for the final HT quench.
After the final pre-heat, bring the steel slowly to 1475°, and hold for 10-15 minutes , then quench if medium speed oil.
 
sounds like i need to put some money into an electric furnace. im just color guessing right now on a bed of coals. i was tempted by a "skutt firebox kiln" sold a cosco for 500$ but something tells me not to jump on that offer.
 
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