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testing grain structure results before hardening?


J.Leon_Szesny

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I'm currently trying to figure out how to know for sure what and if I did wrong.

to do this analyzing it would help if I could tell the grain size before hardening and breaking the piece.

what types of acid could show the grain structure of non-alloy carbon steel, before and after hardening?
all I got is ferric chloride and vinegar...
 

are there any other ways to test grain structure before hardening?

I dont imagine trying to break my soft & bendy annealed steel will give me a clear result.. 

 

this video shows metal grain structure via etching
 

 

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If you don't have a polarizing microscope you won't see much with that method.  Jerrod can tell you more, but you can see grain at any stage in the process by cutting off a small piece, setting it in resin,and polishing it as fine as possible. A quick etch with 3% nitric acid in methanol (aka nital) will then reveal the grain structure under polarized light with enough magnification. 

 

However, the grain of an unhardened sample won't tell you much about what it could be after hardening.  You'll have no martensite or austinite, for example, and your carbon will be in places it wouldn't otherwise.  

 

It's like guessing the inside structure of a loaf of bread by looking at the dough, sort of.

 

For what we do, the easiest way is to harden a sample and break it. You can't see carbides or the different phases of the steel crystals, but you can see the texture. If it looks like sand, you screwed up, if it looks like glass It's excellent. 

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1 minute ago, Alan Longmire said:

If you don't have a polarizing microscope you won't see much with that method.  Jerrod can tell you more, but you can see grain at any stage in the process by cutting off a small piece, setting it in resin,and polishing it as fine as possible. A quick etch with 3% nitric acid in methanol (aka nital) will then reveal the grain structure under polarized light with enough magnification. 

 

However, the grain of an unhardened sample won't tell you much about what it could be after hardening.  You'll have no martensite or austinite, for example, and your carbon will be in places it wouldn't otherwise.  

 

It's like guessing the inside structure of a loaf of bread by looking at the dough, sort of.

 

For what we do, the easiest way is to harden a sample and break it. You can't see carbides or the different phases of the steel crystals, but you can see the texture. If it looks like sand, you screwed up, if it looks like glass It's excellent. 

 

dang, alright well it's good to have confirmation, then I guess I wont waste my time staring at unhardened steel like looking thru a key slot.

I'm struggling with shirogami right now, which is an extremely pure carbon steel and I can get it to be fine grained(better than the factory grains of a good file) but I know its not quite 100%

honestly it's starting to feel hopeless or like I'm going mad but I'm soo close, maybe?! :wacko:
 

 

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On 2/12/2022 at 5:49 AM, Alan Longmire said:

Jerrod can tell you more, but you can see grain at any stage in the process by cutting off a small piece, setting it in resin,and polishing it as fine as possible. A quick etch with 3% nitric acid in methanol (aka nital) will then reveal the grain structure under polarized light with enough magnification. 

 

I've had good results with 2-10% concentrations of my nital, and 4-45 seconds of etch, all depending on alloy and heat treatment.  If the sample is large enough, you don't even need to mount it in resin, that is a trick just to help hold small samples.  As far as I can tell, this is exactly why metallurgical microscopes are of the inverted style, so you can put really big samples up on top of them.  Cutting and polishing is the hard part.  You have to do it all cold (slow and wet).  Generally you take the polish down to sub micron grits on felt wheels.  Then etch and put it on the microscope right away.  You generally etch near the microscope so you can sneak up on the proper etch as you go.  It is often good to grind and polish near a microscope, too, so that you can check to make sure you get all your scratches out as you go.  

 

That video has some pretty good info in it on what is going on with grain growth.  One thing they don't cover though, is recrystallization on sub-grain boundaries.  You can get new grains to form on sub-grain boundaries, which are concentrations of dislocations from coldworking.  They also don't talk about all the junk at grain boundaries, and this becomes important as you are growing and regrowing grains, pushing that junk around (or not, depending on what is there; and not all this "junk" is bad).  

 

On 2/12/2022 at 6:01 AM, J.Leon_Szesny said:

but I know its not quite 100%

Not quite 100% what?  As fine as physically possible?  

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22 hours ago, Jerrod Miller said:

 You can get new grains to form on sub-grain boundaries, which are concentrations of dislocations from coldworking.   

Not quite 100% what?  As fine as physically possible?  

Coldworking or cold forging is something I have to do as part of the process to adjust the tools to the final shape. "New grains from coldworking" is bad right?

 

Jup. I'm trying to get to 100% or atleast 95% of the best possible results for shirogami, fine grain-ness is the missing part...

I tried thermal cycling which grew the grains and tried multiple quenches, which yes, cracked the steel! but refined the grain quite nicely(obvs not a usable technique for me tho) still it ain't there yet...

 

 

Left side is iron right side is shirogami. Cut with a bolt shear like a barbarian.

DSC00403.JPG

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9 minutes ago, J.Leon_Szesny said:

Coldworking or cold forging is something I have to do as part of the process to adjust the tools to the final shape.

A lot of time and energy has been invested by humanity to eliminate this need because it is inherently problematic.  While reproducing things in a traditional manner has its appeal to some, it is important to keep in mind that traditional methods have their own disadvantages and be aware of what all they are.  

 

11 minutes ago, J.Leon_Szesny said:

"New grains from coldworking" is bad right?

You only get the new grains once recrystallization happen.  Upon heating to austenitic temperatures, then the diffusion process will allow new austenite grains to form from nucleation sites and grow along paths that are energetically favorable (everything always comes down to energy).  New grains are new grains.  The potential issues come from grain boundaries.  If you stress grains or grain boundaries too much then you can get failure (cracks); and this is obviously very bad.  If you let grain growth get out of hand then you will also get grain boundaries that are out of hand, as inclusions can get pushed around and end up consolidating.  Few large defects are much worse than the same volume of defects evenly distributed.  

 

When one has very few alloying elements then grain growth happens pretty fast.  I would suggest you look at your process to ensure you are not getting too hot, and not at heat for very long during your grain refinement steps.  

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