The Importance of Snapping Samples to View Grain

If you are going to do hand and eye heat treat, or even any type of heat treatment, snapping samples is a valuable skill. Why? Because with some magnification and pleasant and uncomplicated record keeping, you can actually get a good idea of relative grain size. Larrin Thomas confirmed to me at one point as an amateur if I was willing to snap samples, there's not much reason to bother with etching or something more complicated. If you're following this, I doubt you have a reason to, either.  My experience with grain refined samples has confirmed an observable difference in performance, though - especially in regard to chisel edge performance and plane iron nick resistance.  

To snap samples, you want to save stuff like the following rather than throwing away right away:
* offcuts, slivers, etc
* parts of something that you rejected, like a chisel or knife where you overcut part of the profile

Those odds and ends present you with an opportunity to snap samples and look at the steel grain. If you do this well, you will need significant magnification to see the grain. if you've watched Forged in Fire, you may have seen samples heat treated on the show that have sandy grain that can be seen from 10 feet away. You are never to see something like this, but you may see subtly large grain just barely visible close up with the naked eye, and that seemingly slight growth will lead to a no-good woodworking tool. 

What is snapping then? It's literally breaking samples in the middle to see the smoothness or lack of in the broken section. I'd suggest as follows:
* choose a sample that is either narrow or relatively thin. A 1/8" chisel end 3/4" wide will break if you have a reject chisel to use to test things. A 1x1 inch square bar will not break that easily and would be a bad choice. 
* proceed with how you intend to do your heat treatment, whatever it would be for that alloy, and do it carefully - exactly like you would heat treat a tool. 
* quench the sample but don't temper it
* put the sample in a vise, put a rag or box behind it with part of the sample sticking up and strike it with a hammer toward the box. Untempered fully hardened steel should snap easily. If you have a sample that is difficult to break, then it's not fully hardened or it's not hardened through and through. you shouldn't see this incomplete hardening, but it can happen - and more easily with steels like 52100 that can have a retained austenite issue. 

Look at the sample. A well done 1095 sample at about 70x mangification will look like this:
https://i.imgur.com/fAEI7Xk.jpg

An overheated sample will look like this:
https://i.imgur.com/XcQqRj2.jpg

when you first look at these two images, if it's not obvious, the abundance of white reflective bits in the second photo are facets of enlarged grain shining back at you. It doesn't look that severe and maybe you didn't even notice particularly what the difference is at first, but that is the difference, and it's important. The steel in the second photo will make a mediocre functional tool, but it's coarse enough that the edge will have stability problems and you'll eventually put the tool aside. You should be able to choose your own tools out of preference rather than sentimentality or charity. 

Understanding thermal cycles and how they can bring back enlarged grain is also important. The next three pictures are going to describe intentionally bloating steel grain in 1084 steel, which experiences easy and rapid grain growth. Before you bloat the grain in a sample, take your offcut or scrap as it comes from the mill (no forging, etc, just a piece of bar stock never yet heated to nonmagnetic), heat it to nonmagnetic and quench it in brine or a fast oil. it should look something like this:
https://i.imgur.com/SaYUxnq.jpg

Ignore the bright stuff at the top - that's actually a desk top and it's gray and very finely pebbled and inconveniently similar looking to steel grain. 

At any rate, you're creating a small target to match or better with later work. if you do a bunch of fancy things to the steel cycling and multiple quenches and you can't match the grain size of this sample, then probably all of those efforts result in a worse chisel or plane iron and you need to continue to evaluate. 

One of the things you need to know for each alloy is how much it takes to bloat the steel. it varies, and something 52100 really needs would cause larger grain than shown above in 1084. Fortunately, you don't need a routine for every different steel, just to lump together steels that have similar preference (80crv2 and O1, for example, like the same thing. 1084 and 1095 and W1 are somewhat similar with 1095 slightly less sensitive to bloating)

https://i.imgur.com/ZBoW8ya.jpg (note the bright reflective bits again - large grain!)

The results here from a 15 second moderate overheat let me know that 1084 is pretty intolerant of overheating - and that is true. So, the logical conclusion is the method to deal with it heat treating 1084 to get it to nonmagnetic and only a little past that ...just a little..for about 15 seconds after magnetism is gone, and then quench it. That turns out to be true.  

Practicing thermal cycles should bring the grain size back down. Below is a subsequently broken sample of the same piece of steel after about 5 thermal cycles, or for practical purposes, heating until steel is just transitioning to nonmagnetic and then allowing magnetism to return and the steel to cool to black in still or blown air. Sucking air from around a sample with a vacuum works well. 
https://i.imgur.com/5DrKeO2.jpg

To do these thermal cycles is fast - you heat the steel to nonmagnetic and barely just, allow the steel to cool to fully magnetic (down to black or near black visually is fine) and the process isn't slow like heating from cold because you're only doing about 1/3rd of the temperature change vs. what you'd do from cold. 

I'd take notes of what you find and keep some record - nothing exotic, just keep the pictures in a directory and a notepad file with notes. The cigar shaped hand scopes online are useful and should be really cheap. Read the listing fine print when you buy something like that, though - make sure it works with your PC of phone, whatever is recording the pictures. 

if you manage to keep very fine grain in steel, and still hit a high hardness target, chances are, you'll have a very good tool. 

One last thing - at high magnification, alloys look different. Snapped 1084 and 1095 don't have much in them in terms of carbides, so the steel looks very fine. 26c3, a razor steel similar to white #1, has a lot of iron carbides in it and they can be several microns long. When they are densely packed but uniform, you see a sample like this: https://i.imgur.com/30zf4tY.jpg

This looks less fine than 1084, but the gritty appearance at high magnification is not related to grain but rather the carbide volume. to the naked eye, there is a different color, too, unless you get oil on the steel. The larger the carbides, the lighter in color the snapped sample appears. 

1084, 1095, 80crv2 and O1 will all look about the same - super fine
W1 (1+% carbon), 26c3, 52100 will have an array of carbides and they will look more coarse, with a good target being a match to a commercial file like nicholson or heller. 

Use your time wisely - if you're perfecting things, and you have five small samples, you can organize them and do a whole bunch of stuff at once-  just keep everything straight.  Mark the samples on each end with file notches or something of the sort so you can identify both ends. Five samples, for example, gives you one baseline, one to bloat to check sensitivity and find out your margin for error, and then cycle the other three different ways and see if it makes a difference.