Snapped a blade and would appreciate a diagnostic of the steel

[Rob Davis]

There was a slight bend in this blade that didn't become apparent until after I put the handle on it. I put not a lot of pressure on it in an attempt to straighten it and it snapped.   Granted it's a pretty thin blade.  Does this steel grain speak to anyone??  If so what does it say?   Can anyone tell where I might have screwed up in the heat treating process?  I could jump to a conclusion that it needed more tempering but the grain does not seem to be consistent.   Any thoughts??  As always (with me) this steel started as an old rusty file (Nicholson I think, but can't remember).    Sorry for photo quality, it's a picture of a picture.  

[billyO]

Grain looks pretty big.  I'll re-post a picture that shows this when I get home 

Edited December 20, 2021 by billyO

[Alex Middleton]

I'd agree with Billy that the grain looks pretty large.  It should be much more silky smooth looking. 

Judging by the discolored area towards the top, I would guess that you had some surface cracking going on as well.  It could have been from quenching too hot, or it may have been in the file before you even started.

[billyO]

Here's a link to the picture:

 

[Alan Longmire]

And here's that pic as a stand-alone:

 

[Alan Longmire]

Sorry, I was in a meeting and couldn't concentrate on the important stuff... 

 

From your pic, it looks like your blade was in the same condition as the top file in that stack.  That's classic enlarged grain structure from holding at too high a heat for too long.  Files being the very shallow-hardening steel they are, they have no protection against grain growth.  They can reach that stage in as little as one minute at a light yellow heat.  

 

As that pic above shows, you can fix that by normalizing.  Use the site search for that, and for decalescence/recalescence. If you take it through three or four of those thermal cycles, carefully watching for decalescence each time, you can get the grain back down to the bottom sample. Silky fine and tough, rather than sandy and brittle. 

 

If your grain is huge like that, no amount of simple tempering at 350-400 F is going to fix it.  Multiple normalizations are the key to a happy blade.  I do it as the last step of forging, and I add one more after grinding, right before the quench.

[Rob Davis]

Thank you.  I have some research to do.  I did heat it up to non-magnetic 3 times before the quench.  It sounds like I got it to too hot 3 times.  

 

[Doug Lester]

Non-magnetic shows that it's above the Curry point which is a little lower than austinization.  Watching the shadow of decalesense pass over the blade will show that the steel is changing phase.  Alan gives good advice.

 

Doug

[timgunn]

Hate to be that pedant, but it makes a difference when searching for info on something you are not already familiar with. It's "Curie point."