Etchant question...

[Overmountain]

Ok, I lightly etch a lot of my blades with HCl simply as a means to provide an even purchase for anti-corrosion surface treatment of the steel. (I use a lot of 1095-1075, etc...) My damascus for blade use is someone else's as mine is just not up to snuff yet. I etch this with FeCl. Now, I used FeCl for a standard blade that had been differentially hardened and noted a distinct black oxide on the hardened section and not the body of the blade. Try a full hardened knife, oxide all over.
I know the different etching degree and color between non HT damascus and HT damascus was discussed on the ABS board but didn't see a definitive conclusion reached. I believe, and this is just speculation as my chemistry knowledge ends at raiding meth labs, the FeCl reacts differently to the formed martensite in the hardened carbon steel?
I have been checking my blades via the normal methods, and the FeCl seems to concur. As this etch can be very minimal (microns deep) would this prove a reliable way to verify hardening? I have not experimented with Hamon creation yet, but some of the processes involved in developing the hamon seem very similar, thus I would think it is.
Haven't been able to catch up with my mentors this week so any advice welcome...

 

[Ninja Blacksmith]

Have you tried etching with heated vinegar before? I recall trying several different means of etching, but stopped at rubbing hot - but not boiling - vinegar on the blade with a cloth or cottonball. You can heat a pan with vinegar in it and fully submerge the blade if it's small enough. The vinegar etch takes a long while but I have been very satisfied with the look of my hamons. I actually don't have experience with ferric chloride, so I couldn't give advice about what might be happening there. Maybe someone else who has experience with FeCl will understand more and post a better solution.

 

[Overmountain]

Oddly enough, I have used ferric chloride tempered with vinegar. Same result on plain carbon steel, damascus is just a mix that I use from time to time. Speeds up the 'bite' I think with the same standard oxides FeCl gives me...

 

[javand]

I'm not 100% sure what your question is? If it's whether you can use ferric chloride to verify hardness, I'd say yes and no. Yes harder plain carbon steels like 1080-1095 will show darker etches at higher hardness, and if you notice one exceptionally darker than the rest with the exact same etching regime (time, cycles, oxide removal, etc.), then yes, you *may* be able to make an educated guess that it's harder. But you won't be able to use it to deduce a specific Rockwell C hardness. As a comparative indicator, I think it's a minor tool, only really giving you an idea when something went really wrong, not helping you dial in your HT to the final percentages.

You can use it to showcase differential hardening, and makers do use it as a part of the hamon showcasing process.


Hardened damascus will show blacker blacks, and I was chatting with Jim Batson during a steel making session a week or two ago about a blade he finished, and etched, fully hard, before tempering. The blacks were onyx, and it looked like it was hot blued almost, but the texture and sheen was very different, and exceptional.

 

[Overmountain]

Yes, basically my question is the scenario you laid out. Observing this reaction, and knowing it will highlight hamon, I figure if a blade of plain steel isn't performing as expected this reaction will provide me a very non-invasive sort of check on a reliable basis. I am guessing, yes...

I know it can't help me determine RC or anything, just chucking it in the quick checks toolbox...
Thanks for the reply .

 

[Stacy E. Apelt - Bladesmith]

It will indicate martensite vs pearlite, but it shouldn't tell you how hard the martensite is.

Take a 12" test bar and cut into three 4" pieces. Stamp them A.B.C. Fully anneal one piece and set aside. Fully harden the other two. Temper one at 200F and the other at 600F, Sand them all smooth to clean metal, etch and see how they compare. You should have pearlite, hard martensite, and softer martensite.

 

[Overmountain]

That was my thinking... basically shows martensite formation and degree, of course not the level of the hardness attributed.
Here is the scenario which it plays into with me and why that even matters. I use a single burner forge and heat treat with it as well. I pass longer sections through the forge (12x4x4.5 internal dimensions). I have had a blade that would just show maybe one or two spots on the final edge that didn't perform properly. Generally situations where I tried differential hardening via time/area at temp in the forge, etc. I figured a quick wipe, as long as my conclusions are correct, with the FeCl will provide a quick litmus test of sorts for areas where martensite did not occur at all. I will try the cut bar check you mentione to see what, if any, variations there are in the oxide color on the three...
Thanks all. Cheers.