The telltale etch- Tells Lies

Mark Williams

Mark Williams

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I think I may have run across a discovery here guys. I have a blade that I had forged a couple months ago and ground to a very convex edge just to play with, maybe a thrower.
I decided to at least go ahead and do the heat treatment. After the first temper cycle I etch the blade to see the hardness zone. I plunked the blade into my ferric caraffe and checked it a few minutes later. Everything looked okay except for an area near the tip on just one side of the blade. The best I can guess is a decarbed area? I plunked it back in the etch for an hour or so. Something just wasn't right with the etch, just didnt seem to be doing much. I happened to look at my caraffe in the daylight and duh... I had been etching in hydrochloric acid instead of ferric chloride. I thought hmmm.... lets see what happens in the ferric. I etched for a while in the ferric and the blade looks like there is nothing wrong up at the tip. Is our ferric chloride not giving us a true picture of what happened in the quench? Try it for yourself and see what happens. I used 1/3 reagent grade hydrochloric and soft tap water. I'm going to try it again with distilled water and the acid to get rid of variables.

Edited to add this area near the tip could just be alloy banding. It's new steel 1095 from admiral by the way
 
Considering that it's the tip it's most likely decarburization.That's where you would most likely find problems like decarb or large grain. Don't assume that different etchants will etch the same steel the same way.When we look at steel through a microscope we use different etchants depending on what we want to look for, each one works differently .
 
Is grain growth and decarb synominous Mete? It seems strange that it would be just on one side of the edge. The grain does seem to be consistant size on the surface all along the hardened zone even in this area. I need a hardness tester, besides a file that is. I would love to hear more about what differant etches can tell us about metal properties is you care to explain.
 
I didn't see the "just on one side of the edge" but that would indicate decarb that wasn't ground off on one side .Decarb and grain growth are not synonymous exactly though both become more of a problem with higher temps and longer time. Etchants - macroetching (visible to the eye, without a microscope) typically they use HCl (muriatic or hydrochloric acid) for steel to show things like segregation of alloying elements such as "alloy banding". When we look through a microscope we get into the large subject of micrography, that is the preparation of samples for microscopic examination. That starts out by taking the sample, usually about 1" dia ,and polishing it perfectly flat and a mirror finish. And you thought metallurgists didn't know anything about polishing ! LOL It is then etched in up to about 10% solutions of usually hydrochloric ,nitric or picric acids.Optical microscopes usually use 1000X magnification, electron microscopes 100,000X .The more common uses for microscopes are to look for grain size ,decarb,basic structures such as martensite,pearlite, retained austenite.
 
Mark, I did 4 shear steel blades last week. My sorta normal etching action is to do 4 or 5 etches in 10% nitric and then 3 or 4 more in fc and water. I decided to put the other 3 in vinegar. Think I let them set in the vinegar for at least an hour and I was shocked with what I got. More pattern in the shear than I've gotten anyother way and alot less work. I decided to try some wrought iron in the vinegar and let it soak for 20 hours and it looked the same as if I had done nothing to it.
 
Thats the same issue I was having with my O/A set up......I thought I was just getting it too hot...causing the decarb???? Is that what causes decarb or the spots like you have here?
 
I am a complete idiot. Make sure you grind or scrape off the factory scale from barstock before you forge it.
 
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