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One side harder than the other.

Gabe Rathe

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I am not sure if this topic would go here.  


I was wondering if anyonbe has noticed that one of side of the metal; whether blade or stock, hardened or unhardened; seems to be harder to file or sand over the other.  I have noticed it and it seems that it could be logically possible.  When I first noticed it I thought it was my side vs. side strength, but I usually am ambidextrous.  I tried different tactics to make sure I was experiencing it and seems to be right.  Sometimes after blades are hardened and I am finishing the edge I notice it and sometimes on new piece of metal I cut out for a blank and I am filing it into shape I can tell that there is a difference. 


I usually dont notice as much when I forge the knife unitl after heat treatment.  I which case I theorize it could be because the flame from the forge is "pouring down" on one side while the other is laying on the firebrick which could create minute differences in the metal.  


Just thought I'd ask or if I'm seeing aliens.

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That's classic decarburization from the burner position.  Venturi burners almost inevitably run lean, or oxidizing. Carbon likes oxygen far more than it likes steel, so when high carbon steel is placed in a hot oxidizing atmosphere it will lose carbon from the surface.  You may even notice that the "softer" side can be filed a few strokes and then it seems to suddenly harden up again.  That means you've filed through the decarb layer back to good steel.


That's why I don't like forges where the burner enters vertical top dead center.  The idea of a gas forge is for the burner to heat the lining, which then heats the steel via convection and radiation at the same time.  The people who sell cheap forges don't know this, and assume the idea is to use the burner like a blowtorch and that the forge itself is just there to contain the heat a bit.  


As for how you find it before HT, it could be decarb or it could be mill scale.  New hot-rolled steel will have a layer of hard scale (the smooth dark gray stuff) that is harder than a file directly atop a thin layer of decarb.  Cold-rolled will be harder on the surface via work hardening, even though they usually remove the scale first to get a cleaner finish.  


And then there's scrap steel, in which case you never know what's been done to it before it found you.  Unless you're ordering new steel advertised as spheroidized annealed, you never know what its condition is going to be when you get it. Hot rolled won't have been hardened, but it's not always annealed either.  Same with cold rolled.  Look for an "A" after the condition code.  In the catalogs you'll see things like "1095 HRA" which means Hot Rolled Annealed. It'll have scale and decarb.   If it doesn't have the A, it's just hot rolled and left to cool.  This is why you'll also see tool steel advertised as "precision ground."  It's not just for those who need a plug-and-play dimension for accurate cold work, it also means there is no decarb or scale to mess with.   

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Thank you for the thorough explanation.  I worked in a small town weld shop for while and have been doing industrial HVAC service for almost 30 years; my metallurgy knowledge is lacking, but I have a high tool and crafting skill; I knew something was awry.  Just trying to learn more about metallurgy.  I find it fascinating.  

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