How Hard is this Metal? The File Test

I'm in the beginning stages of building a kindling cracker. I picked up some rebar for the frame, and a chunk of steel to grind down and use as the splitting blade. It got me thinking though: how do I know if that chunk is hardened steel or boring old mild steel? If you don't know the difference, hardened steel can take much more of a beating than mild steel. It is what makes up axe heads, hammers and drill bits for that reason. So how do you tell them apart? One hint is to ask yourself what the steel is or was used for. If it is already used for hammering, cutting, drilling, or otherwise taking a beating, it is most likely hardened steel. That only works for certain pieces of metal though, so what about the rest? That's where the trusty file comes in. First clamp your piece down somehow. Then run the file along the edge of the test piece 3-5 times with firm but not heavy pressure. Mild steel will be ground down and have some material removed, whereas hardened steel will be unscathed.

I put together a basic video for you visual learners out there. I am not a professional video editor if you can't tell (lol) but it helps get the point across. Also I am not an expert in this area, not even close. I just wanted to put some neat info out there that I learned today. If anyone knows another simple way to test the hardness of metal let me know because I'd be happy to know.

when I went to metal fab school they taught us to watch the sparks when grinding a piece of steel
if the sparks kind of explode or starburst it is most likely high carbon steel and could be hardened if heated to red the oil quenched the heat a little again to temper so it will not be brittle.
rebar is not high carbon steel but an old leaf spring would most likely be.

As Bruce points out, what you're looking for is high carbon steel. This is steel that has the chemical composition allowing it to be hardened and take an edge that will last.

Different heating and cooling techniques can be used make such steel nice and soft for easy shaping/grinding or hard for toughness and edge retention.

Thanks for those tips guy that's awesome. I've always loved the sparks. They're like a little firework show every time I cut metal. I've never paid close attention to them though, next time I cut metal I will be sure to!

bruce Fine wrote:when I went to metal fab school they taught us to watch the sparks when grinding a piece of steel
if the sparks kind of explode or starburst it is most likely high carbon steel .

That makes sense! It's classic flint and steel -- hard steel makes hotter sparks.