harder without tempering?

[kogatana]

How can a part of a piece of mild steel becomes hard without me tempering it??

More details: I took that piece of mild steel from an old metal fence in a farm. I believe it is mild steel, and very mild for that because it bends veery easily, is cut with my hack saw very easily and a file bites in it like teeth in butter.

Now I make a nice hot charcoal fire, put there my 1" by 1/4" section of material, and taking it out after some time I notice that it looks as if it had melted. I do not pay much attention, though I'm a bit surprised since I've got red hot steel or iron before but never melted iron. Maybe this was only a partial melt, on the surface.

Back home (I do not forge at home, I'd bother the neighbours), I want to get rid of that ugly end on my small knife. It is then that I notice that it is very very hard, definitely much harder than the remaining of the knife (and it is the side opposite to the edge). I do a spark test, and well, definitely the sparks are way different at that end compared to other parts of the knife. It rings differently also.

I haven't quenched any part of the knife yet. Could it be that some carbon went into the material when it was hotter that usual and made a small area a high carbon steel? Otherwise how come did it become so hard?

Ludo

[HWooldridge]

If you cut a piece of mild steel with an oxy/acetylene torch, you will get a very hard surface next to the cut - something machinists will curse you for if they hit it with a milling cutter. This scale is not very thick but quite hard. It sounds like you have a burned spot on your steel - you need to grind it away to virgin metal.

[kogatana]

It might be scale indeed, but I have cut about 1 inch of the cracked hard part, and when grinding the cut it is still hard.

Why is scale so hard?

[ThomasPowers]

You can carburize low carbon steel into high carbon steel by heating it in a reducing part of a charcoal fire to a high temp for a while. Something thin may carburize all the way through. Of course it is usually un-homogeneous and you are never quite sure what you have got.

[evfreek]

Hi Kogatana. Mild steel can have hardenable spots. I was in a blacksmith workshop making spatulas. They used A36 (kind of like mild steel). I found that if you quenched from red before drilling, some holes could not be drilled. The instructor said that this kind of steel can have hard spots.

But, you said above: "hard without me tempering it?" I think this means hard without quenching????? In that case, maybe the burned end is very hard due to carburization in the fire. I missed a weld, and it just would not stick for the second try. The face of the scarf was silvery, and it definitely was NOT scale. After letting the piece cool, it was too hard to clean off with a file. It required an angle grinder (and a MIG welder). This stuff was good and hard, and no quench.

When you did the spark test, did the spark test look like high carbon steel? Steel with 1%+ carbon has a very memorable bushy appearance.

[bridgeport]

Koga, You have "burned your iron". you will have to grind away all the oxidized parts no matter how deep you have to go... or just throw it away and start fresh. The burned portion is very hard indeed but not in the way you would want it to be.. consider it a "crust" like burned pizza dough.
That is the Non technical explanation... This is not carburization, but oxidation.

[kogatana]

evfreek,

But, you said above: "hard without me tempering it?" I think this means hard without quenching?????

Yes I meant quenching.
It seems you have observed something very similar to what I have. Part of my piece was also silvery, it got dark covered with scale after I put it back into the fire.
The spark test: I'm not expert, but by comparing the sparks on the hard end with those from other part, it is striking that the sparks on the hard part are totally different: the end of the sparks appeared somehow like many snow crystals, while on the other parts, nothing special, the bright rays were more like shooting stars: bright curved lines with no sparkling at the end.

Bridgeport

You have "burned your iron". you will have to grind away all the oxidized parts [...]consider it a "crust" like burned pizza dough.

I have cut the hard part (one low quality hacksaw blade just lost all its teeth in the operation!), and thus had access to bare steel in the thickness of my piece. The bare steel was still very hard, so it was more than a crust. However my piece was quite thin (<1/8"), so maybe the whole thickness got "burned" as you said... But are you sure we can "burn" steel in a similar fashion as we can burn a dough crust??

Ludo

[bridgeport]

Koga, You are blacksmithing.... Forget the hacksaw and cut it on your hardy.