I was making tongs for putting metal pieces into a crucible from two 10mm round steel bars. I had to heat them for forging and bending and did it by holding two carbon rods connected to a welder which heats it quickly and locally, unlike a propane flame. That worked, but after forging and cooling I blunted a drill bit when drilling the hole to attach both tongs together. Using a file also felt like filing hardened steel. So I did it again with a (slow) propane flame and the issue did not appear anymore, it remained normal workable mild steel. Does steel absorb carbon that quickly ?
Rapid cooling causes hardening. I think what might be happening is because the arc heats very locally, when the heat is removed, the rest of the part acts as a heat sink cooling off the heated section more quickly then when a flame is used and heat is more spread out. If you reheat the hardened part and make sure it cools down slowly, it will be workable again.
That sounds plausible, but on the other hand, this issue should also occur after welding, or is it only when carbon is involved ?
You may have, by using a very intense local heat source, nitrided the surface. Same thing happens with plasma cutting. Nitrogen from the air is applied to the surface and will make the surface hard as woodpecker lips. Check this by grinding a 16th off the surface and then drill. When welding shielding gas from flux or argon prevents nitriding. Denis
Yes, it were just ordinary piss pot steel bars. Filing on the non heated parts of the bar sounded and felt 'normal' , while filing on the previously heated part felt like filing rock.
The carbon arc rod could be giving the steel extra carbon to absorb (think case hardening) while you heat it. (This is my theory I have no proof.. but it's exactly what you do when yo case harden mild steel).
There are plenty of reports of cheap steel from china being a mix of unsorted scrap, as identified by half melted ball bearings etc. Maybe your piss pot steel happened to have enough carbon to be hardenable.
Grinding a bit off the surface and then filing or drilling will distinguish between surface hardening (nitriding) and through hardening due to alloy issues. Denis
Yes, this can be a problem with welding. This is why one is not supposed to quench hot welds with water as it will make the weld hard and brittle. quench hot welds with water is a bad habit for a welder. Alex
Maybe.... but it takes a lot of time for the steel to absorb the carbon in case hardening. It's a very slow process. This video is cringey as all hell, but explains the process well enough.
Jason, you're right about case Hardening, but it's my understanding that liquid steel absorbs carbon way easier. In industry they burn off almost all the carbon by adding oxygen into the melt. then add back only the amount of carbon they need for the type of steel they are producing. But it could very well be something akin to nitriding or a combination of the two...or something that we haven't presented yet.
before the blast furnace was invented reverberatory puddling furnaces were used to remove excess carbon to make bar iron and early carbon steel.
