Arc heating mild steel hardens it ?

Discussion in 'Foundry tools and flasks' started by metallab, Aug 2, 2020.

  1. metallab

    metallab Copper

    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 ?
  2. rocco

    rocco Silver

    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.
  3. metallab

    metallab Copper

    That sounds plausible, but on the other hand, this issue should also occur after welding, or is it only when carbon is involved ?
  4. 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.

    Last edited: Aug 2, 2020
  5. Rotarysmp

    Rotarysmp Silver

    Do you know the actual steel spec? Are you sure it is a low carbon steel?
  6. metallab

    metallab Copper

    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.
  7. crazybillybob

    crazybillybob Silver Banner Member

    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).
  8. Rotarysmp

    Rotarysmp Silver

    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.
  9. Grinding a bit off the surface and then filing or drilling will distinguish between surface hardening (nitriding) and through hardening due to alloy issues.


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