| **No, heating mild steel to cherry red and quenching it in oil will not case harden it.**
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| Here is why, and how you actually achieve case hardening:
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| ### 1. Why this process doesn't work
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| * **Mild steel lacks carbon:** Steel can only be hardened if it has enough carbon (typically 0.3% or higher). Mild steel has a very low carbon content (0.05% to 0.20%). When you heat and quench it, there isn't enough carbon trapped in the crystalline structure to form martensite (the hard microstructure).
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| * **Quenching alone isn't case hardening:** "Case hardening" specifically means creating a hard, high-carbon "skin" (the case) over a tough, low-carbon core. If you just heat and quench mild steel without adding carbon to the surface, you are just quenching low-carbon steel. It will remain relatively soft.
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| * **Oil is too slow for mild steel:** Even if the steel had enough carbon, oil is a relatively slow quenchant. Mild steel needs a very rapid quench (like water or brine) to force the hardening reaction; an oil quench allows it to cool too slowly to achieve any significant hardness.
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| ### 2. What *does* this process actually do?
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| If you heat mild steel to cherry red and quench it in oil, you are essentially just **normalizing** or **annealing** it. The oil quench will cool it slower than water, relieving internal stresses and actually making the steel *softer* and easier to machine or bend than it was before.
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| ### 3. How to ACTUALLY case harden mild steel
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| To case harden mild steel, you must introduce carbon to the outside of the steel before you quench it. This is called **carburizing**. The traditional, at-home method is called "pack hardening" or using a commercial compound like Kasenit (or "Cherry Red" case hardening compound).
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| Here is the actual process:
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| 1. **Apply Carbon:** Coat the part in a high-carbon compound (like Kasenit), or pack it entirely in a container filled with carbon-rich material (bone charcoal, leather scraps, or coke).
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| 2. **Heat and Soak:** Heat the packed/coated steel to cherry red (around 1500°F - 1650°F / 815°C - 900°C) and **hold it there for 15 to 30 minutes**. During this time, the carbon from the compound soaks into the surface of the steel to a depth of a few thousandths of an inch.
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| 3. **Quench in Water:** Once the carbon has soaked in, immediately remove the part and quench it in **water or brine** (not oil). The high-carbon skin will instantly harden into martensite, while the interior remains mild and tough.
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| **Summary:** You cannot case harden just by heating and quenching. You *must* add carbon to the surface first, and mild steel requires a water/brine quench, not oil, to lock that carbon in place.
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