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Killed steel is treated with an oxidizing agent to prevent carbon dioxide bubbles from forming, resulting in a denser and stronger steel. Deoxidizing improves hardness and chemical homogeneity, but can cause shrinkage. Stainless and structural steels are usually killed, as are those used for forging and heat treatment.
Steel that has been treated with an oxidizing agent, in order to bind the oxygen molecules present in the molten steel, is called killed steel. The carbon dioxide in the molten steel will form bubbles as the steel cools, which has deleterious effects on many of the steel’s qualities. By adding an oxidizer, such as aluminum, silicon or vanadium, the oxygen atoms in the molten steel are bonded to the oxidizer instead of bonding to the carbon and producing the carbon dioxide bubbles. This results in a denser steel without bubbles. Almost any type of steel can be blast chilled, but intended use generally determines whether a particular steel will undergo this process.
Deoxidizing a particular steel can improve many of its properties, especially hardness and chemical homogeneity, meaning that quenched steel tends to be more consistent, in terms of chemical composition and molecular structure within a particular sample, compared to an unkilled steel of the same formula. A killed steel will be denser than the same steel that is not killed, due to the absence of gas bubbles. For this reason many steel grades are treated this way, although not all applications require killed steel. For example, some steels used for casting are not killed, as killed steel tends to exhibit a greater degree of shrinkage than unkilled steels, which can cause problems with the cast parts.
Steels with a high carbon content, 0.25% or more, are almost always killed, as are steels with a carbon content between 0.15% and 0.25%, which are used as structural steels. Killed steel tends to be stronger and harder than unkilled steel, which is why structural steels are deoxidized. Some low carbon steels are also culled, depending on their intended use.
Stainless steels are almost always killed steels. The presence of free molecules of oxygen or carbon dioxide inside the steel can lead to oxidation of the steel itself, which manifests itself as rust. Stainless steel, by definition, is resistant to oxidation. While this resistance to oxidation is primarily a function of the additives to the steel alloy, the deoxidation process further enhances this quality.
All steels used for applications where steel is forged are usually culled, regardless of their other properties or additives, such as nickel, vanadium, chromium, and others. Bubbles that form in non-blasted steels can cause weak spots and structural defects in the steel. Steels that will be heat treated are also usually killed.
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