Rimmed Steel, Semi-killed Steel, Fully killed steel

Some low-carbon steels are available in three grades: “rimmed”, “semi-killed”, and “fully-killed”. The terms themselves are derived from the action of the steel when it is poured into an ingot mold after leaving the furnace.
In terms of welding, they indicate whether or not there is oxygen in the steel which may cause weld porosity when certain welding processes are used. All carbon steels contain some oxygen. The very nature of the furnaces in which they are made makes this inevitable. It’s what happens to the oxygen when a weld is made that is significant. When a rimmed steel is welded, some of the oxygen will usually combine with some of the carbon to form bubbles of carbon monoxide (CO). These will cause weld porosity if they cannot escape from the molten weld metal before it solidifies. In oxy-acetylene welding, these minute bubbles of gas always have time to escape. In some other processes, such as tig welding (GTAW), however, they may be trapped in the solidified metal. To make a killed steel, aluminum (which has a stronger affinity for oxygen than carbon, manganese, or silicon) is added to the molten steel before it is poured. The aluminum locks up the oxygen, in the form of aluminum oxide, so that it can not form gas bubbles during welding. In a semi-killed steel, silicon may have been used, with or without aluminum, as a deoxidizing addition, and there may be some bubbles of carbon monoxide gas formed during welding.

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