Killed steel is a form of steel which has been treated to fully deoxidize it when it is processed during the casting phase. Deoxidation of steel ensures a more even consistency in the finished product while increasing density and durability. There are a number of applications for killed steel, and certain grades of steel must be killed in order to meet materials standards. Materials testing can be used to determine whether or not steel has been deoxidized if there are doubts about the integrity of a steel product.
When steel is heated for casting, it interacts with the oxygen in the air. Typically, some oxygen becomes dissolved in the molten steel, and travels with it into the mold. Some of the oxygen bubbles out as the steel settles in the mold, but some of it becomes trapped, where it interacts with the carbon in the steel to create carbon monoxide.
The carbon monoxide, in turn, creates small bubbles inside the finished product. These bubbles compromise the integrity of the steel, creating weak points which could break or fracture, especially under strain. The bubbles also disrupt the texture of the steel, and make it less dense. In some cases, this may not be viewed as a problem, but in others, it can become an issue. Steel used in the manufacture of a bridge, for example, needs to be able to withstand stress without developing cracks.
When killed steel is made, a material such as aluminum, silicon, or manganese is added to the steel before it is poured into the molds. This deoxidizes the steel, forcing the oxygen out of the steel so that by the time it hits the mold, most if not all of the oxygen is gone. Some people say that the “killed” is a reference to the fact that the steel does not bubble in the mold once it is poured.
Killed steel has a very even grain and texture as a result of the absence of carbon monoxide bubbles. It is also very dense, lacking the small holes found in steel which has not been killed, which makes it heavier than pieces of steel of the same size which have not been subjected to this process. Killed steel is sometimes subject to shrinkage because of the density, which can be a concern in certain casting applications. This steel product's strength and durability are increased by deoxidation, although factors can influence the qualities of a finished steel product.