Killed steel is deoxidized during casting to create a more uniform texture, increase density and durability. Deoxidizing prevents carbon monoxide bubbles that compromise steel integrity. Killed steel is denser and stronger than non-killed steel, but may shrink during casting.
Killed steel is a form of steel that has been treated to completely deoxidize it when worked during the casting stage. Deoxidizing the steel ensures a more uniform texture in the finished product while increasing density and durability. There are numerous applications for killed steel and some steel grades must be killed to meet material standards. Material tests can be used to determine whether or not the steel has been deoxidized when there is any doubt about the integrity of a steel product.
When steel is heated for casting, it interacts with oxygen in the air. Typically, some of the oxygen dissolves in the molten steel and travels with it into the die. Some of the oxygen escapes as the steel settles in the die, 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 tiny bubbles within the finished product. These bubbles compromise the integrity of the steel, creating weak spots that could crack or break, especially under stress. The bubbles also disrupt the texture of the steel and make it less dense. In some cases, this may not be seen as a problem, but in others it can become a problem. The steel used in the construction of a bridge, for example, must be able to withstand stresses 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 when it hits the die, most if not all of the oxygen is gone. Some say that “killed” is a reference to the fact that the steel does not boil in the mold once cast.
Killed steel has a very uniform grain and texture due to the absence of carbon monoxide bubbles. It is also very dense, lacking the small holes found in steel that has not been killed, making it heavier than similarly sized pieces of steel that have not been killed. Knocked down steel is sometimes prone to shrinkage due to density, which can be a problem in some casting applications. The strength and durability of this steel product is increased by deoxidation, although factors can affect the qualities of a finished steel product.
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