During the steel making process, oxygen may become dissolved in the liquid metal. During solidification, the dissolved oxygen can combine with carbon to form carbon monoxide bubbles. The carbon is added to the steel as an alloying element.
The carbon monoxide bubbles are often trapped in the casting and can act as initiation points for failure.
How Killed Steels are Produced and Their Advantages
Formation of the carbon monoxide bubbles can be eliminated through the addition of deoxidising agents such as aluminium, ferrosilicon and manganese. In the case of aluminium, the dissolved oxygen reacts with it to form aluminium oxide (Alumina, Al2O3). The formation of alumina not only prevents the formation of bubbles or porosity, but the tiny particles or inclusions also pin grain boundaries during heat treatment processes, preventing grain growth. Completely deoxidised steel are known as “killed steels”.
Killed steels as you may know are fully deoxidized steels by addition of certain deoxidizers like Aluminium. They are mostly used in vessels, heat exchangers, oil and gas pipelines, and underground pipelines. As these have no oxygen in them, they are less corrosive hence are used in such applications.