Killed Carbon Steel is a process of deoxidation that is performed during the steel making process. Normally, ASME B&PV Code requires steels that have 0.24% or greater carbon contents to be killed (fully de-oxidized) using either Al or Si additions, and especially those steels that fall under a forging grade.
Steel that has been completely deoxidized by the addition of an agent such as silicon or aluminium, before casting, so that there is practically no evolution of gas during solidification. Killed steels are characterized by a high degree of chemical homogeneity and freedom from porosity.
We say the steel is “killed” because it will quietly solidify in the mould, with no gas bubbling out. As a result, killed steel is dense in structure, uniform in composition, and not so segregative as other types of steels.
- 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 deoxidized steel are known as “killed steels”.
- They have a more uniform analysis and are relatively free from ageing.
- For a given carbon and manganese content, killed steels are usually harder then rimmed steels.
The disadvantage of using killed steels is they often display deep pipe shrinkage.
Steels that are typically killed and are generally killed include:
- Steels with carbon contents greater then 0.25%
- All forging grades of steel
- Structural steels with carbon content between 0.15 to 0.25%
- Some special steel in the lower carbon ranges
Usage of Killed carbon steel
- Killed carbon steel is used for applications requiring uniformity in grain structure.
- Common applications include plates for Heat exchangers, pressure vessels and piping on low temperature service.