KILLED CARBON STEEL-
What is killed carbon steel?
You may come across this term in data sheets. In short it will be written as KCS. From the name itself it is clear that this is a modified form of carbon steel.
What is killed?
By nature any form of material has discontinuities or defect. No metal or alloy that is manufactured is free from defects. We need to define the acceptable level of defect. For example when welding is done on piping, Inspection department caries out radiography or x-ray to check for internal discontinuities.
Whereas cracks are unacceptable, discontinuities like porosities are acceptable to certain level. Similarly component manufactured by rolling, casting forging all have discontinuities. In case of some operating fluids, the discontinuities are not acceptable or should be kept to a minimum so that damages can be controlled.
So what is killed is the discontinuities like porosities, slag inclusion etc. This is a metallurgical treatment given to the molten metal in a steel plant.
Why killed carbon steel is required?
Killed carbon steel is typically specified in hydrogen services or services where atomic hydrogen is generated. If you recall the periodic table in chemistry, Hydrogen is the first element and hence smallest atom.
Small size of atom creates a problem. The atom size is so small that it can penetrate through steel. If there are discontinuities then hydrogen gets locked up at impurities location like slag and porosity and creates problem of blistering.
In such case, where hydrogen blistering is possible, killed carbon steel is used as these have very less discontinuities and hence no problem. Hydrogen will complete its travel through solid steel and escape into atmosphere atom by atom.
People who are aware of ASTM standard
ASTM A53 is a carbon steel pipe.
ASTM A106 is a killed carbon steel pipe.