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Some things you want to have bubbles, some you don't.
Usually, bubbles are good.
In beermaking, yeast consumes the sugars in the wort and converts them into CO2 gas bubbles - carbonation.
In steel making, the main reaction is the combination of Carbon in
the melt with Oxygen to form a gas. At the high temperatures involved,
this gas is very soluble in the molten bath.
If the Oxygen that is available for this chemical reaction isn't
completely removed before the steel is cast, the gases will continue to
be forced out of the melt during solidification, resulting in porosity
in the steel.
Bubbles and where the gas goes can be important in your steel part.
In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium,
Niobium scavenge the available oxygen in the molten steel, react
chemically to form solid oxide particles dispersed throughout the
steel, rather than bubbles of Carbon Dioxide.
The amount and type of deoxidizer added determines the type of
steel. If sufficient deoxidizers are added, no gas is evolved from the
solidifying steel, and the steel is said to be "killed." The ingot
drawing labeled number 1 shows a fully killed (deoxidized) steel
showing only a shrinkage cavity, and no bubbles or porosity. (This
shrinkage cavity would be cropped off in normal rolling practice.)
Because gas is still evolving, this beer is NOT KILLED.
Killed steel has more uniform chemical composition and properties
than rimmed, semi-killed, or non-killed steels, and generally less
segregation. The uniformity of killed steel and and its freedom from
porosity makes these steels more suitable for critical components and
for applications involving heat treatment.
Killed steels generally contain 0.15 -.35 weight percent Silicon as a
deoxidizer, and may contain some of the other elements as mentioned
above. These other elements may be used as deoxidizers or as grain
refiners.
Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as "killed steels."
Free machining steels such as 12L14, 1215, and some 11XX
series steels are not "killed" with Silicon, Aluminum, etc., due to
their deleterious effects on tool life and machinability.
The high amounts of Manganese in these steels form Manganese Sulfides
to promote machinability, and also the Manganese scavenges excess
Oxygen, preventing evolution of CO2.
Killed steel is specified so your critical parts won't have bubbles in them.
Killed steel for critical parts. Non-killed beer for critical after work down time.
Cheers!
Beer Bubbles Photo Credit
Ingot scan from a handout in my files originally after Making Shaping and Treating of Steel.
Beer Head Photo Credit
Bread with Holes
Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which was originally posted here.
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Re: Killed Steel
