Vacuum degassed vs fully killed steel 1

[JasonLouie]

What is the difference between vacuum degassed steel and fully killed steel? We have asked for fully killed steel and have been offered vacuum degassed instead. This is for SA516-70N in sour service.
Thanks.

 

[metengr]

The vacuum degassed steel will be a much better product in terms of overall steel cleanliness (inclusion content) and sulfur removal. Fully killed steel normally will not have the cleanliness as vacuum degassed steel. In this application, vacuum degassing may not be required, but I would take it.

 

[metengr]

To answer your question regarding a fully killed steel, this is a steel that contains either Si or Al and Si additions to deoxidize the melt to avoid outgassing during solidification. You can read more about this and steel making in general, see below;

http://www.key-to-steel.com/default.aspx?ID=Articles#p1

 

[steelmtllrgst]

Vacuum degassing is used primarily for removal of hydrogen from steel. The liquid steel is exposed to an evacuated atmosphere for a period of time, typically reducing the hydrogen value to <2 ppm.

Fully killed steel, as stated, is steel that has been deoxidized using either Al or Si. The deoxidant (Al or Si will create oxides that will for the most part float into the slag, but some residual non-metallics will definitely remain in the final product as inclusions and affect the cleanliness. This prevents blowholes/porosity from forming during solidification due to the low solubility of oxygen in solid steel.

Degassin a fully killed steel (a combination of both) will aid in removal of the deoxidation product and can aid in obtaining excellent cleanliness as well as low dissolved gas content.

 

[steelmaking]

Vacuum degassing gives opportunity to use technology "vacuum carbon deoxidation" - it means more clean steel, because the product of deoxidation is CO instead of Al2O3 or SiO2. The adding of Al and FeSi is only to cover needings required to the steelgrade. Also content of hydrogen is less than 2.5 ppm and the level of the nytrogen is 20-30 ppm, which is not possible to reach in full killed steels in normal atmosphere

 

[steelmtllrgst]

The only problem with VCD is that you are still working to the C-O equilirium, it is just under low pressure. This method is excellent for steels where deox products are not wanted at all, but in most cases precipitation deox gives lower total oxygen.

 

[INPIDI]

IF U CHECK ASTM 516 GRADE 70 SPEC U WILL NOTICE THAT BY DEFINITION IT IS KILLED STILL(THE NON KILLED/RIMMED EQUIVALENT IS 515 GR 70