Nitrogen removal during degassing

Aug 04, 2004 5 Replies
L

I can't find any textbooks to help me out, so I want to ask the group: In what way does sulfur impede or accelerate the out gassing of nitrogen during degassing of carbon and 8600 grades of steel. The degasser is a tank unit and the pressure is lowered to 0.5 mm Hg for 10 minutes. A porous plug is used in the bottom of the ladle to stir the heat so all of the steel is exposed to the lower pressure. We want to add N to the steel, (up to 150 ppm) and I understand the S content during degas affects the N removal. If someone can help me, I'd appreciate it.



Thanks



Lloyd Hanning


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A

Cool question! :)

Which 86xx?

8670-modified used in saw blades is the only one I'm familiar with. For the most part, what use/s are your 86xx going to go too? Bearings? Saw blades? Springs?

Alvin in AZ

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L

Hey Al: The end use is automotive gears, for transmissions. The grades I meant were

1538, which is a high Mn and this particular spec also has .08% V and 110 ppm N. There's also a 1545 with a .21 V and 160 ppm N.

Any help would be appreciated.

Lloyd Hanning

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A

Cool, thanks. :)

Neither of those steels are listed in my Metals Handbook exactly. Where can a guy find a list that includes 1538 and 1545 etc? Not that I need them, I'd like more information about others.

Also what do you mean by "8600 grades" if it's the high Mn 1500 steels?

I'd also like to understand the other steel classification systems they use in the US that aren't listed in my Metals Handbook.

Hmmmm... maybe a I have new access to some of information since I just joined ASM and haven't bothered to look into it yet? :/

Two steels that are "named? numbered?" 0186 (8670-modified) and

0170-6 (50100-B)... are they just that? Names? :/ Or is there a classification system that they belong to? :)

Alvin in AZ ps- even my dumb response is good for an original post ;)

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L

Hey Al: Go to

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's some pretty good info that may interest you. As you can see, I haven't had too many replies to my question. Maybe I'm in the wrong newsgroup? I'll try some of the steel producer sites, just might find something there.

Lloyd

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E

I guess your original question was related to how sulfur would impede or accelerate the out gassing of nitrogen during degassing.

The mass transfer for nitrogen is normally given due considerations with respect to the actual contents of sulphur and oxygen in the metal bath. Oxygen and sulphur are very surface active (only surpassed by selenium) and will thus have a strong impact on the removal rate of dissolved gases with relatively large atomic radii. If using Fick's law of first degree for estimating the removal rate of nitrogen during the vacuum treatment, then the nominal mass transfer coefficient, kN, should be corrected according to the concentration of oxygen and sulphur in the steel, e.g.: kN,corr = kN / (1+corrFact)

- where corrFact could typically be something like corrFact = 770 * a[O] + 620 * a[S] a[O] and a[S] are here the Henrian activities of sulphur in the steel.

This explains why one should vacuum degass the steel after it has been desulphurized and in a highly deoxidized state. The rates for removal and dissolution of nitrogen are both affected the same way. So, if your intentions are to increase the dissolution rate of nitrogen by purging N2-gas through the porous plug, then the steel should first be desulphurized and have a lowest possible oxygen content, preferably by addition of CaSi.

EMJ

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