Nitrogen in Steel 1

[UKCats]

I notice in many of the duplex steels, and alloy steels, higher concentrations of nitrogen are used (say 0.2-0.3%). As a Chemical Engineer, I can't comprehend how gasious nitrogen can stay in solution of steel at 2000F. Boiler Feedwater is stripped essentially all of its O2 and N2 (50 ppm) at 300F.

Can someone explain how that much nitrogen stays in the steel during a 2000F manufacturing process.

Thanks

 

[CoryPad]

A nitrogen atom is much smaller than an iron atom, so it diffuses into interstitial sites with the iron lattice, similar to a carbon atom in iron. Once inside the lattice, the iron and nitrogen bond. These bonds can be overcome with sufficient conditions (temperature, atmospheric nitrogen partial pressure). To maintain equilibrium, temperature, pressure, and nitrogen concentration must be balanced. During manufacture, the atmospheric nitrogen partial pressure must be kept high to prevent nitrogen loss.

 

[UKCats]

Excellent response... So if the N2 partial pressure is kept high, that tells me the steel is manufactured in a closed vessel and the N2 diffues into the steel from the top surface and what keeps the N2 concentration uniform in the steel is the temperature is kept very constant over a long period of time to ensure chemical homogenaity. Comments

 

[arunmrao]

It is true that Nitrogen is introduced into steel in closed vessels under presure. These are known as convertors. In open air melting it is difficult to introduce Nitrogen as an alloying element. I am told that high Nitrogen releasing compounds are available commercially but the efficiency of such a process is low.

 

[mcguire]

It isn't hard to keep nitrogen in some steels. Carbon steel, alloy steel, and ferritic stainless steel accept virtually none. If it can't get out of the steel, it preciipitates, like carbon does, as a separate phase or compound.
In austenitic steels or in the austenite phase of duplex stainless steels, nitrogen is quite soluble. At atmospheric pressure of nitrogen, 304 is at equilibrium when it has about 0.25% nitrogen at solidus temperature.
Manganese ehhances nitrogen solubility as does molybdenum.
So you really don't have to try hard to keep the nitrogen in. In fact, it would take a vacuum treatment or flushing the molten metal with inert gas to get it out.

 

[bruv]

Arunmrao

We add nitrogen into our steels using nitrided ferro-chrome, which typically contain 3% or 6% nitrogen. We are induction melting, in air, and our losses aren't that great.

 

[arunmrao]

Thanks Bruv,
Unfortunately this is not available in India and I do not know if anyone would be interested in supplying small batch quantities. Please try and help me in sourcing this material.

 

[aaranyi]

Dear Arunmrao & Bruv:

As a stupid chem. ing., heavily infected by mechanics, I am theoretically interested in the process of making nitrided ferro-chrome. If it is similar to nitriding steel, so the process seems to be rather simple, it can be easely accomplished practically anywhere.

Best regards Andras

 

[bruv]

Arunmrao

How small is a small quantity?

I will make enquiries and see if anybody here in the UK can make any suggestions. Perhaps somebody nearer to you than I am may have an idea - any Australians or Japanese out there who can help??

 

[bruv]

Apparently, making nitrided ferrochromium is very easy. Just smelt ordinary FeCr and bubble nitrogen throught the liquid. Chromium has a great affinity for nitrogen. It's just a matter of knowing when to stop.

Bruv

 

[bruv]

Oops, nearly forgot:

Arunmrao (if you see this - you didn't mark the thread)

I've had a word around and most nitrided FeCr in this part of the world comes from one of two sources, either South Africa or Germany. The South Africans are at Middleburg Steel & Alloys, but I've not yet found a decent link to them. Links you might find useful are:

http://www.aimcornet.com/metalagentxt3.htm

(South Africa)

http://www.hernic.co.za.

(South Africa)

http://www.elektrowerk.de/en/index2.html

(Germany)

Hopefully one will be able to help

Best of luck

Bruv

 

[arunmrao]

Thanks Bruv for the information. But I am yet to get hold of the N rich FeCr. Thanks again