N2 Backing Gas For Stainless Steel (austenitic)

By zmxhzyc Date 03-04-2009 09:07

it is suitable generally, but normally the nitrogen hydrogen mixture is the option.

By Lawrence (*****)

Date 03-04-2009 12:50

Terry

Nitrogen is used often enough for purge gas instead of argon on 300 series austinetic stainless...

This being said the two gasses are not necessarily interchangable.

The decision is an engineering level call that must consider service conditions.

By Metarinka (****)

Date 03-05-2009 07:08

someone correct me if I'm wrong, but I had it stored in my mind that nitrogen content of the shielding gas had a serious effect on ferrite formation?

By js55

Date 03-05-2009 13:51

It can if ionized, which is why it isn't used too often in shielding gases unless that effect is desired like in super duplexes. It is generally restricted to purging for austenitics where its difficult for it to find its way into the arc column to be ionized because of the positive and massive gas flow of argon from the torch.

By Metarinka (****)

Date 03-05-2009 18:22

thanks, I'll be sure to read up this some more.

By Chin Terry (*)

Date 03-05-2009 08:57

will N2 backing gas affected the weld penetration, weld pool viscosity? and finally affected the welding position?

By Nanjing Date 03-05-2009 14:10

It is generally used for two main things. It is easier to purge in larger volumes and it is cheaper. Downside is the quality of the purge is not so good. So if your welding standard 300 series stanless steels and you are not too concerned about the standard then nitrogen will suffice.

By Chin Terry (*)

Date 03-10-2009 15:42

meaning that there will be some oxygen inside pipe line while purging? therefore, pipe internal will be oxided? then how about add some volumn of helium? will it help?

By js55

Date 03-05-2009 18:29

Can't say as though I have ever visibly noticed such.

By Shane Feder (****)

Date 03-05-2009 19:53

js55,
As you are probably well aware argon and nitrogen as purging gases work in different ways. Argon is heavier than air so once your argon level has risen above the weld level you should have 0% oxygen, whereas the nitrogen combines with the air to change the composition of the gas in the chamber. Therefore it is harder to confirm complete removal of all the oxygen (if not using a purge monitor) and if welding commences prior to complete removal of oxygen the weld may be acceptable but not as higher standard as an argon purged weld.
Hope that makes sense.
There may be other metallurgical reasons that Nanjing was alluding to.
Regards,
Shane

By js55

Date 03-05-2009 22:14

Shane,
Thats a good point. It will take longer.

By Nanjing Date 03-06-2009 10:27

Should we not all be dead then? If we take it that the half way point in the atmosphere is 18000 ft and that the argon content is 0.9% then according to your theory that argon does not mix with air and is heavierthen we should have approximately 162 ft of argon from sea level!

By Lawrence (*****)

Date 03-06-2009 12:11

I don't know what your talking about man..........

Of course the Argon is heavier than air..

Why else do I stand I my head for 15 minutes after every shift and 10 minutes out of my lunch break so that the argon drains out of my lungs!

Ok.. This thread is really making me think about regular atmospheric argon, gas density, and purging procedures in general.

Come on you physics egg heads... splain

By Nanjing Date 03-06-2009 16:00

Lawrence, what have you being inhaling man? So simple, just like playing the flute.

By Lawrence (*****)

Date 03-06-2009 17:54

Apparently I've been inhailing Argon!

Since it is more dense than air my flute is now playing contralto!

By Shane Feder (****)

Date 03-06-2009 19:24

Mr B,
You been on the Tsingtao again ????

By Metarinka (****)

Date 03-06-2009 19:22

there's two separate things at play here Dalton's gas law if i remember correctly states that a static mixture of equal gases will uniformly distribute. We know this to be true, because if it wasn't Argon/helium gas mixtures or virtually any gas mixture would separate out into it's components in our gas cylinders. And the low spots of earth's atmosphere would be full of the heavy gases like argon and xenon.

That is the assumption that they are a STATIC mixture in a container given enough time to fully diffuse (Which doesn't take that long). This diffusion issue is key in high performance engines where the oxygen and fuel vapor only have a fraction of a second to completely mix. Ideally the fuel mixture would be completely mixed before entering the cylinder.

Now lets investigate purging.

In purging we are now talking about displacement in which a very heavy gas such as argon displaces the lighter air mixture over a short period of time. Therefore you would bleed the air out of the top. Some diffusion will take place but the purge volume is generally cycled several times and doesn't have to be 100% perfect for most welding operations.

IF you were to say displace all the air in a fish tank with argon, and wait several hours with an open top, they would slowly diffuse and the oxygen level would slowly increase. IN critical materials that can't handle even very low PPM's of oxygen the atmosphere chamber is either placed under a high vacuum, or purged many times as there will always be some diffusion.

By Lawrence (*****)

Date 03-06-2009 21:23

Ok

That sounds feasable..

But please don't publish this too widely or we will no longer be able to get green apprentices to stand on their heads at the end of the day.

By DaveBoyer (*****)

Date 03-07-2009 05:36

Or roll the mixed gas cylinders around on the floor at the beginning of the day. :-)

By Metarinka (****)

Date 03-07-2009 07:28

your secrets are safe with me. I must be working with nice people because in my green days as a welder no one ever tried pulling my leg or asking me to grab 710 nuts from the tool crib.