Effect of purging gas in stainless steel

Date 06-23-2009 11:40

Crashproj,
Use the search engine.
Just type in purging of stainless steel and you will have loads of info.
Chears,
Shane

By 803056

Date 06-24-2009 01:26

The end use dictates the degree of "cleanliness" required. An aggressive environment, such as deionized water or certain corrosive electrolytes, requires the welds to be tint free, whereas an application that is dry may operate perfectly fine with some tint present.

Best regards - Al

By crashproj (*)

Date 06-24-2009 10:41

Shane and Al

Thanks a lot for the info.
But how to control the discoloration is it because of the insufficient gas purge flow that produce the yellow, bluish, brown color in the HAZ or it is simply because of the heat input during welding. My PMC is saying that the bluish color we can see in the HAZ, is it because of the purge gas was shut off after the weld was completed and the pipe is still hot..

Al,

All this stainless pipe will be in the process line for gas to liquid project.

Thanks a lot guys

By 3.2 Inspector (***)

Date 06-24-2009 11:14

Crash,

Check this out, it might be in danish :)
But in the second link you can see different grade of discoloration, and if you put on a pair of glasses, you can also se the O2 content

http://www.polysoude.com/documents/english/Influence_%20of_residual_oxygen_on_the_welding_result.pdf

http://www.forcetechnology.com/NR/rdonlyres/E3B9C9C6-378E-4BE3-9865-ED8BEAF967AA/2197/20542da.pdf

You can order the full reference chart (it cost about 30$) by writing to jvh@force.dk, J Vagn Hansen (jvh) is considered a true expert in welding of SS in my part of the world.

3.2

By Shane Feder (****)

Date 06-24-2009 11:30

Crash / 3.2,
We had our very own stainless steel "guru" on this forum for many years who sadly passed away a while ago. He was a friend and mentor to many of us here and was world renowned when it came to s/s.
Enter Chuck Meadows into the search engine and you will have access to an amazing quality of information.
I would be surprised if there wasn't an answer to every conceivable question on s/s.
RIP Chuck,
Regards,
Shane

By 3.2 Inspector (***)

Date 06-24-2009 12:16

Shane,

I have read some stuff from him, great stuff!
Ups....I still owe you some documents, sorry for the delay.

3.2

By 803056

Date 06-24-2009 12:43

I believe the PMC may have a valid opinion on the cause of the tint assuming there was sufficient purge to begin with.

My experience is that most welders do not allow sufficient time to adequately purge the pipe spool. Next would be the failure to "seal" the system from leakage to prevent oxygen and nitrogen from leaking into the spool faster than the purge gas can displace it.

I had one case where the pipe spools consisted of several joints each with 1/16 inch root openings at each joint. They didn't tape the open root joints and they used a 1/4 inch diameter hose to feed the argon purge gas into the pipe spool. They did switch to a liquid argon supply and they installed a filter on the liquid argon tank to no avail (no kidding!).

The contractor insisted there was no way to obtain a proper purge and my client would simply have to live with the results they obtained.

I disagreed, changed the purge setup, timed the purge to displace the volume six times and obtained perfectly silver root surfaces.

They then claimed that they wouldn't make any money on a job if they followed my instructions.

The system they installed was completely dismantled. They followed my instructions on the re-installation and got the silver root surfaces required by the client and contract. As they say, "There isn't enough time or money in the bid to do it right the first time, but there is to do it right the second time."

Darwin's theories were right and they hold true in the business world as well as the natural world.

Since Chuck is no longer available to provide us with his council, I suggest some literature available from the Nickel Development Institute. They have several papers available on the affects of root oxidation (heat tint) in corrosive environments.

May our good friend Chuck rest in peace.

Best regards - Al

By Shane Feder (****)

Date 06-24-2009 13:44

3.2,
It is OK, thank you anyway.
Even though I am not doing the course they have given me the full course notes.
Sit the exam in 2 weeks, think I should be OK
Cheers,
Shane

By crashproj (*)

Date 06-25-2009 09:24

Al,

Thank you for the info, but i do not agree with the PMC reason that this bluish color with in the HAZ area(tint) is due to the purging gas was shut off right after the weld was completed. Because the welder purge the pipe 5 mins. for just 14"lenght 2"dia. before he start welding and still the same result we've got no oxidation on the root pass , the HAZ from inside and outside the pipe is bluish in color even we shut off the argon purge gas 15 mins after the weld joint completed.

Thanks and Regards,

By 803056

Date 06-25-2009 20:06 Edited 06-26-2009 03:30

Sorry, but I'm not sure I understand your response.

If the pipe was purged before welding, during welding, and for a short time after the weld was completed and still there is a heat tint there are only three probable causes I can think of. They are:

1) Insufficient purge or leakage that prevents a proper purge.

2) The purge was discontinued while the pipe was still above some temperature correlating to the temperature at which the temper colors observed form. They would be light straw at around 400 to 425 degrees F, dark straw at about 500 degrees F, brown at about 550 degrees F, purple around 575 degrees F, dark blue around 600 degrees F, and black around 800 degrees F.

The color of the tint provides a very good indication of when the purge was interrupted. In other words, the purge must continue to be maintained until the temperature of the joint and surrounding metal drops below 350 degrees F to prevent some level of discoloration.

3) Contamination on the inside or outside of the pipe. Contamination can discolor the weld and base metal beside the weld. The contaminate can be oils, drawing lubricants, solvents, paints, or anything that can be volatilized by the heat of welding. The maximum affect would be the weld that was liquid at some point and capable of reacting with the volatiles in the pipe. In other words, the pipe has to be swabbed to ensure it is clean. I've encountered this problem on several jobs. The "cure" was simple: clean, clean, clean until the solvent soaked swab shows no discoloration and allow ample time for the ID/OD to dry completely before assembling the spool.

If the purge and cleaning are done properly, there is no reason for discoloration on the ID. One way to prevent discoloration on the OD is to use a purge that envelops the entire diameter of the pipe. This is not uncommon when welding titanium.

Best regards - Al

By crashproj (*)

Date 06-25-2009 20:57

Al,

thanks a lot for the information it would be a great help to me.

Best regards

By labib

Date 06-27-2009 08:15

Without inert gas purging oxidation is one of the major concerns which can lead to
corrosion ,reduction in mechanical properties and content of nitrogen can lead to brittleness
This might give rise to cracking during or after cooling.

Presence of Oxygen and lesser extent Nitrogen around the molten weld
can lead to wide ranging defects. Discoloration is unsightly and in some
instances might reflect metallurgical imbalance, especially with some stainless steels.

We hope this helps