Purging Chromolly

Date 03-05-2005 09:39

Hi Guys,
I posted this response on "Shop Talk" but I am still keen on seeing if I can get info on my last question and thought if I posted it twice I would double my odds,
Thanks and regards,
Shane Feder

Theophilus,
Greetings from "Down under".
My first question is does nitrogen purging gas contain any non-inert gases.?
The reason I ask is I am currently working on a 1 1/4 chromolly welding procedure and I am researching addition/deletion/composition of purge gases.
QW 256 (GTAW welding variables) lists relevant Gas variables as 408.2,408.5,408.9.
408.2 covers shielding gas.
408.5 (...."a change in backing gas composition",.... is a non essential variable.
408.9 " For groove welds P41 through P47..... (you mentioned P43 material),the deletion of backing gas or a change in the nominal composition of the backing gas from an inert to a mixture including non-inert gas(es).
If nitrogen is a full inert gas does this mean that changing between Argon and Nitrogen does not require requalification of welding procedure ?
Also,are there guidelines anywhere on what percentage of chromium is considered the minimum before purge is required on chromolly piping ?
Regards,
Shane Feder

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By George-kh (**)

Date 03-08-2005 00:20

you don't need to purge a 1 1/4 chromolly material.
and NEVER use nitrogen for purging of steels except series 300 stainless steels. best composition is "90% N2 + 10% H2" for series 300 stainless steels, if you want to use nitrogen.

By Shane Feder (****)

Date 03-11-2005 00:18

George-kh,
Thanks for your response.
It was never my intent to purge 1 1/4 chromolly.
I have been supplied with welding procedures for a project by my companies head office on the other side of the country and these had been accepted by the client prior to my commencement on site.
The PQR had been qualified using argon purge gas and the Senior Inspector who qualified the procedures agreed that I could delete the purge as per QW 408.5 but he would "prefer" that we use purge.
All the CWI's that I have discussed this with agree with me but I was hoping to obtain some sort of written document through this forum to take to the design engineers to prove that it is not necessary.
We are welding 12 metre lengths of 250 nb Sch 80 chromolly together (no heat resistant purge dams available) and the cost of fully purging these pipes would be phenomenal.
Hope someone can help,
Regards,
Shane Feder

By Jim Hughes (***)

Date 03-11-2005 00:58

It's just like what George-KH said, you do not have to purge 1 1/4 chrom. In fact you don't need to purge up to 2 1/4 chrom. The way I see it if you can't convince the senior inspector as per QW 408.5 that you can drop the purge than your stuck. When I was working in the petro-chem industry we were building a ASME B31.3 1 1/4 line that 100% x-ray. We purged it with argon because the film came out looking cleaner. It was only 4" Sch 80 40 foot long joints with 180's welded on. Did you try looking on the internet under "welding of 1 /1/4 Chrom.?

By George-kh (**)

Date 03-12-2005 14:02

If you read “ANSI/AWS D10.8-96, Recommended practices for welding of Chromium-Molybdenum steel piping and tubing” in section 4.2 “root pass consideration” is mentioned that:

“…… the following common practices, which are based upon chemical composition of the filler metal, are often selected:
(1) under 4% Cr no back purging
(2) 4 – 6% Cr sometimes back purge (based on service requirements)
(3) over 6% Cr always back purge"

I can mail you the page from ANSI/AWS D10.8-96, if you need it.

By Shane Feder (****)

Date 03-13-2005 23:50

Thanks for your help guys,
I have had a win and engineering have accepted a revised weld procedure.
Much appreciated,
Shane Feder

By George-kh (**)

Date 03-14-2005 23:38

May you tell us what is your new procedure?

By Shane Feder (****)

Date 03-15-2005 23:28

George-kh,
As per QW 200 (c) I have amended the original WPS to reflect the fact that purge has been deleted as allowed in QW 408.5.
Regards,
Shane Feder