Hi Mr Lakhsman and Mr Nopadon,
Thanks for your query and thanks to Mr Nopadon for sharing his experience.
Usually 3 factors contribute to SCC, which had been observed for Grade -91 steel in specific instances (please see below):-
- Stress .High residual tensile stress is prevalent in untempered or Non PWHT-ed grade 91 welds. This is a significant factor to generate cracking.
- Specific environment: Humid environment , close to coastal areas ( where the salinity is always high) could be a contributing factor for generating SCC. Also SCC significantly accelerates when the ambient temperature is always high 20 Deg C or more for grade 91 steel.
- Also CSEF steels e.g. grade-91 steel does not have enough resistance to SCC.
The as welded hardness in this steel is easily above 300-350 HV or more, hence leaving thus steel untempered , in any of the above environments would aggravate the probability of cracking .It’s the precise reason for controlling environments in case the PWHT is delayed.
People often talk about DHT:- DHT is good to mitigate the probability of Hydrogen cracking. This would work better even when the welding consumables are strictly H4 category or less (see attached). However performing DHT alone and delaying the PWHT does not solve all “cracking propensities” for this grade of steels.
These are my thoughts, which may be conservative. But cracks in Grade-91 steel could be a nightmare and needs to be avoided at all costs.
Please see highlighted text in the attached publication from TWI. Though written for Grade-92 steel, all rules are the same forGrade-91 also.
Thanks
Pradip Goswami, P.Eng,IWE
Independent Welding & Metallurgical Engineering Specialist & Consultant.
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Hi Mr. Pradip Goswami,
Thanks a lot for the info,
Shall we need to reconfirm the soundness of the weld and HAZ area by any means of NDT prior to PWHT once again? As it was delayed for more than a week, any specific requirements are their?
I’m not connected to this subject, but for my information I am requesting you….
Thanks & Regards,
Lakshman Kumar B,
+91 9440031459.
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of c.nopadon@vipco-thai.com
Sent: Friday, February 26, 2016 12:47 AM
To: materials-welding@googlegroups.com
Subject: [MW:24376] RE: 24374] RE: 24373] Time between Hydrogen Bake Out and PWHT
Once our shop got a relax from client to keep it as long as one week if the storage was done with assurance that relative humidity would be low and not create risk of cracking due to water.
What we did during that time was using heater in storage shade and monitoring humidity.
Anyhow, no body is comfortable to see 15E welded joint stayed without PWHT too long. PT/MT and volumetric inspection after PWHT is highly recommended.
Regards,
C.Nopadon
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of pgoswami
Sent: Friday, February 26, 2016 11:59 AM
To: materials-welding@googlegroups.com
Subject: [MW:24374] RE: 24373] Time between Hydrogen Bake Out and PWHT
Hi Mac,
One week without PWHT on 3.5” think Grade -91 Steel is too long. In spite of hydrogen bake out one may get cracking. Did any one check the hardness of weld and HAZ by any means (just for curiosity??).
Also prior to the DHT the welds should be cooled to Mf temperature which is generally the ambient temperature. In absence of it full martens tic transformation won’t happen causing poor creep strength in these steels. DHT may be adopted before the PWHT, however not advised to delay PWHT so long.
It’s worth looking at.
Thanks.
Pradip Goswami, P.Eng,IWE
Independent Welding & Metallurgical Engineering Specialist & Consultant.
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com]
Sent: Thursday, February 25, 2016 3:47 PM
To: Materials & Welding
Subject: [MW:24373] Time between Hydrogen Bake Out and PWHT
Dear Sirs,
We welded a 15E Pipe material (16 inch - 87mm thickness).
Our procedure will be preheated with 205 deg centigrade then welding and will be
( hydrogen ) bakeout and perform PAUT, If accepted in PAUT, immediately followed
by PWHT and will perform MT.
Process is B31.1
The joint has been accepted by PAUT, my query is, how long should be acceptable if there is a delay in performing PWHT?
Right now it is almost 1 week that the welded joint is not yet PWHT.
Need your advice.
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