Dear Mr.Pradip Goswami,
Thanks for a wonderful explanation. It was very informative. Do you still think solidification cracking can be ruled out in the present situation? ESAB product catalogue gurantees ferrite content of FN 6-12 for E347 consumable which is definitely good to resist solidification cracking. But dont you feel that checking the ferrite content in the weld metal will give us an idea. I think too low ferrite (to avoid solification cracking) or too high ferrite (transforming to sigma) will influence the cracking of the weld. With Regards, A.Praveen
--- On Sun, 1/9/11, pgoswami <pgoswami@quickclic.net> wrote:
From: pgoswami <pgoswami@quickclic.net>
Subject: RE: [MW:9156] MW:9116-PWHT Of 347H
To: materials-welding@googlegroups.com
Cc: KBazrafshan@BPC.CO.IR
Date: Sunday, January 9, 2011, 8:37 PM
Dear Mr. Praveen and Mr Bazrafshan, SS 347H is a very proven material for high temperature application in petroleum/ petrochemical and thermal power plants.However there are proven examples of fabrication problems, especially cracking in after PWHT, commonly called "Reheat Cracking".The mechanisms behind it are as follows (Ref: Welding Metallurgy of Stainless Steels ( Dr JOHN C. LIPPOLD ,Dr DAMIAN J. KOTECKI)-:- -
Reheat, or stress-relief, cracking is unusual in standard grades of austenitic stainless steels· but may occur in alloys that form MC-type carbides during the stress-relief thermal cycle. Type 347, which contains Nb and forms NbC, is known to be susceptible to this form of cracking . Higher-carbon heat-resisting alloys such as 304 Hand 316H may also be susceptible to reheat cracking, During welding, alloy carbides dissolve in the high-temperature HAZ adjacent to the fusion boundary. The weld metal also contains carbon and carbide-forming alloy elements in solution. When the weldment is reheated during a post weld;stress relief, carbides precipitate in the grain interiors and strengthen these regions relative to the grain boundaries. If significant stress relaxation occurs in this same temperature range, failure can occur preferentially along the grain boundaries. This phenomenon has been observed in both the HAZ and weld metal. The combination of stress relaxation and NbC precipitation in the grain interiors promoted cracking in this weld metal. NACE RP-170-2004 recommends stabilization Anneal @ 843-900 deg C for a period of 2-4 hrs for re-formation of Nb-Carbides in 347H S.S and avoid formation of Cr-Carbides (which will destroy the basic corrosion resistance of 347 H S.S). See the attached Q&A from Damian Kotecki as illustration. However selection of the right PWHT cycle is left up to the fabricator. In you case a combination of one or more of the recommendations below may help to solve the problem:- -
Check the Max, Nb+Ta in the base metal and welding consumable.For base metal the limit is 10x %C and not more than 1.00%. for weld 8 x %C and not more than 1.00%.For weld metals the first choice, 8 x %C would be preferred. -
What's the ferrite level in welding consumables, typical requirement for SS 347 is 5FN, Minm, 8 FN max. Higher ferrite may cause sigma phase during service exposure between 425-800 deg C. For the present case i.e stabilizing anneal at 900 deg C if the initial ferrite in the weld is greater than the specified range above , there would be chances of sigma phase, which would embrittle the weld and reduce ductility. -
Change the stabilizing cycle.Instead of soaking for 3 hrs @870 deg C.follow a reduced duration PWHT cycle, e.g 870deg C, 1hr typically -for this thickness. Note the minimum thickness is 6mm which is the governing thickness for PWHT soaking time. If your PQR was qualified with maximum time(6 hrs or more) at 870 deg C then a requalification would not be required. Last but not least, send the failed sample to a lab for a failure analysis, and see if the root cause determination is tied to one of the facts I've mentioned above. You may find the article from Dr Damian Kotecki, as quite informative on this issue. Thanks. Pradip Goswami,P.Eng. Welding & Metallurgical Specialist & Consultant
| Dear Mr.Bazrafshan, There may be two reasons why your weld would have cracked. First it can be weld solidification cracking. what is the orientation of the crack? is it a longitudinal centerline crack, running along the weld or is it transverse? If you find a longitudinal crack running along the weld centre line, then it shoudl be weld solification crackng, Please find attached suutala diagram for predicting weld solidification cracking from weld metal composition, you need to calculate the Cr and Ni equivalents and also P + S content of the weld metal. You also need to check the ferrite content of the weld metal using magnegage or ferrite scope adjacent to the crack, you'll need a flat surface for you probe to sit on it. If cracking is visible on the joint, I think that shoudl be on your SMAW weld.Too low ferrite can do the damage. You stabilizing heat treatment would have opened up the crack making it more visible. Try LPT on one of the as welded joints before heat treatment for screening any cracks. I will recommed you to make a weld pad for both your GTAW and SMAW welding consumables and do the chemical analysis of the undiluted weld metal. You can also check the WRC 1988 or 1992 diagram to understand expected ferrite content and the solidfication mode of your weld metal, if you solification is fully austentic or austenite + ferrite (AF) mode then chances of a solification cracking is very high. if it is FA mode then it cna be ruled out. Second reason may be you would not have maintained cleanliness of the joint and consumables because oil / grease can introduce P & S into your weld metal and give rise to solidification craking. Try depositing a convex weld, which is better than a concave weld in these kind of situations. Hope this helps. Regards, A.Praveen Senior Welding Engineer Muscat
--- On Fri, 1/7/11, pgoswami <pgoswami@quickclic.net> wrote:
From: pgoswami <pgoswami@quickclic.net>
Subject: [MW:9120] MW:9116-PWHT Of 347H
To: materials-welding@googlegroups.com
Date: Friday, January 7, 2011, 9:09 AM
Mr Bazrafshan, The recommended PWHT cycle looks like a stabilizing anneal.You may check the precise requirements of this type of PWHT in NACE RP-170-2004.The PWHT time looks extremely high ( 3 hrs for a thickness of 12.7mm/6.0 mm).Prolonged exposure at this temperature would be detrimental to toughness of the weld. Also Nb:C ratio is very important to prevent hot cracking or stress-relief cracking of 347H. Feel free to exchange information, should you wish. Thanks. Pradip Goswami,P.Eng. Welding & Metallurgical Specialist & Consultant Thanks for support. I am worry about mechanical and chemical properties of material after repeating Heat treatment. You know after new PWHT the material will affected for 6 Hours. also May be we will found new cracks after PWHT, what can we do in order to prevent new cracks. Regards K.Bazrafshan From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Karthik
Sent: Thursday, January 06, 2011 12:24 PM
To: materials-welding@googlegroups.com
Subject: Re: [MW:9115] PWHT Of 347H | Hi,
PWHT can be repeated as per ASME Section VIII,Div.1-UHA-32 (e).Please find below the extraction.Holding time shall be the same as the original/as required by project spec.
UHA-32 (e) Vessels or parts of vessels that have been postweld
heat treated in accordance with the requirements of this
paragraph shall again be postweld heat treated after repairs
have been made. 379,Moo6,Soi8,Nikhomphatana, Phone: 0066 38 897035-8 (Off) Hand Phone: 0066 892512282
--- On Thu, 1/6/11, Bazrafshan,kazem <KBazrafshan@BPC.CO.IR> wrote:
From: Bazrafshan,kazem <KBazrafshan@BPC.CO.IR>
Subject: [MW:9114] PWHT Of 347H
To: materials-welding@googlegroups.com
Date: Thursday, January 6, 2011, 3:28 PM
An urgent question: Base on Project tech Documents we have to do PWHT( in 870°C for 3Hours,Air Cooled) for welding of 2 parts as A312-347H(12.7mm) to A182-347H(6mm,) together((A182 is as a branch to A312). After performing PWHT we found some cracks(dept of crack are from 3 to 8 mm) Just in weld seam. Joint Information: Welding process: GTAM + SMAW. Design Tem: 780°C and Design Pressure is 5.3Barg in convection coils. Welding consumables are :E347H Esab, Wire ER347 esab. Shall we repeat PWHT after repairing or not? If reply is Yes, how much time shall be Holding time ? Please let me know your STD reference As well. Thanks Bazrafshan --
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