Hi Naveen

I am keen not to give any advice just in case something goes wrong, I am sure you understand.  Nevertheless, I can suggest you read some papers by KC Mitchell and SJ Brett

Also Jon Parker, might be able to help.  

http://www.twi-global.com/technical-knowledge/faqs/material-faqs/faq-what-is-controlled-deposition-repair-cold-repair-without-a-subsequent-post-weld-heat-treatment-and-how-is-it-carried-out/

https://scholar.google.co.uk/scholar?as_q=weld&as_epq=&as_oq=&as_eq=&as_occt=any&as_sauthors=%22kc+mitchell%22&as_publication=&as_ylo=&as_yhi=&hl=en&as_sdt=0%2C5

http://www.tandfonline.com/doi/abs/10.1179/1432891713Z.000000000253

https://www.researchgate.net/profile/Jonathan_Parker5/contributions

Hi,

I do not really understand why you use Alloy 82 for repair welds. Due to the fact that Alloy82 is much more expensive i would suggest welds with similar filler metals. Neverless, if you are in the compressive zone it should not be that problem to use Alloy82 from mechanical behaviour. But i donn't know the long therm creep behaviour about this dissimliar welds. And mention that the different chemical potential of those different metals might cause soon or later corrosion.

Dear Hannes Pahr,

I think i could not put the problem definition very properly, my mistake. Let me explain it again.

Casings (castings) which we use in our supercritical steam turbines are made of 9Cr steel. If some small defects appear on these castings AFTER FINAL MACHINING we use alloy 82 for repair, as it don't need any PWHT and hence final machining dimensions remain unaffected. As you have pointed out rightly normally similar filler electrodes are used for repairing 9Cr steel with PWHT, but that is done if defect appears before final machining as deformations that will occur after PWHT will be taken care during final machining.

Here I want to mention, we use alloy 82 only for very small size defects only. Like any pore opened during final machining will be filled with it, as customers like to see a smooth surface. only for cosmetic kind of repairs. Till now we have not allowed such repair in highly loaded areas like blade grooves on inner casing etc.

My question is - could I allow a repair of this type in my casing in the areas which are highly loaded. you got it right I am worried about the behavior of this dissimilar metal weld joint mainly. In addition the difference in coefficient of thermal expansion between these two metal is causing additional stresses in this zone.

Hope I made the problem more clear now and will get some good responses.

Regards

Naveen

Hi Naveen

I am keen not to give any advice just in case something goes wrong, I am sure you understand.  Nevertheless, I can suggest you read some papers by KC Mitchell and SJ Brett

Also Jon Parker, might be able to help.  

http://www.twi-global.com/technical-knowledge/faqs/material-faqs/faq-what-is-controlled-deposition-repair-cold-repair-without-a-subsequent-post-weld-heat-treatment-and-how-is-it-carried-out/

https://scholar.google.co.uk/scholar?as_q=weld&as_epq=&as_oq=&as_eq=&as_occt=any&as_sauthors=%22kc+mitchell%22&as_publication=&as_ylo=&as_yhi=&hl=en&as_sdt=0%2C5

http://www.tandfonline.com/doi/abs/10.1179/1432891713Z.000000000253

https://www.researchgate.net/profile/Jonathan_Parker5/contributions

@ Michael William Spindler - Dear sir thank you very much for your suggestions. I will definitely go through the papers suggested by you. The paper "Weld repair of grade 91 steel without post-weld heat treatment" was available with me sharing it here.

Indian standard IS5530 : 2005 (Recommendations for production, rectification and repair of steel castings by metal arc welding process) recommends the repair of 9CrMo castings with similar metal electrodes with minimum preheating temperature of 300 deg C and PWHT temperature range is 620-720 deg C. As per my current knowledge we do not have any standard regarding the cold repairs (Not very sure what ASME tells), but we are doing these based on our experience (mild repairs). The recent snag that came in one of our casings gave me a feeling that our manufacturing shop is pushing the use of alloy82 beyond the limits earlier decided.

We got some references where alloy82 (NiCrFe-3) filler is used to create dissimilar metal weld joint between austenitic and ferritic steels and same filler has been used with 9CrMo steels also. In the paper “Weld repair for creep applications (shared here; section 3.2.2)” evidences of use of same filler are given (I think they used it with low chrome steels not with 9CrMo). In the paper "SELECTION OF MATERIALS FOR PROTOTYPE FAST BREEDER REACTOR " (http://www.igcar.gov.in/igc2004/PFBR.pdf)  Inconel 82/182, welding consumable is recommended For welding Alloy 800 to Modified 9Cr-1Mo,.

During literature survey I have found use of this filler with 2.25%CrMo steels more in open literature. I have a general feeling that use of this electrode on 9CrMo is ok if zone is not stressed, but could it be used to fill a high stress high temperature zone….. I feel more insight needed…

I am sharing some more papers here not very much directly linked to my question, but may be useful for someone interested in the topic

Regards

Naveen

i think this can help you 

http://link.springer.com/article/10.1007/s11665-008-9349-7

Dear Naveen,

It is a long time that I didn’t handle welding procedure problems in power plants ....

As I know, Alloy 82 is excellent for dissimilar welds : welding Nickel based alloys (Inconel, Incoloy, Monel, …) to stainless steel or stainless steel to carbon steels. It is not really to weld Cr-Mo steels (9Cr1Mo, …) to each other : but that is for cosmetic repair purpose if very small defects as you explain it.

However, if it is to be used to repair defects in zones subject to stress/temperature, using Alloy 82 on 9Cr1Mo-steel will create a local dissimilar weld. With additional problems in the HAZ due to the dissimilar welding. Microcracks and cracks can thus appear in the HAZ e.g. at transitions with base metal and coarse grained HAZ.

Therefore, at first sight, the reaction would be to encourage post-weld heat treatment (PWHT). This is to reduce residual stresses and produce acceptable hardness across the weldment. In this context, you allude to an Indian recommendation (for grade 91 and welding with 9Cr1Mo filler metal ?) : « minimum preheating temperature of 300 deg C and PWHT temperature range is 620-720 deg C”. This could inspire a welding procedure with Alloy 82. However it is probably not a good idea as the indicated range is close to the transition ferrite – austenite. During the PWHT, one could thus have (depending on PWHT precise parameters, speed, stay at temperature, …) local nucleation of austenitic structure then return to the ferritic one. Thus something like : ferrite -> austenite -> ferrite which would induce problems of mismatched micro-structure with additional stresses etc.

One can thus understand that, as referred to in the links in your and Michael’s posts, one should better just consider cold repair welding, without PWHT. In addition, it is known that the weld metal is of higher toughness than the ferritic weld metal (see Michael’s post with FAQ on link : www.twi-global .com …). Also, residual stresses will relax at service temperatures in the more ductile HAZ and near parent metal zones (your ref. A. Klenk). To conclude : as you mention a service temperature of 590 deg C, close to the 620 deg C minimum for a PWHT, the best thing would probably be not to make a PWHT. Residual stresses will relax at the service temperature.

Anyway, the use of Alloy 82 in zones subject to stress, creep, … should then be limited to very temporary repairs. Alternatively, repairs in these zones should be restricted to 9Cr1Mo filler metal according to the Indian Recommendation.

Now, if the manufacturing shop is pushing a lot, I only see 2 solutions (for the present) :

-          Use the EPRI qualified welding procedure for dissimilar weld with ENiCrFe-3 filler metal as given in the document “Well-Engineered Weld Repair of Grade 91 Steel: Welding Procedure Qualification to ASME Boiler and Pressure Vessel Code Section IX, Part II” referred to by Michael. There is no access to this document of 246 pages (dated Feb 2016), but you could try contacting Jonathan David Parker who is on RG and ask for a copy of the few relevant pages.

-          Another solution would be to make an attempt to notwithstanding use Alloy 82 as filler metal for repair welding : but then on a zone with sufficiently low stress. To decide on the maximum stress limit, you could then start with Fig. 1 of your enclosed document “Inconel 82 – 9 Cr1Mo weld.pdf” (+ any other similar information maybe in your possession). It can be seen on Fig. 1 that the rupture lifes of HAZ under creep for a P91/Inco82 dissimilar weld at 823 K are shorter than 3 000 hours above 220 MPa and that, after 3000 hours service, the stress drops very quickly. To allow for 1 year service, the stress should not be higher than 150 MPa. As this is for 823 K while you intend to use such repair welds for service at 863 K and in order to take also account of uncertainties, I would suggest a safety factor of 40 % on these figures. Thus, 130 Mpa instead of 220 Mpa and 90 Mpa instead of 150 Mpa. The weld repair should of course be done with all needed precautions. In addition, the installation should be shut down after 3 000 hours (maximum stress of 130 Mpa) or 1 year (maximum stress of 90 Mpa) to allow NDT on the repaired zone : magnetic particle for surface cracking and ultrasonic for internal cracking (in and along the HAZ). The tests should be performed by an ASNT Level III expert in NDT. If nothing is detected, there should anyway be a systematic follow-up afterwards.       

I hope this helps a bit and sorry I have no additional information on related literature !

Dear Guibert Crevecoeur…thank you very much for your very detailed reply…It was very much useful…

I don’t have access to the EPRI document suggested by you, will try to get it and contact Mr. Parker…

I could get EPRI doc - 3002003833 “Best Practice Guideline for Well-Engineered Weld Repair of Grade 91 Steel (http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000003002003833)” I was just scanning it and in section 4 of it, ERNiCr-3 is considered as a filler material with controlled filled GTWA, but still could not get EPRI qualified welding procedure for it.

I also feel if the primary stresses are low (in the limits specified by you) in a zone, repair could be allowed.

Please keep posting if you get any additional information.

Regards

Naveen

Hello everyone

many of the steam turbine components work at very tight tolerance and with a very good surface finish. During the interaction with some of my friends I came to know about the incidents of component deformation in the cold reworked components during operation at high temperature (I could not fine any documentary evidence for it, but looks logical to me). Also these deformation (if they occurred) did not lead any subsequent failure, which may occur some time.

Take care of functional failures also (along with mechanical failures) while allowing a rework. Especially if deformations may lead to subsequent failures e.g. leak of joint, jamming of moving components etc.

Regards

Naveen

You should check the EPRI findings on weld repair of Grade 91 steel, many papers and reports are listed here. For superficial type repairs or repairs where the creep load is low epri favor a B8 filler - that is 9Cr steel but without the alloying elements which form precipitates. We believe that all repairs should be 'well engineered' so the more you can tell us regarding the specific application the better.

@ Jonathan David Parker

Dear sir,

Thank you for your reply. Work done by EPRI and especially by you on weld repair of Grade91 steel is very enlightening and useful.

I agree that all repairs should be well engineered and we also deal them on case to case basis. For superficial type of repair we are using Alloy82 frequently. We are planning to study the issue in light of our products (we are in commercial manufacturing) so that we can give a go/nogo call for such repair in minimum time, so that production line doesn't halt.

Regards

Naveen

One problem you will face during operation is that the Ni base repairs are very hard to accurately inspect using traditional NDE. Thus, it will be difficult or in worst case impossible to check for in-service damage

That's a valuable input. We have also experienced difficulty in inspecting such repairs. I agree that in service inspection of these repairs in a turbine casing will be  impossible with the existing commercially available technologies. Prima facie seems a big constrain for using such repairs aggressively. Their use should be limited to cosmetic repairs only.