Dear Experts, I need your help and 3

[Tomy-Met]

Dear Experts, I need your help and i am thankful in advance to everyone especially Sir Metengr.

In our boiler steam drum we found some cracks in fillet welds (propagating from HAZ to Base metal) of feed water inlet nozzles.
Drum material= WB 36
Shell thickness= 65mm
Penetrating nozzle thickness= 18mm
Crack excavation depth= Full fillet weld and 25mm in base metal
We excavated those cracks and welded with qualified WPS, We maintained Prehaet of 100C during welding, after welding hydrogen bake out was continued for 3hrs at 300C.

After hydrogen bake out visual inspection, DPT and PAUT conducted and found acceptable.

AREA OF CONCERN:

We decided to carry out localized PWHT on all 11 nozzles at once, heating band on shell horizontal direction, installing heating pads inside and outside around nozzles.
Contractor has now raised the concern that localized PWHT in this way will cause more stress around nozzles (penetration) and it can cause more cracks or deformation.
Contractor is justifying pwht waiver by saying, hardness values are under limits so no need PWHT.
And i am sure they have not applied temper bead welding as recommended by NBIC and API 510.
Hardness values in base meta 250 and at HAZ 310 maximum.
My concerns are as follows:

- Can we waive the PWHT? aligned to international industrial best practice international codes and standards where applicable, for in-service repairs of high integrity pressure equipment
- What will be the effect if we conduct PWHT and if we eliminate PWHT?

- How can we justify the tempering of the beads by any examination or testing?
- Can we measure the residual stresses or residual stresses are directly related to hardness?

I have suggested them to carry out PWHT for individual nozzle but the concern will be number heating cycles.

I have attached Nozzles and drum photo.
Please let me know if some more detail required.

Please suggests us best solution. Shall be thankful on immediate reply by seniors

 

[MJCronin]

Background information on WB 36:

http://www.ijceronline.com/papers/Vol6_issue5/G065036052.pdf

Consider posting on the "Metal and Metalurgy" forum .... you may get better answers ...

MJCronin
Sr. Process Engineer

 

[DekDee]

I will not comment on whether or not you should PWHT - more experienced vessel personnel on this forum.
What I will give you is some food for thought.
Cracks are a material relief system - the worst metallurgically affected part of the material (or the weakest) will release the stresses via cracking before the surrounding areas are affected.
How large are the affected areas ? - no idea without extensive hardness testing and that is not always conclusive.
You remove the cracks and adjacent material and reweld.
Have you removed all the material that was highly stressed previously and led to the cracking in the first place ?
If they were my vessels I would definitely be recommending PWHT - ( stress relieving)

Note: When a Contractor maintains you shouldn't do something it is usually because it will affect their bottom line or their schedule. Time to investigate further.

Regards,
Shane

 

[r6155]

I can see in the photo excessive fillet weld.

Regards

 

[Tomy-Met]

Dear Shane, thanks for your reply and valued comments.

Hardness values in base metal are satisfactory. But we are getting higher values at new weld HAZ like 310HV. As per contractor the maximum allowable hardness is 340HV for WB36 material, i could not find a standard to justify hardness limits for WB36.

i am not sure about cause of these cracks development because we have not done any RCA study, in my opinion it could be due to thermal fatigue, vibration induced fatigue(water hammer) and precipitation hardening due to presence of co

Now we are trapped that what to do, either exempt PWHT or conduct PWHT?

 

[DekDee]

Rana,
Huge amount of info on Google.
Here is an example:


Study of PWHT of EN-WB 36 Welded Material for High Pressure Application in Power Plants
1512
83
Abstract:
Article Preview

The heat-resistant, copper-alloyed ferritic steel 15NiCuMoNb5 (EN-WB36, material number 1.6368) has been widely used in European nuclear and conventional power plants for decades for feed water system. This widespread application is due to the toughness and strength, caused by the precipitation of copper, that are exhibited even at elevated temperatures which other fine-grained structural steels have at room temperature. The common problem faced on the construction sites is the embrittlement due to Cu-rich precipitates and the high hardness in welded joints that affect the weld quality and the premature joint failure can occur in service. Weld hardness has close relationship with the strength and ductility of the welded structures. Hence it is important for any weld to achieve certain level of weld hardness. The aim of this study is to see the impact of the impact of post weld heat treatment (PWHT) on the welded joints of WB 36 and find the optimal PWHT conditions that will produce the required hardness to overcome the problem of embrittlement in the welded material. Samples of different thicknesses were welded, keeping all the parameters same, PWHT carried out with induction heating equipment, joints were radiographed for any defect, macro-etched to confirm complete fusion and joint free from crack, and the hardness values were measured in base metals, heat affected zones and weld metal. It was found out that the embrittlement of WB-36, due to Cu-rich precipitates and high hardness, in welded joints can be brought into the lower and safer limits by applying PWHT at 570 °C for 1 hour min. This, for sure, will reduce the risk of premature joint failure and will have, comparatively, safer weld joint.

Regards,
Shane

 

[TGS4]

A local pwht conducted in accordance with WRC 452 will generally have good outcomes, and not cause additional problems. Your contractor may be doing the local pwht wrong if they find it causes problems.

 

[r6155]

I suggest Acoustic Emission Examination during pressure test.

Regards

 

[EdStainless]

Not sure if it could be effecting your situation but I dealt with one recently where the edge of the PWHT was so abrupt that it cause local high stresses and eventually (2 yr) cracking. These areas were adjacent to a patch. any patching and welding will cause high local stresses.

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P.E. Metallurgy, Plymouth Tube