Microfissure

[AWeld1]

Good Day!

I am working in a Power Plant and as we have conducted an in-service Borescope inspection in the tube to header connection, some discontinuity was observed. The HRSG was operating for more than 2 years now.

As per our clients report based on the photos taken during the inspection, they have mentioned that there were microfissure on the root weld measuring 0.2mm

The materials are P91, P22, P11

I would like to ask:

1) What causes microfissure?
2) Which reference can I read in order to understand microfissure?
3) How do we measure microfissure and what method or procedure?

Thank you very much.

 

[weldstan]

Were the welds dissimilar metals welds? In weld metal or fusion line? How did they measure so accurately with fiberoptic scopes?

 

[weldstan]

One other question. Was operation base load or intermittant (peaking)?

 

[metengr]

What was the orientation of the visual indications relative to the weld root?

 

[metengr]

Also, for a typical tube to header connection for an HRSG, the weld joint is typically a partial penetration weld between the tube and header and visual examination of the weld root would not be possible. Instead, the so called microfissures could be ligament cracks in the header bore hole. This would need to be confirmed by someone.

 

[AWeld1]

Sorry for the late reply, kinda busy this day.

Material: same material was used, P22, in this particular issue of microfissure.
Operation: base load
Visual indicationn the weld root

With regards to the measurement, they just sent us the report that the microfissure measures 0.2mm but not mentioned how they came up with that measurement.

History:
There has been a contamination of seawater which causes the refurbishment. During the inspection of the headers (using borescope,)it was found that there where lack of root fusion, incomplete penetration, cracks and excess filler cuts.
Due to the discovery of the these flaws/discontinuities, all of the 8 HRSG units where scheduled for tube-to-header borescope inspection.

Sample photo is attached for reference.

As per ASME Section I, there is no mandatory visual inspection for tube-to-header connection and therefore the is no acceptance criteria.

Regards,
AWeld1

 

[weldstan]

Intergrannular cracking is indicated. It appears to be in both the weld metal and HAZ. Possible environmental cracking. More info is needed. How did you introduce sea water into the boiler feed stream? In this case most likely all of your water treatment is in question - pH, Oxygen content, etc.

 

[AWeld1]

There was a leak in the surface condenser which caused the seawater ingress.

How dangerous is these intergranular cracks?
Another information is that these intergranular cracking was also present in the tube-to-header weld in the new harp that will be used as a replacement.

What causes these interganular cracking?

 

[weldstan]

For the new Harp, hydrogen cracking. Inadequate preheating. Delays between weld completion and PWHT. Finally inadequate PWHT and highly dissimilar heating and cooling rates of tube and header.

High temp Creep is intergrannular.

 

[metengr]

How dangerous are the cracks?

Well, at this point corrosion fatigue crack propagation (start-up and shut downs) can drive existing cracks through-wall to produce tube leaks. This will reduce reliability of the HRSG. Are these tube to header weld joints actually full penetration welds?

I agree with weldstan that these cracks probably occurred from environmental-assisted cracking associated with the ingress of seawater.

 

[weldstan]

How was the new harp hydrotested/drained. Water quality, inhibitors added?

 

[EdStainless]

Regardless of how these initiated (I would side with EAC) the risk of thermal fatigue causing a failure from these is very high.

If you know the stand-off distance on you fiber scope you can calibrate the image magnification.

And what has been done to the condenser?
As someone who sells condenser tubing for a living I am always curious.
Leaks in a seawater condenser? sounds like very poor workmanship. This is serious premature failure.
Condenser life should be in the range of 20-40 years.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[weldstan]

Ed,
I've seen some major workmanship problems with Condensers in the past two/three years. Owners and their Engineers are often highly remiss in their attention to Supplier surveillance.