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Technical Discussions
faran
Engineering
FFC Pakistan, Pakistan, Joined May 2005, 17

faran

Engineering
FFC Pakistan,
Pakistan,
Joined May 2005
17
06:54 Mar-17-2009
HTHA Inspection

In our ammonia coverter we suspect HTHA, The reactor has a multilayered shell. Now would the WFMT (wet flouroscent MT) be suitable enough to detect any damage by Hydrogen Attack. How about doing UT based inspection from the inside.
I mean is there any chance that we miss something in WFMT which we can pick up in HTHA survey (while performing both from the service side)

    
 
 
Norbert Trimborn
Norbert Trimborn
09:44 Mar-17-2009
Re: HTHA Inspection
In Reply to faran at 06:54 Mar-17-2009 (Opening).

Dear Faran,

I am a specialist in detecting and sizing of HTHA. I do these inspections worldwide.
Tomorrow (18-03-09) I will give a presentation about detection and sizing of HTHA at the Pearl-continental Hotel Karachi. May be you manage to come. Otherwise please contact me and I will answer your question per e-mail.

If you are interested, it is a pleasure to invite you.

Kind regards,

Norbert Trimborn
SGS Industrial Services.

    
 
 
J. Mark Davis
Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama, USA, Joined Mar 2000, 85

J. Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
12:43 Mar-17-2009
Re: HTHA Inspection
In Reply to faran at 06:54 Mar-17-2009 (Opening).

For these HTHA and HIC examinations using ultrasoncis, you might look at Phased Array. Using the power of the number of the elements with sectorial scans (S-Scans) and the increased frequency, I have seen much better detection and sizing capabilites using Phased Array.

In addition, instead of using a Spot inspection method with a velocity ratio method, you can actually scan an area or band of the vessel to detect the small mid plate segregates.

The best part is that you can sweep the sound beam an angles to detect any step wise cracking that may occur between the mid plate segregates.

If you will send me an email, I will send you some Phased Array images of HTHA and HIC that I have from my test samples.

    
 
 
Luis ganhao
Engineering,
USA, Joined Sep 2008, 25

Luis ganhao

Engineering,
USA,
Joined Sep 2008
25
15:42 Mar-17-2009
Re: HTHA Inspection
In Reply to faran at 06:54 Mar-17-2009 (Opening).

HTHA and HIC are both totally different mechanisms of degradation and the crack propagation in the material is different too, however both you can detect and sizing by UT with techniques such as; TOFD, Phased Array, Backscatter, velocity ratio and attenuation measures. Now if you have access from the inside probably you can detect the HTHA with WFMT only if the attack is advanced ( I mean cracks) in the case if you only have is decarburization or voids the best technique is in situ metallographic evaluation.



Regards





Luis A. Ganhao B.

Sr. Materials Specialist

ASNT-TC-1A-L-II-UT-ET-PT-MT

TOFD-L-II, API-510, AWS-CWI

JACOBS, Inc. Calgary AB-Canada

luis.ganhao@jacobs.com

403-692-1037

    
 
 
P V SASTRY
R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D, India, Joined Jan 2003, 195

P V SASTRY

R & D, NDT tecniques metallurgy
TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D,
India,
Joined Jan 2003
195
15:15 Mar-20-2009
Re: HTHA Inspection
In Reply to faran at 06:54 Mar-17-2009 (Opening).

Dear Mr. Faran,
Since you are suspecting HTHA in your Ammonia reactor and asked the question whether WFMT from inside will detect the damage? Instead of answering your question straight away I take the liberty of briefly touching on the HTHA first.
HTHA or The High temperature Hydrogen Attack depends on the temperature, partial pressure of hydrogen, time of exposure, the steel composition (plain carbon or alloyed steel) and the alloying elements, the heat treatment given to the plate/ the micro structural constituents/phases, the residual and operational stresses and a few other things.
Depending on the combination and number of the variables, the hydrogen damage may be classified as shown below:
 Hydrogen Embrittlement,
 Hydride Embrittlement,
 Solid Solution Hardening,
 Internal Defect formation,
While in any case the first three effects in their primary state of their existence cannot be detected by WFMT, we will see about the last mechanism that is “Internal defect formation”
The Hydrogen at higher temperatures (beyond 350°C) tends to go into highly mobile atomic stage and diffuses into the metal in contact. The diffusion rates are governed by temperatures, pressures stresses apart from others.
In plain carbon steels the microstructure usually consists of ferritic and pearlitic phases. The pearlitic phase consists of plates made of iron carbide (Fe3 C). The hydrogen attacks this carbon and forms methane gas (CH4). Consequently the carbides are depleted and the structure becomes week.
When it comes to alloy steels the story is a little different in that part of the carbon is held as Chromium Carbide, Molybdenum carbide or other alloy carbide and they do not part with carbon as easily as the Iron carbide. However the time and temperatures and other local conditions like large pressure build ups can still make this reaction possible. In any case all the carbon is not held by the alloying elements and the iron carbide in any case, continues to be attacked.
Other factors that drive the HTHA are the stresses and micro structural constituents. Steels with Bainitic and martensitic microstructures appear to be more prone for HTHA than those with ferrite pearlitic phases. Similarly HTHA cracks have been observed in highly stressed regions of otherwise resistant material. High thermal stresses, localized cold work, stress concentrators and welding residual stresses are known to accelerate HTHA attack in local regions.

When this attack happens at and near surface the generated methane gas escapes into surroundings and does not cause much physical damage. When this reaction takes place a little below the surface of the metal the generated methane gas cannot escape/ diffuse, the way atomic hydrogen does. So the concentration of the methane increases and forms tiny bubbles near the grain boundaries as well as other inclusion sites and thus voids are formed. Eventually the pressure builds up and the voids join to form fissures. The growth of fissures and voids weakens the metal and the fissures eventually develop into major cracks.

Please understand that the whole process is occurring well below the surface of the metal ( About 1/3 to 2/3 depth/thickness) the depth depending on so many other factors like temperatures pressures, time etc.
Since HTHA is time dependent amongst other factors, we can conceive different stages of the attack.
1. The initial incubation period when changes are hardly noticed. In this period the methane gas pressure builds up without causing noticeable damage. Voids which are microscopic in size, appear. There are empirical curves which indicate the incubation time for several steels at operational conditions.

2. The second stage is when the micro voids coagulate to form micro fissures and these micro fissures in turn join to form cracks. This Hydrogen induced cracking (HIC) is also known as lamellar blisters, mostly located near former inclusion sites. Please note again that this action is taking place inside the metal.

3. The last stage is when all the carbon is exhausted and the micro fissures and the cracks have caused loss of strength. The reduced strength of the material causes the stresses to go high, further driving the cracks. So we have a stage where the cracks formed at different depths are eventually joined in stepwise progression.

Since you said your vessel is of multi layer construction no test for HTHA or HIC can be done from outside. All tests are to be done from inside only. For all practical purposes only the first inside layer is attacked and all the remaining layers should be free until unless there is a major leak through the first layer and vents in the subsequent layers are blocked.

Now, if we come back to your first question about WFMT, it must become obvious by now that it cannot detect the HTHA in its incubation period, because there is hardly any linear defect.

In the second and third stages the defects/cracks are lamellar and internal. So the WFMT is not exactly relevant with respect to HTHA and HIC.

Having said that I still recommend doing WFMT for detecting possible (conventional) cracks near the highly stressed regions and welds.

For detecting the HTHA damage, particularly in the incubation stage I recommend that you should start with In-situ metallographic examination by replication method or direct photography. By this you will be able to see the surface decarburization clearly, more so if it is carbon steel plate.
A host of Ultrasonic Techniques are available for detection of HTHA and HIC particularly in the second and third stages. While it is not possible to elaborate on them I shall just make a mention.

The Ultrasonic attenuation measurements.- Here the loss of back wall amplitude is measured as a function of the Hydrogen damage. Normally a 5 Mhz probe is used.

Ultrasonic Amplitude backscatter technique- After proper gain setting on a reference sample, the presence of hydrogen damage gives plenty of backscattered signals from the coagulated void and micro/macro fissures.

Velocity Ratio technique- The velocity ratio of the shear wave to the longitudinal wave is taken to indicate the presence of Hydrogen damage. While longitudinal wave velocity is taken using a normal probe the shear wave velocity is measured using 0° shear wave probe. Special couplant has to be used for coupling this probe to the surface. Ratios beyond 0.55 indicate hydrogen damage.

Normal Pulse echo technique can very well be employed for detection of HIC or lamellar cracking. Only point to be remembered is that this should not be confused with other plate defects like laminations.

Here ends my first hand experience. Other techniques available are

1. Creeping waves/Time-of-Flight Measurement,
2. Pitch-catch mode shear wave velocity
3. Ultrasonic method based on backscatter and velocity ratio measurement,
4. AUBT - Advanced Ultrasonic Backscatter Techniques,
5. Method based on TOFD, Thickness mapping, backscatter and velocity ratio.

In his technical paper “PORTABLE PHASED ARRAY APPLICATIONS” presented in 3rd MENDT, Mr. Michael Moles, has given an excellent account of how phased array technique can be gainfully employed for detection of HIC amongst other things.
He makes a mention of the difficulty of detecting the SOHIC (Stress Oriented Hydrogen Induced Cracking or other wise known as step wise cracking) with conventional UT and gives a beautiful illustration of how a modified Phased Array technique has vividly shown SOHIC or the step wise cracking.
It is also possible to use other techniques based on Eddy current and Barkhausen noise principles but one practical difficulty may arise in their application. Since the whole (interior) of the vessel may be affected by HTHA the reference cannot be set on the vessel. If external reference plate is taken it should be of exactly the same material and heat treatment. Again these techniques may not be good enough for HIC.
In any case there are plenty of proven UT techniques available for detection of HTHA and HIC as mentioned earlier.
Good luck and best wishes

P V SASTRY

    
 
 
J. Mark Davis
Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama, USA, Joined Mar 2000, 85

J. Mark Davis

Teacher, And Consultant
University of Ultrasonics, Birmingham, Alabama,
USA,
Joined Mar 2000
85
17:31 Mar-20-2009
Re: HTHA Inspection
In Reply to Luis ganhao at 15:42 Mar-17-2009 .

PV,

That was an Excellent write up on HTHA. I appreciated the read.

Sincerely,

J. Mark Davis

    
 
 
hando
hando
13:47 Jul-13-2009
Re: HTHA Inspection
In Reply to P V SASTRY at 15:15 Mar-20-2009 .

How do u actuall measure the longitudinal and shear wave velocity on an unknow test sample? Using modern UT flaw detector.

And lets say i have 2 probes with no markings, 1 is 0deg longitudinal and 1 is 0deg shear, how can i tell them apart?

    
 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
03:18 Jul-14-2009
Re: HTHA Inspection
In Reply to hando at 13:47 Jul-13-2009 .

I will answer your second question first . You can check out the probes on known specimens and depending on the echo positions, you will know which is which. Having done that, you can now use the known probes on the unknown specimens (specimens with unknown velocity but known geometry).

Best regards.

Swamy

    
 
 
hando
hando
06:54 Jul-14-2009
Re: HTHA Inspection
In Reply to S.V.Swamy at 03:18 Jul-14-2009 .

Thanks Swamy. When it comes to velocity ratios i understand that you divide the longitudinal velocity by the shear velocity. This is measured on the same spot location with viscous couplant for the 0deg shear wave.

How do you measure the velocity of both waves?

    
 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
06:01 Jul-16-2009
Re: HTHA Inspection
In Reply to hando at 06:54 Jul-14-2009 .

With a specimen of known thickness, the echo positions will be different and the distance between two echoes can be used to measure the velocity. Am I missing something here?

Swamy

    
 
 
hando
hando
14:25 Jul-19-2009
Re: HTHA Inspection
In Reply to S.V.Swamy at 06:01 Jul-16-2009 .

ok, i will answer my own question here. To calculate velocity ratios. You need
1.Manual UT Flaw detector
2.0deg Longitudinal wave probe
3.0deg Shear wave probe

Set the unit of measurement to time. Measure the time between the second and third backwall echos on the test sample. Using the same spot location with both probes. Then divide the longitudinal time by the shear time, this gives you the velocity ratio of the sample. As is published a ratio of above 0.550 is typical of HTHA.

    
 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
07:53 Jul-21-2009
Re: HTHA Inspection
In Reply to hando at 14:25 Jul-19-2009 .

Yes, that is what I implied if you read my posting. So, where is the confusion?

Best regards.

Swamy

    
 
 
KP
KP
04:12 Jul-29-2009
Re: HTHA Inspection
In Reply to S.V.Swamy at 07:53 Jul-21-2009 .

You would be crazy to do anything but pull a replica first since you have access to the insde surface. Take multiple replicas from different predicted hot spots on the vessel. If there is no fissuring or decarburization then you have no problems. If you find fissuring you can quantify with UT, however, UT will not detect decarburization. If you have damage you may have a hard time quantifying below 20% of your wall thickness in the frst place. My company uses5 &10 MHz backscatter, frequency spectrums, velocity ratios, and spatial averaging all together to determine if damage is present. Just because you have a slightly high velocity ratio, or a really nasty backscatter, doesnt mean you have damage. You must use multiple techniques together. Anything above the magical .550 ratio number is NOT a sure bet for damage. A conventional Scope would be sufficient for backscatter...but spectrums, ratios, and spatial averages should be done with an oscilloscope with an external pulser utilizing full RF waveforms. Detection and quantification of HTHA is very hard to call with confidence with UT...thats why I say PULL REPLICAS FIRST!!! When done properly they will not lie. Good Luck

    
 
 
Vikram
NDT Inspector, - Phased Array Specialist
Mistras Group Inc., USA, Joined Jun 2009, 9

Vikram

NDT Inspector, - Phased Array Specialist
Mistras Group Inc.,
USA,
Joined Jun 2009
9
00:10 Aug-04-2009
Re: HTHA Inspection
In Reply to KP at 04:12 Jul-29-2009 .

I am facing similar issues with some vessels here. They suspect SOHIC/HIC in the vessel and they have asked me to inspect the Weld/HAZ for possible cracking. I went in with a sectorial straight beam 5MHz and a sectorial Shear wave, 2MHz. I am seeing indications which have the specific morphology for HIC but the question remains as to what level of amplitude you will see them at with respect to the backwall/ reference SDH, etc.

    
 
 

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