Hydrogen Induced Cracking, 13Cr steel 2

[bigboys]

Hello my friends,

I would really appeciate if you can help me with my problem. we are having some problems with 13Cr steel so
I need information about HIC effect to the material.

please take a look about details:

Material: 13Cr steel (Martensite) for Turbine Blade in
Thermal power plant (if you cannot find 13Cr steel it is ok similar one.)

HIC (Hydrogen Induced Cracking) is one of SCC (Stress
Corrosion Cracking). Turbine blades are used under steam (high pressure and high temperature)

Under what kind of condition does HIC start happen?

Steam Temperature, Pressure, pH etc..
What Impurity in Steam cause HIC (H2SO4, HPO3 etc.)
Amount of the Impurity in steam
Amount of hydrogen that appear form those impurity.

please if you have any solution, answer or any hint where I can search about it I would be very happy.

Thank you very much
Aleksandro

 

[metengr]

From my experience with large steam turbines in Power Plants, it is rather doubtful that HIC is of any concern. The operating temperatures and hydrogen partial pressures are too low. Stress corrosion cracking (SCC) of the 4XX martensitic stainless steels used in last or second to last stage buckets or blades in large low pressure (LP) steam turbines is a more probable scenario. Also, corrosion fatigue can play a role as well.

I know of instances related to SCC in L-0 (last) and L-1 (2nd to last) stage blades caused by impurities in wet steam. These impurities are typically chlorides or sulfates and have more to do with carryover in your feedwater circuit. As far as hydrogen induced cracking, you can discount this scenario.

What exactly is your situation? Are you suffering from root cracks in the last stage blade row?

 

[bigboys]

dear metengr,

Thank you very much for this valuable and detailed information.

Well you are right, my search gave me the answer that this material is very resistant to HIC and that probably it is not the case, but my manager is insisting on this.

Currently we have problem with root cracks in the last stage blade row as you mentioned.
The point is that I was asked to find out about HIC for this material. In which temperature and pressure occurs, amount of the impurity in steam that can cause it, as well as amount of hydrogen that appears form that impurity.

Your information is very helpful, so I think he can go in different direction now. However if you have some information about temperature and pressure let me know.

I highly appreciate your help.

Thank you so much

Aleksandro

 

[metengr]

As you now confirmed, the root cracks in the last stage blades are, in my opinion, either caused by SCC or corrosion fatigue from impurities or contaminant carryover in the steam. The normal operating conditions in this section of the LP turbine are approximately 100 deg F and vacuum, which cannot and will not sustain hydrogen induced cracking. The last stage blades are normally the largest size and would be exposed to elevated stress levels in service, making them more susceptible to SCC or corrosion fatigue (CF) crack propagation under wet steam conditions.

I believe there have been a number of technical papers published in the public domain regarding SCC/CF of last stage turbine blades in LP steam turbines. You might want to try an internet search for this information.

 

[bigboys]

Dear metengr,

First of all I appreciate your advices and help.

Now I have one similar previous example.
Please not that it was not high temperature and not high pressure.

Background was like that,
There was an accident before. The last stage blade (L-0) under LP steam broke from the root.

Conditions of saturated steam are:

Temperature: approximately 30 to 40 degree centigrade
Pressure: approximately 5.6 kPa
Wetness: approximately 5%

Someone in our company thought HIC happened under above conditions. HIC caused the small crack-HIC was trigger of the accident. Then the crack grew due to SCC. The crack reached level which can make the root of blade break due to SCC effect in the end.

I understand your previous explanation
However today I found information about SSC (Sulfide Stress Cracking) which is one of HIC, and it is opposite of what we think and discussed before.

"SSC occurs under coexistence of H2S and H2O and with following values for 13Cr stainless steel (for piping but not for turbine blade) and atmosphere.

The mechanism of SSC is
H2S => H+ + HS-
Fe + HS- => FeS + H+ 2e-
2H+ + 2e- => 2Had (ad: adsorption) => 2Hab (ab: absorption)

Rockwell hardness of material: larger than 22
Temperature: under 100 degree centigrade and 25 degree is most incident temperature.
pH: under 6

Martensite is difficult to be discriminated between HIC and SCC.

I’m confused!!!!!

I’ll continue searching today and wait for your kind answer.
What do you think?


Thanks and regards,
Aleksandro

 

[metengr]

Most likely, the SCC had initiated from a corrosion pit not from HIC cracks. Hydrogen induced cracking is normally observed in petrochemical reactors where you have hydrogen partial pressures that are greater then 25 bar and above 500 deg C. You need to have temperatures in excess of 250 deg C to support the diffusion of hydrogen atoms into the steel.

For low pressure turbines the last stage blades see condensed (wet) steam that is relatively pure and contains little if any dissolved gases at 30 deg C. The conditions of moisture will, however, support surface corrosion pitting during off-line conditions.

If the corrosion pits are located in areas of high tensile stress, they become crevices that can concentrate contaminants like chlorides or sulfates. SCC requires three conditions working together - tensile stress, contaminants and susceptible material. Tempered martensite is susceptible to SCC under these conditions.

To prove this point, I would suggest you have a detailed failure analysis performed on any cracked last stage blades that are detected during in-service inspections. If you had one blade failure, chances are you have other cracked blades, as well.

I would suggest you perform a wet fluorescent MT of the last stage blades during a schedule outage. Any cracked blades should be removed and a failure analysis performed of the cracked area to confirm the presence of contaminants.