×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pearlite dissolution in RH tubing
2

Pearlite dissolution in RH tubing

Pearlite dissolution in RH tubing

(OP)
guys,

my client recently took over an old oil fired power station circa 1970. We recently did some met work on the RH tubing, and discovered that the Pearlite in the stub to header weld has almost completely dissolved, spherodoized or whatever it does. The material is 10CrMo 40mm diameter. I'm guessing welding was standard TIG process, but with the age of these, records are dubious at best!!!!!!!

It was a bit of a rush job, so we didn't really get a chance to do a full analysis. We are planning a complete retube using X20 or P91 in the near future, but have a couple of queries anyway.

1- with the pearlite dissolving, should we expect a significant decrease in hardness. I'm just working on the assumption here that as carbon is responsible for the hardness, if it dissapears, so should a level of hardness. I am not taking into account the lifetime operational conditions of the boiler............

2- will this condition significantly affect the strength & creep resistance of these tubes.

3- The welds in question come out of a header in the horizontal plane for approx 300mm then turn vertically down. We did some sample replicas on the bend, and here there is at least a discernable amount of pearlite. There is a good degree of deterioration however. Anybody know why, two locations in such close proximity display such differences - perhaps just a function of heat from welding!

regards

Declan

RE: Pearlite dissolution in RH tubing

Quote:

1- with the pearlite dissolving, should we expect a significant decrease in hardness. I'm just working on the assumption here that as carbon is responsible for the hardness, if it dissapears, so should a level of hardness. I am not taking into account the lifetime operational conditions of the boiler............

Yes.

Quote:

2- will this condition significantly affect the strength & creep resistance of these tubes.

Yes, as the spheroidization process continues, the elevated temperature strength will decrease and reach a minimum value. The creep life will also decrease proportionally over time at temperature.

Quote:

3- The welds in question come out of a header in the horizontal plane for approx 300mm then turn vertically down. We did some sample replicas on the bend, and here there is at least a discernable amount of pearlite. There is a good degree of deterioration however. Anybody know why, two locations in such close proximity display such differences - perhaps just a function of heat from welding!

Yes, the heat from welding alters the original microstructure.
 

RE: Pearlite dissolution in RH tubing

I'm not familiar with non-ASTM grades, but googled and found info indicating it could be 2¼Cr-1Mo; is that correct?   

RE: Pearlite dissolution in RH tubing

(OP)
thats correct 2 1/4Cr-1Mo more commonly know as P22......thanks for your comments metengr

RE: Pearlite dissolution in RH tubing

Yes.

RE: Pearlite dissolution in RH tubing

This 2¼Cr-1Mo steel never had pearlite, but nevertheless it takes a lot of doing to completely spheroidize the steel.  The room temperature and creep properties will have decreased significantly, and no doubt they need replacing.  

But put things in perspective: at 40 years the tubes have had a very good life.  The damage could have been caused by many, cumulative over-temperature events rather than one continuous period of not so high over-temperature.  So before you go to a more expensive and difficult alloy (and this depends on the new operating conditions), the same material may well suffice* for the refit.  You might also consider the more user-friendly T23 alloy.  

About the weld, it has an affect on the base metal, but only to a few mm away, certainly not 300mm.  So the operating conditions 300 mm away had to have been different.  


* If: Time to Your Retirement < Anticipated Time to Tube Retirement, then select that grade of tube  ;)

 

RE: Pearlite dissolution in RH tubing

brimstoner;
2.25% Cr-1% Mo, Grade P22 can indeed have pearlite colonies along with ferrite/bainite. I have seen annealed pipe and tubing with islands of lamellar carbides that resemble pearlite colonies.
 

RE: Pearlite dissolution in RH tubing

Yes metengr, I should have said 'probably', because pearlite is of course possible even if uncommon.  
 

RE: Pearlite dissolution in RH tubing

Yes, I should modify my last statement regarding the tube bend at 300mm. I was thinking less distance. Two possibilities - PWHT from the welding operation and a more likely scenario, the tube bends were formed hot with no subsequent thermal treatment because the code would not require a post bend thermal treatment for this material. As a result, the cooling rate from bending would alter the final structure.

RE: Pearlite dissolution in RH tubing

We can both just blame it on the metric system then  
;)

RE: Pearlite dissolution in RH tubing

(OP)
Guys,

thanks very much for both your inputs............

I agree that the condition is more likely to be old age than any actual overheating. I also concur that the bends are hot bends and as such affect the microstructure that way. I wasn't thinking that the weld had anything to do with the structure of the bend. I havn't measured it, but I'm assuming the weld zone is only a couple of mm from the edge of the cap.

As I am new to the met side of things, and I'm hoping this isn't too complicated a question........but when the carbon dissipates ( not necessarily from this material) where does it go? does it just completely dissolve into the structure, grain boundaries or what.....again sorry if that is a dumb-ass question

Declan






Declan

RE: Pearlite dissolution in RH tubing

deco0404;
What happens is that most of the carbon forms a fine distribution of carbides during the original heat treatment of Grade P22. However, over time at elevated temperature exposure this fine distribution of carbides, which provides creep strength, begins to coalesce and migrate to grain boundaries. Toward the end of useful service life for P22 you will see networks of grain boundary carbides and zones within the grains next to the grain boundaries will be free of necessary carbides. It is the regions next to the grain boundary networks of carbides that suffer from creep damage.
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources