Pearlite dissolution in RH tubing 2

[deco0404]

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

 

[metengr]

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.

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.

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.

 

[Guest102023]

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

 

[deco0404]

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

 

[metengr]

Yes.

 

[Guest102023]

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

 

[metengr]

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.

 

[Guest102023]

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

 

[metengr]

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.

 

[Guest102023]

We can both just blame it on the metric system then

 

[deco0404]

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

 

[metengr]

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.