Microstructure for low alloy steel SA213 T11/T12

[engineermat]

Does any one know the microstructure for low alloy steel SA213-T11/T12? ASME code shows that these alloys should be fully/isothermally annealled or normalizing + tempering. Can some one advise what microstructure formed after the above heat treatments, or show me some photos? Thanks a lot.

 

[mrfailure]

Initial structure usually starts as a combination of ferrite and fine pearlite. Elevated temperature service degrades the pearlite into carbides which over time will migrate towards grain boundaries. EPRI has a rating scale of 1 to 7 that describes the severity of this degradation (1 - new tubes; 7 - carbides disappear).

Aaron Tanzer

http://www.lehightesting.com

 

[metengr]

The initial microstructure of Grade T11 or T12 can consist of ferrite/pearlite for annealing or ferrite/bainite only for N&T – depending on mill cooling rates.

If you know the hardness, you can use the following table (EPRI Publication on T11 and T12 using a CCT diagram). The normalizing cooling rate would most likely result in ferrite and bainite with some carbide colonies. You get the picture.

Pearlite Ferrite Bainite Hardness
0% 35% 65% 217 VHN
30% 70% trace 160-170 VHN



The elevated temeprature exposure results in degradation, which is called spheroidization, and results in several interesting features; carbide coalescence and diffusion to the grain boundaries.

 

[engineermat]

Thanks guys.

The attached are some photos of a reheated tube (T12) that was in service for a period of time.

 

[engineermat]

Thanks guys. Very useful information.

I attached three photos in three separate replies. These photos show microstructures for a reheated tube (T12) that was in service for a period of time. Seems like it has spheroidized carbide colonies and ferrite matrix. But some colonies has lamellar or needle like features. Are these lamellar or needle like structures Widmanstätten structures? or the bainite (such as upbainite) that formed during original heat treatment, such as normalizng + tempering. I am not quite sure if this lamellar structure caused by initial heat treatment or some overheating during service.

Thanks again.

 

[engineermat]

I attached three photos in three separate replies. These photos show microstructures for a reheated tube (T12) that was in service for a period of time. Seems like it has spheroidized carbide colonies and ferrite matrix. But some colonies has lamellar or needle like features. Are these lamellar or needle like structures Widmanstätten structures? or the bainite (such as upbainite) that formed during original heat treatment, such as normalizng + tempering. I am not quite sure if this lamellar structure caused by initial heat treatment or some overheating during service.

 

[engineermat]

Sorry, accidentally click "submit post" with no attachment.

I attached three photos in three separate replies. These photos show microstructures for a reheated tube (T12) that was in service for a period of time. Seems like it has spheroidized carbide colonies and ferrite matrix. But some colonies has lamellar or needle like features. Are these lamellar or needle like structures Widmanstätten structures? or the bainite (such as upbainite) that formed during original heat treatment, such as normalizng + tempering. I am not quite sure if this lamellar structure caused by initial heat treatment or some overheating during service.

Thanks.

 

[metengr]

This looks like residual bainite colonies from original heat treatment in conjunction with spheroidization of original carbides, after exposure to elevated temperature service. What is the hardness of the microstructure? Hardness is also a good indicator of level of spheroidization damage based on softening.

 

[metengr]

In the 30+ years of having performed condition assessments and failure analysis of power boiler tubing, long term overheat exposure will result in a gradual coalescence of original carbides and the bainite will continue to degrade into larger carbides, as well. The lamellar appearance of the original colonies is from cooling and is not related to in-service exposure, unless the tube material had been exposed to excessive heat exceeding the lower critical transformation temperature. In this case, tube failure from stress rupture occurs and the microstructure contains a wide range of rapid cooling transformation products (mostly ferrite and bainite with no carbide colonies), not like what you have shown.

 

[engineermat]

metengr, thanks.

I don't have the equipment for hardness testing, so don't know the hardness.

Actually, microstructure shows some long term overheating characteristics. But I was wondering if it is possible that the tube was overheated to temperatures above lower critical transformation temperature, that sometime cause the formation of Widmanstätten structures. That's the reason I need to know if these are Widmanstätten structure or bainite.

By the way, do you have the experience that what temperature range can cause Widmanstätten structure when the tube is cooled from the temperature, near A3 or could be lower.

Thanks.

 

[engineermat]

Add little bit: the tube examined was not bulged or ruptured. There may be some overheated tubes elsewhere in the reheater (not sure bulged or ruptured).

 

[mrfailure]

Degradation from service temperature exposure is normal. Even eventual complete dissolution of carbides into the matrix over a long period of time can be normal. Instead of microstructure degradation, I would look at loss of wall thickness and ID scale growth to determine degree of tube degradation and to calculate tube metal temperatures and remaining life. This type of evaluation can actually be done nondestructively in the boiler.

Aaron Tanzer

http://www.lehightesting.com

 

[metengr]

engineermat

Actually, microstructure shows some long term overheating characteristics. But I was wondering if it is possible that the tube was overheated to temperatures above lower critical transformation temperature, that sometime cause the formation of Widmanstätten structures. That's the reason I need to know if these are Widmanstätten structure or bainite.

As I mentioned previously, if the temperature of the tube reached the lower critical from upstream blockage of steam flow and was pressurized in service, the tube would probably fail from stress rupture (axial-oriented rupture with obvious tube swell). The stress rupture would result from loss of tensile strength at elevated temperature under pressure and would produce the following microstructural characteristics; blocky ferrite grains with islands of bainite and carbides. These features would be formed by accelerated cooling from contact with reverse steam flow during de-pressurization of the tube. You don't have this situation. Even if the tube were to reach the lower critical temperature for whatever reason with no internal pressure or accelerated cooling, the partially austenitized tube metal most likely revert back to ferrite/pearlite and possible trace amounts of bainite.

Why are you worried about Widmanstätten structures? I would be more concerned with estimated remaining service life of the tube material as mentioned above using internal oxide thickness readings with an algorithm that predicts remaining useful service life.

 

[engineermat]

metengr, thanks.

My concern is if the temperature reached lower critical temp. or upper critical temp. before.

If these structures are Widmanstätten structures (since some colonies have this features), then I would know the tube temperature at least close or above upper critical temp, possibly due to an overheating event in service.

If these lamellar features are from original heat treatment (possibly residual bainite), then I know the tube temperature is much lower.

 

[mrfailure]

I seriously doubt you will ever see original structure in service-removed tubing T22/12 tubing. It is the nature of the beast - the bainite or fine pearlite degrades from service temperature exposure. Back in the day I would call these structure spheroidized pearlite (it did not matter if the tube began as bainite or not - the carbide spheroidization process already degraded this structure).

As metengr stated, you are unlikely to have reached such high temperatures as reaching transition temperatures without being on the way to an overheat (specifically a short term overheat) failure and you really will not be able to tell that you hit that temperature if for some reason the tube did not rupture as phase transformation product will not be present during boiler cooling when you get to see it. Hence, knowledge of original structure and whether or not Widmanstatten ferrite was in the original structure really is not useful. In failure analysis of the rupture, you will be able to perform a temperature estimate that is particularly useful in short term overheating based on structural transformation that results from quenching of the surface on rupture by steam. (You should still not see martensitic/bainitic transformation product during long-term overheating, but at least this helps differentiate from short term).

Aaron Tanzer

http://www.lehightesting.com

 

[mrfailure]

And I should clarify my last posting before someone else corrects me: The lack of microstructural transformation on rupture to harder bainitic/martensitic structures is not by itself the only differentiating factor between short and long term overheat mechanisms. You also have to look at other issues including wall wastage, ID oxide scale growth, boiler location (for example, superheater/reaheaters are more likely to fail under long-term mechanisms while waterwall tubes under short-term, but there are exceptions to both), and presence and degree of creep damage.

Aaron Tanzer

http://www.lehightesting.com

 

[engineermat]

Thanks, mrfailure