Microstructure for low alloy steel SA213 T11/T12
Microstructure for low alloy steel SA213 T11/T12
(OP)
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.





RE: Microstructure for low alloy steel SA213 T11/T12
Aaron Tanzer
www.lehightesting.com
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
The attached are some photos of a reheated tube (T12) that was in service for a period of time.
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
RE: Microstructure for low alloy steel SA213 T11/T12
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
RE: Microstructure for low alloy steel SA213 T11/T12
Aaron Tanzer
www.lehightesting.com
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
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.
RE: Microstructure for low alloy steel SA213 T11/T12
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
www.lehightesting.com
RE: Microstructure for low alloy steel SA213 T11/T12
Aaron Tanzer
www.lehightesting.com
RE: Microstructure for low alloy steel SA213 T11/T12