T23 Boiler Tube with Longitudinal Crack & Hidden Gems 2

[Guest102023]

The micrograph is in the HAZ very near unaffected base metal (~235 HV). We are seeing a number of these TiN (or TiCN) particles throughout the tube wall, even embedded in the 2 mil thick oxide covering an old fracture surface. The tube has a short (1/4") vertical tear just above the header weld, and it has been there for some time gauging by the oxide thickness. The tube is in one of the outer rows and is bent in the region just above the header. The crack is exactly on the intrados side.

Q1: Are the particles abnormal?
Q2: What could cause a vertical tear? My only guess is that the problem started at the factory - perhaps a mistake with the bending tooling?

I have a suspicion the two conditions are related ... chemical analysis has been ordered. For those who appreciate the difference, this tube did not originate from either V&M or Sumitomo.

 

[metengr]

brimstoner;
There is no attachment to view. Based on your stated description of the 1/4" long axial-oriented tear and particles;

Q1: Are the particles abnormal?

Yes, particles of this size are not normal. Titanium should be added at 0.020% by mass to combine with nitrogen to allow boron to do its thing for hardenability in T23 creep strength enhanced ferritic steels. First off - how did you even confirm the composition of these so-called particles, did you use an SEM/EDS or other method? You say nothing about the actual microstructure -was it bainitic, does it contain ferrite and bainite? How long was the tube ebnd in service? When was it found - because of an in-service leak?

Q2: What could cause a vertical tear? My only guess is that the problem started at the factory - perhaps a mistake with the bending tooling?

The cause of a tear could be from original forming conditions due lack of working temperature or original defect in the tube wall. You say nothing about the tube OD, wall thickness, or bend radius. Was their evidence that the tear was present prior to final heat treatment for this tubing material? Were the tube bends subjected to a post forming thermal treatment?

 

[Guest102023]

2nd attempt -- looks like I omitted the final step. Nothing worse than a geek with poor geek skills Thanks for the reply.

TiN is just my educated guess, having seen them elsewhere in steels like 321SS and knowing the ingredients of T23. My experienced metallographer had the same reaction. The microstructure does not seem abnormal (The section is transverse to the tube, about 0.2" away from the weld toe.)
So Q1 was somewhat rhetorical; I just don't yet know HOW they messed up the thermal treatment. I probably should order N and B with that chemical analysis to get to the bottom of this.

The plant later reported to me there were indications of poor penetration in the groove portion of the weld (it should be CJP + fillet with conventional B9 filler to the P22 header). I could not picture how that could cause a nearly perpendicular crack in tube base metal, which I why I speculated about a forming problem. The crack (actually ~15° off of longitudinal) is on the intrados side where bending deformation is greatest. I can't even guess about any post-forming thermal treatment, but telling you the fabrication of these harps was subbed out to Korea might already give you the probable answer to that. These T23 reheater tubes have a short but painful history, having seen one incidence of creep in a tube-header weld and two cases of SRC in the T23 tube HAZ. I ascribed the latter to poor PWHT practice.

I will also try to get a measure of the local bend radius. I will check with the literature and the Code, but IIRC 5% is the limit w/o thermal treatment.

 

[mrfailure]

The orange particles are characteristic of nitrides as you suggest. Based on composition, they most likely are niobium carbides (T23 doesn't have any titanium specified but niobium is present).

Regarding the tear: Can you see closely whether this was more characteristic of being tool induced as opposed to service formed?

Aaron Tanzer

http://www.lehightesting.com

 

[stanweld]

We have found that some T23 tubes have exhibited a high number of non-metallic inclusions at specific tube ends attributable to the tube manufacturer's need to increase its yield (not scrapping enough material from the base stock). This has resulted in a number of repairs thereto. Baased on your described crack, I would guess origination during tube bending.

 

[Guest102023]

mrfailure,
Unfortunately the outer surface has been ground off during extraction, but I will investigate the inner surface (the sample is 130 miles away ...)
Could the particles be vanadium CN as well as niobium, since it is more prominent? Previous analyses show about 0.21 V, 0.08 Nb

stanweld,
Interesting suggestion. Could an out of spec (or at least out of balance) chemistry have affected the ductility or response to banding?

I will post the results of previous analyses on tubes of the same reheater section for comment.

 

[Guest102023]

Tube bending of course. And the fracture shows no evidence of necking in the cross section, so we must describe it as brittle.

 

[mrfailure]

In answer to your question brimstoner:

Vanadium nitrides are submicron sized and, as such, are not visible using optical microscopy. Niobium nitrides are visible and consistent with the photomicrograph you posted of the orange cubes.

As for the tear: Given the previous creep failure and not knowing much about the geometry or boiler design, you may want to determine if the region is seeing long-term overheating by measuring ID scale and wall thickness using UT. If the photomicrograph is represented, original structure looks to has spheroidized and most of the carbides have already diffused into solution (a decomposition often observed prior to creep void formation, though its presence does not necessarily mean anything is wrong).

Aaron Tanzer

http://www.lehightesting.com

 

[Guest102023]

mrfailure,

These RH tubes operate at well below their material limit, though I can't recall the design temp at the moment.

I have not noticed precipitates like these in any other investigations.

 

[metengr]

brimstoner;
The chemistry reported for the Grade T23 is lacking a beneficial alloy element, titanium. From what I have seen with recent correspondence of problems with hardenability in T23, some heats of this steel were produced with no titanium resulting in ferrite during quenching from original heat treatment. This was found after quenching because the hardness was below 300 HV10.
This material contains M₂₃C₆ carbides from Fe, Cr, Mo and MX carbides from Cb/Nb and V with N. The Ti addition was discovered as being critical to tie up aluminum and N to allow boron to increase hardenability and avoid ferrite formation.
You should confirm if ferrite is present by using TEM. Also, the microstructure shown does not look typical of T23 quenched and tempered.

 

[Guest102023]

I dug up another analysis of a tubes from the same reheater where Ti was determined = 0.03%.

I know from past research on grade X-70 steel welding that Ti addition is necessary to protect B, which suppresses coarse ferrite nucleation at the grain boundaries, but in these steels I understand that it also has a strong effect in controlling carbide coarsening. I assume the goal in making T23 is to maximize the bainite and minimize ferrite.

Any suggestions for a good chemical analysis lab? I am not entirely satisfied with the pricing and results where I am. Accuracy is more important than speed.

 

[metengr]

http://www.exova.com/industry-sectors/aerospace/services/metallurgical-testing-lab-services

 

[Guest102023]

Thanks metengr, pricing and delivery were excellent, with no more suspicion that the results may not be accurate. Failure analysis is tough enough without introducing doubts about the lab results ...

 

[Guest102023]

The chemistry appears quite normal, with 0.04 Ti and 0.001 B, 0.009 N. So what could the mill have done wrong to produce large TiN nuggets, and what other side effects might there be? To slow cooling at some point?

BTW, where could I obtain the current code case for alloy T23?

 

[metengr]

brimstoner
Unless you are a code volunteer or owner/user or manufacturer, you can obtain the code case directly from ASME.
Have you evaluated the mechanical properties of the subject material? Also, have you performed TEM of the microstructure in the locations of the tube bend? The reason for my asking is that we found small amounts of ferrite dispersed along prior austenitic grain boundaries that would not be observed using the optical microscope.

Last, you realize that creep strength enhanced ferritic steels are very susceptible to cracking in service after cold or even warm forming. If the tube bends were not subject to controlled post thermal treatment after forming you will increase susceptibility to altered microstructures resulting in low temperature creep and softening due to accelerated precipitation coarsening. I would carefully examine the tube bend location intrados, extrados and neutral axis to evaluate the microstructure and obtain microhardness traverses.

 

[Guest102023]

A through-wall hardness traverse is a useful suggestion. So far I have only done a longitudinal traverse from weld fracture out to the base metal, along the tube mid-wall. All numbers seen unusually high.

The little fragment of tube I am examining has multiple issues, from basic metallurgical condition to creep-fatigue through the (possibly incomplete penetration) header weld, to the longitudinal tear, which I am certain originated no later than the tube bending operation (the oxide thickness in the crack is identical to that on the inner tube surface). Unfortunately the sample is not good enough to permit measurement of the bend radius, so some speculation will be inevitable.

TEM is not in the picture at this point. The GB ferrite you mention could be a consequence of the boron being tied up when it should not be. Since the composition appears OK, heat treatment becomes the prime suspect in the large TiN precipitates.

 

[Guest102023]

metengr et al,

Without having seen the more recent T23 code case revisions, I will guess that, similarly to P-91, cold forming above some % kicks in a stress relieving requirement. I understand that composition was also tweaked.

This is not the first failure in this harp, and I am getting a more comprehensive picture of how (badly) this equipment was manufactured. I am guessing that high general hardness in the formed tube base metal near the weld (not just HAZ) is the result of cold work that was excessive and/or not stress relieved out. This construction was made around 2002. I don't have a good enough sample to evaluate the bend radius, but I would be interested in finding out the work hardening characteristics of T23. Any good references on that?

Question:
Would you say that such excessive residual work hardening could enhance the chances of stress relief cracking? I have now encountered two cases of SRC, which I have blamed on the fabricator.

 

[metengr]

Question:
Would you say that such excessive residual work hardening could enhance the chances of stress relief cracking? I have now encountered two cases of SRC, which I have blamed on the fabricator.

Yes, based upon my involvement with review of present and past Code Cases for T23, and more recent industry problems. This material can exhibit significant secondary hardening that will strengthen the grain interiors and leave the grain boundaries in a lower strength state, susceptible to intergranular failure.

 

[Guest102023]

Thanks metengr,

I should have added that these T23 tubes do not operate at a high enough temperature for secondary hardening in service, which is what pointed to the fabricators. Probably improper workpiece support during PWHT that led to tensile stresses across the welds.

Live by the carbides, die by the carbides, it seems.

 

[metengr]

I should have added that these T23 tubes do not operate at a high enough temperature for secondary hardening in service, which is what pointed to the fabricators.

Incorrect. I have seen data that indicates otherwise for T23 used in as-welded waterwall panels, and having PWHT may be necessary to reduce hardness.