T23 Boiler Tube - Curious Fracture
T23 Boiler Tube - Curious Fracture
(OP)
T23 Reheater Tube Fracture in Weld -- Anyone seen this fracture morphology before?
Several T23 reheater tubes failed in a similar manner after less than 6 months and ~250 cycles from commissioning of the HRSG. The circ. fracture originates in the weld, at the front (where you would expect bending force to be greater) then moves into the base metal as it propagates around to the back side. Meanwhile, a ~4" long tear happened on the front side. It seems like there is more than one thing wrong here.
Unfortunately I did not see the header side of the story (another case of repair first, ask questions later), so it is hard to confirm the lack of penetration that me and the repair contractor suspected.
Particulars: 2" T23, welded to P22 header with manual TIG, ER9018-B3, PWHT. Unknown tube origin, but they are significantly older than the plant. All zones appear to have normal microstructure (no ferrite at least). The correct filler was used. Weld hardness is 285HV1, HAZ up to 335HV1, base ~192HV1. I have found both tube surfaces decarburized. Scattered corrosion pits up to 3 mils deep on outer surface. A few very short secondary cracks and sometimes oxide along prior austenite GBs.
Any thoughts or suggestions? The hardness seems on the high side for a PWHT joint. I thought about reheat cracking, but did not think that can affect the weld also(?)
I am a newbie here so I hope this gets posted correctly ...
Several T23 reheater tubes failed in a similar manner after less than 6 months and ~250 cycles from commissioning of the HRSG. The circ. fracture originates in the weld, at the front (where you would expect bending force to be greater) then moves into the base metal as it propagates around to the back side. Meanwhile, a ~4" long tear happened on the front side. It seems like there is more than one thing wrong here.
Unfortunately I did not see the header side of the story (another case of repair first, ask questions later), so it is hard to confirm the lack of penetration that me and the repair contractor suspected.
Particulars: 2" T23, welded to P22 header with manual TIG, ER9018-B3, PWHT. Unknown tube origin, but they are significantly older than the plant. All zones appear to have normal microstructure (no ferrite at least). The correct filler was used. Weld hardness is 285HV1, HAZ up to 335HV1, base ~192HV1. I have found both tube surfaces decarburized. Scattered corrosion pits up to 3 mils deep on outer surface. A few very short secondary cracks and sometimes oxide along prior austenite GBs.
Any thoughts or suggestions? The hardness seems on the high side for a PWHT joint. I thought about reheat cracking, but did not think that can affect the weld also(?)
I am a newbie here so I hope this gets posted correctly ...





RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
I should add the HRSG is also experiencing chronic fractures of 1" drain lines below these headers, so we know thermal stress is the driver (surprise).
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
just trying to understand the appearance of a fillet weld configuration on the tube or was this the previous attempt you mentioned to repair the leak?
The initial observation has the appearance of a rapid overheating but is tempered by the proximity to the header.
Very good pictures.
RE: T23 Boiler Tube - Curious Fracture
Do you mean rapid overheating in service? There was no information given to indicate tube temperature excursion.
RE: T23 Boiler Tube - Curious Fracture
It also appears there is no shear lip on the longitudinal fracture surface while there is on the circumferential fracture surface is this true.
You mention failure of other components near this failure is it possible that this tube was quenched, or dry, for lack of a better word?
I'm looking for some very old pictures of 2 similar failures in a waste heat boiler. If my recall is correct it was quickly decided to plug the header and make no attempt at a repair.
RE: T23 Boiler Tube - Curious Fracture
On the longitudinal fracture there are small shear lips, aided by the soft decarburized surface layers - hardness was as low as 129HV1 there. (Makes one wonder how they checked the hardness after N&T and after PWHT.) I am looking at this condition not so much as a possible cause but as an indication of a lower quality tube supplier. But the weld fracture appears fairly brittle; what you are probably looking at is the fillet on top of the groove weld. I am awaiting better micrographs of some of the fracture surface detail.
RE: T23 Boiler Tube - Curious Fracture
Has anyone recently altered the hot gas flow path in this area?
Two reasons I ask, one is the possible overheating and the other is a remote possibility of fatigue as the cause of the original failure. The fatigue aspect comes from your mention of brittle looking failure. We had a fatigue failure in a screen tube on another waste heat boiler right at the header.
If you can get some thickness measurements of the failed section along with the data you have already have it might give some indication of the time frame.
The additional photomicrographs you mentioned will be interesting.
RE: T23 Boiler Tube - Curious Fracture
The axial-oriented fracture surface looks like it exhibits chevron markings that point back to the circumferential weld, and the fracture looks rather flat and brittle-like in appearance(hard to tell with the picture).
I will tell you this; I have seen similar failures on conventional Grade T22 material where we had a serious water hammer event during a hot start condition in of our HRSG's. The water hammer event was so localized and severe we sheared tubes from the header and several split axially. This does not appear to me to be the result of overheating - just a guess and based on what I have seen and read thus far.
RE: T23 Boiler Tube - Curious Fracture
Fatigue is an interesting suggestion, and would explain a few observations. Certainly the cyclic thermal strain is substantial. I just have found no evidence to justify the lack of penetration I had expected to find. It could also be both, of course. The metallographic section suggests part of the fracture surface was exposed for a period of time.
RE: T23 Boiler Tube - Curious Fracture
No reports of water hammer; I think someone would have noticed. As for the vertical tear, I think that should not have happened unless something was very wrong - either insufficient tube properties or excessive loading (such as water hammer). The tube appears normal, decarb notwithstanding ... makes the question of which part of the fracture came first a little trickier. Another pic attached.
RE: T23 Boiler Tube - Curious Fracture
Not necessarily a true statement for many of the plants I have dealt with over the years. Keep in mind that if your header drains are not routed or vented correctly and condensate pools in one location of the header and you start-up in this condition, you stand a high chance of a local water hammer event or subsequent shock wave that will fail thinner wall tubes versus a header. I have seen this before.
Unfortunately decarb is typical for most boiler tubing unless it is dealt with by customer specification. Second, your close up view of the axial rupture does not appear to be from exposure to elevated temperature service. The Grade T23 materials have high allowable stresses which mean they have good high temperature strength and creep strength, if heat treated correctly.
I believe your failure initiated from the axial rupture and the only type of operating event that can cause this in service is shock loading from a water hammer or over temperature excursion. I don't see any evidence that this tube saw excessive metal temperature short or long.
RE: T23 Boiler Tube - Curious Fracture
Even fatigue does not explain the vertical tear (no evidence by SEM fractography, although there was much post-failure damage).
RE: T23 Boiler Tube - Curious Fracture
Thanks to all - I will reciprocate where possible. My specialities are welding engineering and failure analysis (mainly refinery). I just started career #3, working for myself.
RE: T23 Boiler Tube - Curious Fracture
Just out of curiosity you mention two things that If possible a little more information would rev up my windmills.
The first was that was the manufacturer flattened the header and set-on the tubes and welded. Was that his method of attaching the tubes to the header?
The second was that they start up the boiler dry?
If you get any more information from observation or analysis please post.
RE: T23 Boiler Tube - Curious Fracture
first question, I wrote 'flattened' when I meant to say they ground off header material to present a planar surface to the tube end. Sloppy writing on my part. But this was hearsay from the repair contractor; I have not seen it personally. Don't know if that is standard industry practise; it would be simpler and more reliable than dressing the tube end to a saddle shape I suppose.
second question, I cannot answer that one
If anyone can point me to good references that address these HRSG operational issues I would appreciate it greatly. I have downloaded all the available issues of Comb Cycle Journal and a ton of EPRI stuff.
One more thing - I would like an opinion on the weld zone hardness I reported previously - it seems a bit high for a PWHT joint ...
RE: T23 Boiler Tube - Curious Fracture
Thanks for the kind words.
This paper would be of interest to you on Grade T23 material (welding and fabrication). Also, best of luck to you and your new career.
ht
RE: T23 Boiler Tube - Curious Fracture
You might want to rummage around on this site:
http://www.hrsgdesign.com/
Best of luck.
rmw
RE: T23 Boiler Tube - Curious Fracture
I now know what you mean by 'cut your way in' - I have some pics from the repair contractor on this job. I look at the MBA beancounters on this projects as my job security, because they are so fixated on upfront and material costs.
In the refinery business they use the term 'opportunity crude'. The opportunities are for corrosion engineers and failure analysts ... ;)
cheers
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
Here is a little information on welding of Cr-Mo boiler steels.
http:/
http://www.t-put.com/EN_welding_CrMo.pdf
RE: T23 Boiler Tube - Curious Fracture
The weld was PWHT due to P22 on the header side of the joint. The filler is -B3.
I noticed a few prior austenite GB penetrations along the weld fracture, the most prominent of which is in the attached pic. This one has oxide, which is very interesting. I cannot be sure if the fracture (which was ductile but not very) was intergranular or not. I would certainly think that impact properties are closer to 20J than 200J ...
On the longitudinal tube tear, very rough chevron marks point to an origin at or near the weld. The fracture surface had a lot of ups and downs of up to almost 1 mm in scale; I'm wondering if this suggests anything untoward in the tube condition. Comparison with the textbook 45° shear edge at the circumferential terminus confirms it was very dynamically loaded.
Thanks for the references. I would be interested in opinions/philosophies on filler metal for this joint - matching for the tube or for the P22 header. Also I am interested to know failure mechanisms in these joints after 5-10 years (including type IV creep I assume).
RE: T23 Boiler Tube - Curious Fracture
a)reheater spray water causes alernating dry-out overheat folowed by wet subcooling of teh tube. Whiel the tube itself can accept fast temperteru cahnges, the close vicinity of the thicker header surface will not respond to the fast heat-quench-heat cycles the same way. The result can be severe alternating shear stresses at teh stub to header interface- the telltale indication of shear is that it is a maximum at the wall centerline thus the crack should originate there.
b) same alternating tube temperature variations as (a) , but if also causes a high tube to tube temperature difference which can lead to a high alernating bending moment on the tube if there is inadequate flexibility in the tube bundle design. For example, 3 rows of tubes from inlet hdr to outlet hhdr with no tube bends intervening is a low flex- high bending moment configuration that is not tolerant of such temp unbalances.
c) Water layout on the bottom of header or a header which has tubes stubs only welded on the bottom side of a top supported header- fast heatup of the header can lead to a top to bottom header temperature differential that would tend to cause the header to temporarily bow or "bannanna" , and if this deflection is restrained by the tubes, then it is overstressing some tubes. see below (d) for one such event .
d) If unit has a dry reheater during startup ( ie does not have a HP steam to reheater bypass system ) the tubes will be heated to 1150 F prior to turbine synchronization, and the instant after synchronization, the initial flow of steam thru the reheater will cuase sudden downshock of teh tubes and sudden upshock to the header.
e) typical causes of water layout in the header or alternating wet/dry conditions of the tubes are (1) OEM's are notorious for using insufficient straight transfer pipe lengths downsteram of the spray water attemporator, leading to un-evaporated spray water slugs to etner the inlet header that distribute unequal enthalpy fluids to the neext ros of tubes. Some tubes get hto steam, some get wet steam , and some get water. The min lenght of straight pipe downsteram of thw spray nozzle should be 0.35 seconds min transit time to next elbow and 0.7 secs min transit time to downstream header. (2) connecting the reheater header drains to the blowdown tank is asking for trouble- the reheater can operate under vacuum prior to turbine synch on a dry reheater startup system and it sucks water from the blowdown tank up into the header( as much as a 25 ft draw) .
Put some thermocouples on the headers and tubes and monitor with a Digital data aquisition system , with fast trace reset on a 1 sec basis, and prepared to be very surpised at the massive temperature differences you will see during startup. Don't even think of pluggin these values into a finite element model of the tube/header joint- you won't believe the stresses.
RE: T23 Boiler Tube - Curious Fracture
Thanks! Thats a lot to think about but clearly you know your way around a HRSG (and that startup is a tricky business).
Could you define 'water layout'?
What is the specific function of reheater spray water (besides temperature/thermal expansion management in general)?
I revisited the many photos and see that the 3rd tube that was sent elsewhere did not have a vertical tear, but otherwise similar morphology in the weld fracture. It is located at the end of a tube row, while the two tubes I have (one damaged beyond usefulness) are from near the middle - the one I am examining in the middle row, the other in the front of three rows).
It is clear that operational issues are at the root of these failures, as the customer has acknowledged. I will offer the last piece of your advice as a recommendation.
RE: T23 Boiler Tube - Curious Fracture
The water layout at the header invert is just remnant moisture that was separated from a 2-phase flow mixture that may have discharged from the smaller tubes. IN the individual tubes, the steam+ water velocity may have been high enough to entrain the water mist as droplets, but the lower velocity in the header may allow the water to drop out of entrainment and layout on the header bottom.
Also, water can be inducted up the header drain connection into the reheater header if the header drain was erroneously intertied to a superheater drain line or if the reheater is under vacuum during startup.
RE: T23 Boiler Tube - Curious Fracture
So, water is going where it should not, or else its presence (if unavoidable) is not being handled correctly?
RE: T23 Boiler Tube - Curious Fracture
If the data shows 200-400 F temperature differences between the top and bottom of the header , or between an individual tube stub and the header or to the average of the other tubes, then it is pretty certain that liquid water found its way to where only superheated steam is supposed to be.
It goes without saying that the reheater tube bundle was not designed for such temperature diffences. But it gets better- the temperature differences may be alternating at dozens of cycles per second , adding a prompt fatigue failure component to the problem.
So, add the thermocouples, and have fun figuring out where the water is coming from.
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
BTW, can someone tell me the status of grade T23 in the Code? I don't have access to the most recent editions. I saw SA-213, T23 on a data sheet - can I infer T23 is no longer a Code Case?
RE: T23 Boiler Tube - Curious Fracture
Could I consider the weld and HAZ somewhat high for PWHT material? The literature indicates HAZ hardness for T23 of under 350 HV in the as-welded condition.
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
We recently did a couple of new builds on HRSG boilers, and where we did find difficulties was in the PWHT of stubs to headers. The difficulty is that in order to do a proper PWHT, the header needs to be heated up as well as the stub, and it is also quite difficult to get good contact between heaters and stubs, and even thermocouple placement is difficult and ultimately very critical on these. With these materials it is easy to get up to & through soak temp, the trick is the controlled cooling, and 95% of the difficulties we had came from problems related to cooling............
RE: T23 Boiler Tube - Curious Fracture
I am aware of the difficulties of effective PWHT on some of these components - in the past it has kept me gainfully employed doing failure analysis and field hardness testing. (Don't even get me started on the kids swinging their 'calibrated' Brinell hammers for some heat treaters.) As-welded P-91 hardness is somewhat higher than that of T/P23.
For this job I had hardness testing done on a prepared metallographic section using Vickers (1kg). I am aware 1kg may overstate hardness relative to the standard 10kg, but from my experience I think it would be very minor. I have compared the mean weld and HAZ hardnesses to 4 available PQRs, which are all significantly lower. Except for the 1kg test load and variations in PWHT times/temps it is an apples to apples comparison.
Also, my results are similar to published data (simulated HAZ) for non-PWHT P23. I am trying to locate a CCT curve for T23.
Starting to look as if PWHT was not very effective. Not surprising in the field, but a bit shocking for an OEM shop welded component. But its all very consistent with the brittle-looking weld fracture.
RE: T23 Boiler Tube - Curious Fracture
.
Did you obtain a copy of the paper I referenced above? There is a CCT for Grade T23 on page 3!
RE: T23 Boiler Tube - Curious Fracture
Interesting but useful that this material is so insensitive to preheat; it allows one to make a better conjecture about the PWHT. With conventional steels it can be more difficult to know whether high HAZ hardness was caused by insufficient preheat or PWHT.
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
I would hope that under shop conditions the entire header would be preheated using electric resistance elements with adequate thermocouple control. For field repairs things get more difficult of course.
In this case I am sure that PWHT, or lack thereof, is the culprit. The HAZ hardness level in this tube is typical of as-welded construction when it should have been ~30-40 points lower. Also, T23 as-welded hardness does not seem to be particularly sensitive to preheat level. Some ferrite probably is not uncommon in T23 base metal, but I did not see it anywhere in this case.
RE: T23 Boiler Tube - Curious Fracture
if you ever need to PWHT something like your headeer you might want to discuss the problem with these people.
http
RE: T23 Boiler Tube - Curious Fracture
I would appreciate that ...
Different materials, widely different thickness and thermal mass, high PWHT temperature, 45 foot long tubes ... I am curious about the PWHT procedure that is used by OEMs.
RE: T23 Boiler Tube - Curious Fracture
These people can heat some mighty large boilers and vessels with excellant control. Remember that you can overcome a lower temperature by extending the time.
In the majority of materials about 95% of your PWHT is accomplished in the first 15 minutes or less.
RE: T23 Boiler Tube - Curious Fracture
BTW, what is the optimum PWHT temperature for T23? And does it depend on thickness at all?
RE: T23 Boiler Tube - Curious Fracture
Looks like improper PWHT problems to me. It can be very tough to PWHT the header and not overheat the tube just past the socket weld, especially if PWHT is performed on in-boiler repairs. Many times the PWHT techs will move the TIs down the tube to a "cooler" spot. This avoids over-temps showing up on the chart, but the tube directly above the socket weld is cooked and/or cooled down too fast.
RE: T23 Boiler Tube - Curious Fracture
HAZ hardness just next to the fusion line was 330-335 HV, as I have since learned a little high for welds that were PWHT. The hardness is in fact similar to the as-welded condition. But metallurgy notwithstanding, the immediate root cause appears to have been water hammer. Failure
analysis gets tricky when more than one thing goes wrong!
But I agree TC placement is crucial, and I have witnessed 'temperature management' by PWHT techs who are mainly concerned with obtaining a perfect-looking chart.
This is another example of where having an appropriately trained engineer overseeing a task would be better - someone with a knowledge not just of welding but heat transfer, distortion, etc. It is near impossible to ensure correct heat treatment by reviewing a generic procedure while sitting in my office. One of the unfortunate trends with EPCs is cutting back on direct surveillance of fabricators ... maybe a good idea for a whole new thread!
RE: T23 Boiler Tube - Curious Fracture
HAZ hardness just next to the fusion line was 330-335 HV, as I have since learned a little high for welds that were PWHT. The hardness is in fact similar to the as-welded condition. But metallurgy notwithstanding, the immediate root cause appears to have been water hammer. Failure
analysis gets tricky when more than one thing goes wrong!
But I agree TC placement is crucial, and I have witnessed 'temperature management' by PWHT techs who are mainly concerned with obtaining a perfect-looking chart.
This is another example of where having an appropriately trained engineer overseeing a task would be better - someone with a knowledge not just of welding but heat transfer, distortion, etc. It is near impossible to ensure correct heat treatment by reviewing a generic procedure while sitting in my office. One of the unfortunate trends with EPCs is cutting back on direct surveillance of fabricators ... maybe a good idea for a whole new thread!
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
Anyway, their staff is bare-bones and I'm sure they can afford to hire the experts.
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
Another source would be the EPRI workgroup on HRSG's .
If you only wish to focus on the above described failures, I would suggest the followign field T/C's in addition tothe standard operating T/C's:
a)10 inlet tube stubs , as close to the header as feasible
b) 2 on header top CL, 2 on hdr bottom CL
c)1 at reheater spray water pipe as close to spray attemporator nozzle as feasible
d) one on each header drain pipe, 1 ft below header connection
In the old days, field T/C's installed in thick walled tubes or headers would be simplydrilled and peened , to avoid welding. But this might not be feasible with thin wall reheater tubes. There may be a tack weld pad type available for these thin tubes, but you need to do some homework on weld procedures on P91 tubing.
RE: T23 Boiler Tube - Curious Fracture
These are T23 tubes, so I am investigating the implications of attachment by welding. I think B31 may have something to say about it.
A question about b) - how close to the tubes should the header TCs be placed?
Thanks very much!
RE: T23 Boiler Tube - Curious Fracture
a) top to bottom DT- layout of water on the bottom can cause the bottom to operate much colder than the top during water induction events ( from the drains or from overspray). A top to bottom DT of opposite sign occurs during initial synch of the STG on units with a dry reheater ( ie no steam to reheater steam bypass system)
b) tube stub to header DT- header top T/C's minus temp of coldest or hottest stub T/C provides some indication of shear stress imposed at stub to hdr weld. The stubs can change temp perhaps 25- 50 times faster than the header ( varies by the 2nd power of wall thickness, (0.15" /0.75")^2 = 1/25 . A fast scan of stub temps is required to indicate how many cycles are occuring due to alternating wetting- dryout during overspary events. The header T/C does not need to be near the stub weld.
c) spray water t/c- will show the severe subcooling at this location at teh initiation of spray use- very subcooled spray water does not evaporate easily and leads to carryover of liquid to the downstream tubes and header.
d) drain line t/c near hdr- will show water induction from blowdwon tank or superheater drain manifold tie-in.
RE: T23 Boiler Tube - Curious Fracture
You might want to give these people a call about their
Acoustical Pyrometry
http://www.enertechnix.com/
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture
[This is the kind of professional development my previous employer should have paid for, but the manager spent the money on new office furniture instead ;) ]
RE: T23 Boiler Tube - Curious Fracture
RE: T23 Boiler Tube - Curious Fracture