T23 Boiler Tubes - Fracture in HAZ
T23 Boiler Tubes - Fracture in HAZ
(OP)
I suspect reheat cracking as the failure mechanism in a 2"Ø T23 reheater tube that failed in the HAZ. Following are the findings so far:
(1) The tube is joined to a P22 header with ER70S-B3 filler (undermatching for hardness and creep strength w.r.t. the tube).
(2) The fracture surfaces are well oxidized (leak-before-break it appears).
(3) There are almost no indications of creep microcracks sub-fracture surface.
(4) HAZ hardness approaches 300HV, decreasing to 205HV in unaffected base metal.
I am not looking for anyone to solve my problem, but I would like to know what conditions could predispose T23 to reheat cracking, and what is the incidence of this type of failure in HRSGs?
(1) The tube is joined to a P22 header with ER70S-B3 filler (undermatching for hardness and creep strength w.r.t. the tube).
(2) The fracture surfaces are well oxidized (leak-before-break it appears).
(3) There are almost no indications of creep microcracks sub-fracture surface.
(4) HAZ hardness approaches 300HV, decreasing to 205HV in unaffected base metal.
I am not looking for anyone to solve my problem, but I would like to know what conditions could predispose T23 to reheat cracking, and what is the incidence of this type of failure in HRSGs?





RE: T23 Boiler Tubes - Fracture in HAZ
Normally, reheat cracking is intergranular for this alloy is in thicker wall sections.
RE: T23 Boiler Tubes - Fracture in HAZ
Puzzling to me that the tube could have been leaking for what looks like a long time before complete fracture. But then it may have been broken a long time; it was a single fracture, and one leaker might have gone unnoticed.
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
For example,
was it a full penetration weld with nozzle reinforcement or was it a partial pen weld with add'l header wall thickiness for reinforcement?
What was its operating life to date- is this an infant mortality or has it been thru a series of severe startup/shutdown/spray water cycles?
T23 is bainitic and requires a fast post weld cool down to ensure bainite- if the header was very thick, then the cool down rate may have been inhibited.
RE: T23 Boiler Tubes - Fracture in HAZ
(1) Full pen + fillet to ~1" thick header. The site pics just received show fracture coinciding with the weld toe.
(2) Operating cycles are in the low hundreds; under 3 years in service. They start the unit too hard (more recently backed off somewhat) and have had severe draining/water hammer problems.
(3) Bearing your comments in mind, the high hardness is a bit puzzling, but it has previously been found at the opposite end weld, where there were several weld metal creep-fatigue failures ~1 year ago. That was less surprising considering the undermatched B3 filler.
I am now wondering how much tube vibration could affect random tubes. Tube length is >40 ft.
What is the 'optimum' heat treat temperature for T23? I have googled for lots of information, but there's not a lot of fundamental data out there; maybe its time to shell out for the V&M book or the HRSG Users Group text ...
RE: T23 Boiler Tubes - Fracture in HAZ
The most important feature associated with this alloy is the chemical composition to ensure a fully hardened bainitic or bainitic/martensitic microstructure, prior to tempering for original heat treatment. Subcritical PWHT is not as critical to exceeding the lower critical transformation temperature, as compared with Grade 91.
RE: T23 Boiler Tubes - Fracture in HAZ
The very fast startups of these units plus the disparity in the tube wall thickness vs hdr thickness plus common occurrence of overspraying of the attemporator often leads to fatigue failure at the tube stub to header weld.
The rate at which the metal response to temperature fluctuations varies by the square of the wall thickness. The tube is likely 6 times thinner than the header, so it responds 36 times faster to transients, such as caused by subcooled water spray hitting a hot tube or header during startup . The tube responds quickly and cools down to 650 F, but the header remains at 1050 F ( guestimate), leading to a high shear stress at the weld interface between the tube and header.
You can prove or disprove this theory by placing a few thermocouples on the header and also monitor 10+ tube stubs close to the header- the plot ot T vs time during startup ( with 1 sec scans) will show how severe is the magnitude of the temperature difference during cold startups as the superheater spray is activated.
So, it is just as wild to assume the failure is fatigue as to assume it is weld reheat, only the metalllurgiacl analysis and field test data can yield a correct answer.
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
They have run the duct burners very aggressively (for the 1st year at least), but of course they are not monitoring tube and header temperatures. I am not sure where the duct burners are situated - the upper end? [I don't want to spell out the name of the constructor, but you may have guessed it reading between the lines.]
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
davefitz, temperature monitoring would have been invaluable in solving this case (or even preventing it).
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
RE: T23 Boiler Tubes - Fracture in HAZ
I can't top that; you win. Did they give you tubes too?