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Overheat damage of carbon steel tubes

  • 1.  Overheat damage of carbon steel tubes

    Posted 05-08-2021 08:39

    The images shown here are replica micrographs for carbon steel pipe that was subjected to an overheat incident:
    Material: ASTM A106 Grade B
    Design temperature for the tubes is 335 °C.
    Temperature excursion exposed tubes to a temperature of 600 °C for 16 hours.
    Location: Base material of tube

    Description: ferrite and pearlite microstructure showing pearlite decomposition and spheroidization of carbides

    Comment: Overheating exposure caused pearlite decomposition (partial metallurgical damage)

    Degree of damage: Moderate damage
    What was recommended is to repeat replication after 3 years without any immediate action . How do feel about this recommendation?


    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection and Welding
    Maadi, Cairo

  • 2.  RE: Overheat damage of carbon steel tubes

    Posted 05-11-2021 09:19

    I would approach it like this for starters (this is by no means comprehensive):

    • Look into the original heat treatment of the pipe. ASTM A106 is fairly vague about heat treatment and just says that, if it's performed, it shall be done at 650C or higher (the operator may have their own heat treatment specification however). So this could mean normalize, normalize and temper, etc. If the original heat treatment was to normalize and then temper at 650C, then this exposure won't drastically change the microstructure. If it was just normalized, then it will.
    • If there's a section of affected pipe you can cut out and take tensile/bend tests from, that can serve as a quick pass/fail to see if you've fallen below the A106 minimum strength/ductility values. If you have, then it would be a defensible decision to replace the affected pipe.
    • Regarding the recommendation to just let it ride for 3 years, I would question the rationale. Was this recommendation founded on any technical grounds, or was it just a managerial decision made out of convenience? If it's the latter, I would push back and ask for justification. What is expected to be different in 3 years? Do they expect the microstructure to revert back to how it was before the pipe was overheated? Is there going to be any reduction in service intensity? If that pipe has been severely compromised, it could fail tomorrow, let alone 3 years from now.

    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX

  • 3.  RE: Overheat damage of carbon steel tubes

    Posted 05-12-2021 02:01
    The issues with carbon steel tubes include:
    -limited temperatures are allowed in service because of graphitization tendency.
    -decreased strength at elevated temperatures beyond the original design.
    -decreased strength if in service with other than the expected microstructure (pearlitic).

    If the material was under stress/pressurization when at 600C, substantial creep may have occurred.  This will increase the diameter of the tube, which will both decrease the wall thickness and increase the unit stress under pressure.  If the diameter or wall thickness have changed outside of the original design requirements, you may want to perform a new design analysis for suitability.  Excessive oxide thickness also can play a part in tube service suitability.

    The microstructure shown (not the best micros) appears to be substantially degraded from the original.  You may want to remove some tubes and do mechanical testing to determine the current mechanical properties and compare them to the original specification.  The microstructure shown has a much lower percentage of pearlite than I would expect under Grade  B which may mean that the supplied tube is on the low side of strength to begin with, though Grade B has a very low mechanical property requirement to start with.  In any case governing Codes generally assume that service stresses are being applied to material fully meeting the original specification.

    I hope that these comments are useful.

    Paul Tibbals

  • 4.  RE: Overheat damage of carbon steel tubes

    Posted 05-12-2021 09:56
    Hi Waleed,
    You did not mention at what condition the steel was used. Is it normalized or normalized and tempered. If it is tempered, I am guessing that the temper temperature is higher than the design temperature. If you are worried about the temperature excursion of 600°C, I would guess that it is higher than the temper temperature used.
    The spheroidization of Pearlite is a measure of temperature effect if we know what was the original condition, thus the original strength/hardness you begun with and that was needed for your design. Longer duration will lead to carbides dissolution 
    Mechanical properties of ASTM A106 decreases with Temperature/duration. It is an Arrhenius law. First, spheroidization of pearlite occurs and then carbides dissolution.

    I agree with Sean statements, especially the one regarding the recommendation. Please look at the study made on this steel in the following link (it is open source) on how the microstructure of this steel changes with temperature/time and its impact on erosion.

    The data in this article could be used to determine the Arrhenius parameters of this steel (data of as-normalized condition and 530 and 630°C at different durations), and then applies it to your situation. You can then have an educated decision​ on what to do, and challenge the recommendations if needed.

    Hope this helps.

    Nihad Ben Salah
    NBS- M&P Consulting

  • 5.  RE: Overheat damage of carbon steel tubes

    Boston Chapter Admin
    Posted 05-12-2021 16:09
    For appropriate recommendation, there needs to be application context on usage, temperature, pressure, applicable Codes & Regulations, and potential safety and health consequence.  How much thickness and t_min values?


    Dan D.

    Daniel Denis
    ASM Chapter Council Vice Chair
    Hartford Chapter
    Senior Consultant - Metallurgy & Materials
    Structural Integrity Associates, Inc.

  • 6.  RE: Overheat damage of carbon steel tubes

    Posted 05-14-2021 05:29
    Hi Everyone 

    Thank you for your comments and the shared resources.
    DAN, I believe this piping work was done according to the process piping code B31.3.  I am not sure of the initial heat treatment condition of the pipes . The heat damage occurred during commissioning of the heater, so the heater was not in service before the temperature excursion. 

    The scope of this work was initially based on characterizing the microstructure and the hardness at several locations of the hot oil heater. The Investigation covered locations of pipe base metal, HAZ, weld metal, and base metal at a bend location. Out of 36 locations, 21 locations did not show any significant microstructural changes. Slight metallurgical damage was observed in 12 locations, while only three locations showed moderate metallurgical damages similar to what appeared in the micrographs.  The hardness values were all above the lower limit (120 HB), but at the lowest range which might agree with low carbon level as noticed by PAUL . Based on these results, and after discussion with User, the three years of re-inspection was suggested. Additional work was done after that to confirm these recommendations.

    This is to physically simulate this heat overshot via heat treatment of an identical pipe spool.  This might be in line with SEAN commendation. The replicas of the simulated spool showed a moderate damage at three locations, and only the weld metal was free did not show any  microstructural changes.  The hardness were all above the lower limit as well. Tensile specimens were cut from the simulated pipe spool, but I am not aware of the results yet. 

    I reviewed the paper that NIHAD recommended: the paper showed that mcirotsrctural changes occurred after 50 hours at 630 C. I am not sure if this can be useful to compare with our case, since this was the shortest treatment time in the paper at this temperature. This means the changes might occurred  at shorter times but not recorded or observed. The starting microstructure of the pipe material of the paper might also be different from that of the oil heater. Even with this, the paper was very useful to understand the rate of changes in A106 grade B materials.        

    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo

  • 7.  RE: Overheat damage of carbon steel tubes

    Posted 05-15-2021 13:49
    I'm wondering why this is such a big deal.  600 -650°C is the standard range that carbon steel (P-1) is heat treated at under B31.3.  That is done all the time, and while you don't usually PWHT for 16 hours, the effects are predictble.   The only thing that will happen to carbon steel at this temperature over a long time is the carbides will spherodize and the steel will become softer.  That appears to be what the microstructure shows.  
    Hardness testing will show if the steel has been softened too much, and it does not appear that it has.  Since it has not softened too much, the tensile and yield strength can be expected to be acceptable.
    Waleed, as long as you do not find any geometric distortions that could create local over stress conditions, I think you are good to put this in service.  The only reason to look at it at some later date is that 335°C is hot enough that high-temperature oxidation might be an issue over a long time.  The microstructure and properties you have today will not change further at 335°C.

    Walter Sperko
    Sperko Engineering Services Incorporated
    Greensboro NC
    (336) 674-0600