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In situ etchant for P91 steel

  • 1.  In situ etchant for P91 steel

    Posted 09-18-2020 08:12
    I am looking for a suitable etchant for field replication of the P91 steel. The picric acid and its reagents are restricted. I would appreciate if you can share some of your experience.


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    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
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  • 2.  RE: In situ etchant for P91 steel

    Posted 30 days ago
    I had relatively little experience with P91.  With the 2.25 and 5 Cr steels, for field replication, the concern was always to quantify the grain boundaries and cavitation, and we always used nital as the etchant.  When temperatures were relatively low we added some H2O2 as an "accelerant"; I am not sure whether the peroxide or just the water content was what increased etching.  I believe that we did some P91 with nital.

    It is certainly inconvenient when picral-containing etchants are not available for general use.  I have never heard of an actual picric laboratory explosion in a metallurgy lab, but occasionally with bio students who weren't caring for their picric staining solutions.

    When we first started evaluating the Cr-Mo steels, we followed the practice of a consultant who used electropolishing with perchloric mixes.  For various reasons we moved away from that practice to mechanical polishing and chemical etching with nital:
    -Perchloric has its own significant safety and handling issues;
    -Electropolishing prepared only very small areas, and there is value in getting a larger view of an area;
    -Nital is relatively safe to use and transport;
    -Picral mixtures are not considered as good for grain boundary structure.

    I recommend lab tests on known microstructures for comparative purposes.  9Cr steels may be low enough in Cr to still give good results with nital.  You could increase the concentration of nitric to see if it provides an improvement.

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    Paul Tibbals
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  • 3.  RE: In situ etchant for P91 steel

    Posted 27 days ago
    Hi Paul

    Thank you. It is really rich information. I am going to do some tests on lab for the concentration and time..

    ------------------------------
    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------



  • 4.  RE: In situ etchant for P91 steel

    Houston Admin
    Posted 29 days ago
    I suppose the best etchant will depend on what specific detail of the microstructure you are trying to evaluate (grain size, sensitization, etc.). It's unfortunate you can't use picric or compounds containing it because Vilella's ​is pretty standard for higher chrome / stainless steels. I recommend referring to ASTM E407, which is a standard for etchant selection; certainly not exhaustive but it gives several options for 9Cr1Mo, and not all of them contain picric. Unfortunately I don't have much experience with these alternative etchants to recommend one over the other since I usually just use Vilella's but you could try Fry's or Glyceregia. I probably wouldn't recommend etchant 210/211 (sodium thiosulfate / potassium metabisulfate, which is a color-contrast etchant) for field microscopy like this because it's applied by immersion (not swabbing) and it would roll off your pipe.

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    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    773-524-8985
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  • 5.  RE: In situ etchant for P91 steel

    Posted 27 days ago
    Hi Sean

    Thank your for your comment. I am going to check the ASTM E407 and try the other etchants...

    ------------------------------
    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------



  • 6.  RE: In situ etchant for P91 steel

    Posted 24 days ago
    Glyceregia works best when it's freshly mixed. If it cannot be used within one hour, it's usually best to discard it and mix a new batch. If that is not possible, it's probably best to avoid using it. Once it turns orange, it generally produces an unsatisfactory etch. 
    Glyceregia may be too aggressive for use on P91. I've generally used it on very high alloy stainless steels (6% Mo, superferritic, etc.). I suggest starting with nital etching. 
    You may want to experiment with small pieces in the laboratory before trying large pieces in the shop or field.

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    John Grubb FASM
    New Kensington PA
    (724) 448-5272
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  • 7.  RE: In situ etchant for P91 steel

    Posted 14 days ago
    Dear John

    thank you for the advice, I am familiar with nital and it may be the best to start with

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    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------



  • 8.  RE: In situ etchant for P91 steel

    Gujarat Admin
    Posted 20 days ago

    The selection of etchant is recommended to be done based on the phase to be investigated/highlighted. Long time back (~2012) I had explored use of nital etchant (two types: 2% and 4%) to see microstructure of 9Cr steels. Nital helps in highlighting the prior-austenite grain boundary area and those with carbides or MX type precipitates. I had tried to do heat tinting (i.e. after etching sample with 4% nital, the samples were subjected to heating in oven at 260C for 1 hour) so as to get phase contrast in microstructure. In other words, it does not etch the carbides and MX type precipitates. A publication (of 2012) indicating this is attached herewith for your reference.

    However, if you want to see the martensite phase, then you will need to use Villela's reagent. Have a look at the attached paper wherein we tried to identify delta ferrite within P91 microstructure (2019 paper). 

    You may use ASTM E-407 which will give you a detailed idea of which etchant to be used for emphasizing which phase in microstructure.

    Hope this helps.
    with best regards,

    Nirav Jamnapara, Ph.D.
    Chairman - ASM Gujarat Chapter



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    Nirav Jamnapara
    Institute for Plasma Research
    Gandhinagar
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  • 9.  RE: In situ etchant for P91 steel

    India Admin
    Posted 19 days ago
    Dear Dr. Jamnapara,
    This is just to say that I liked your reply and the details of the steel, the structure and the problems in steel grade P-91 that may be encountered in your share. It made a fascinating read and thanks for the addition to information

    Sincerely

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    Vivek Singal
    Mumbai
    919769208775
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  • 10.  RE: In situ etchant for P91 steel

    Gujarat Admin
    Posted 14 days ago
    Thank you, Sir. 
    It is always a pleasure to interact with materials professionals here !
    Best regards,
    nirav.

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    Nirav Jamnapara
    Institute for Plasma Research
    Gandhinagar
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  • 11.  RE: In situ etchant for P91 steel

    Posted 15 days ago
    Nirav Jamnapara. Thank you for the details and the attachments..

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    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------