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Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

  • 1.  Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

    Posted 03-09-2021 11:44
    Hello,

    I am working with low-carbon, low alloy steel samples in the quenched and tempered condition and would like to know your thoughts/recommendations on how I could prepare and examine the metallographic samples in order to be able to identify with certainty upper and lower bainite, retained austenite, un-tempered martensite, and tempered martensite microstructures...I have the possibility of using Light Optical Microscopy and SEM.

    Thank you in advance for your valuable recommendations.

    Tomas Padron 


  • 2.  RE: Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

    Posted 03-10-2021 09:29
    Thomas, there are a lot of different resources available to you and historically optical metallography was a primary tool.  At EPRI, we have unfortunately seen a lot of bad optical metallography from laboratories working on CrMo steels and bad interpretation.  I think for some phases a combination of hardness and optical metallography can give you useful information, but depending on what you are trying to do, today's SEM's and in particular the use of EBSD can be highly valuable for such an analysis of steel samples where the subtle differences will not be apparent optically.  Unfortunately I don't have an atlas of microstructures to point you to with these newer techniques, but to give you an example for a steel we work with frequently, the images (sorry post makes them poorer quality than they really are) shows that optically (in this case using our laser scope) you don't see a large difference in 4 out of the 5 different material conditions for the same heat of material heat-treated in different ways, but EBSD allows you to clearly see differences.  While hardness differences are moderately different for some of the cases, the high-temperature performance (creep) in this alloy system can vary by a factor of 100 based on this starting structure.  This may be more than you need, but this is what we are working to.  I would be interested to know what kind of atlas of microstructures would be useful to you; I personally believe ASM could build such tools to help us all building on much of the historical data but also using these newer methods beyond Optical metallography.



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    John Shingledecker
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252
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  • 3.  RE: Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

    IMS Board Member
    Posted 03-10-2021 09:54
    John's email on EBSD is certainly useful (and a great example), as some of these things are definitely difficult to interpret with optical microscopy - and it is not always done well.  However, those are often still the tools available to us right now, and a great deal faster than EBSD.  Sadly, I have neither a confocal microscope or an SEM myself!

    George Vander Voort and I are presenting a webinar covering preparation, etching and identification of microstructures in Carbon and Alloy steels with light optical microscopy.  This is part of a series of webinars on the last Wednesday of each month - all on etching and microstructure identification for different material groups.  We will have Q&A and follow up any questions raised in the webinar. I'm running these webinars because there are always a lot of questions about selection of etchants and microstructure identification!  Buehler Upcoming Webinar Sign up 

    This article on the ASM website is some good reading also.

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    Michael Keeble
    Buehler a Division of ITW
    Lake Bluff IL
    (847) 393-3645
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  • 4.  RE: Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

    Posted 03-10-2021 10:23
    Thank you Michael,

    Already registered for the Webinar and looking forward to it.

    Thanks,

    Tomas





  • 5.  RE: Lower and Upper Bainite / Martensite / Retained Austenite - Metallographic Analysis

    Posted 03-11-2021 07:59
    Great Michael, I will pass this onto some of our metallographers and I appreciate the references.  One challenge we've been having is our safety team always seems to be changing their opinion on 'safe' etchants.  Many of the older etchants are no longer available for us to use; the confocal microscopes have been a great addition in this case; with proper sample preparation (sometime a 20hr vibratory polish) we can actually see the grain boundaries and some structure without even needing to etch.

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    John Shingledecker
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252
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