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Case depth call out for carburized components

  • 1.  Case depth call out for carburized components

    Posted 03-16-2021 14:36
    As I understand it, the total case depth call out (in addition to effective case depth) is helpful for thin walled parts where the intent is to retain a core and prevent through hardening.  Am I correct?  Is there any purpose to call out total case depth in addition to effective case depth where there is no risk of through hardening? 

    Also, I read that the total case depth is usually measured optically after etching.  Is there an industry standard that defines this?  We have been measuring the total case depth by checking micro hardness, and I wonder if we are doing it wrong.  Thank you.

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    Sree Menon
    Vice President, Engineering
    Interstate-McBee, LLC
    Cleveland OH
    2168810015
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  • 2.  RE: Case depth call out for carburized components

    Posted 03-17-2021 09:22
    I've always been used to measuring the fully martensitic zone as the specified case depth (also noting any surface decarb, oxidation or austenite) and then the total depth of carbon penetration and this was optically after etching.
    Microhardness testing is also a good measure of the effective case and also a better way to identify any surface anomalies from decarb after carburising or retained austenite.


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    Martin Reeves
    Owner
    fontec-global LLC
    Holland MI
    (616) 635-4283
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  • 3.  RE: Case depth call out for carburized components

    Posted 03-17-2021 10:09
    I prefer to specify effective case depth (ECD) as determined by microhardness because the results are much less open to dispute or disagreement. Optical measurement of total case depth is very subjective. I don't know if one is preferred over the other for specific applications, like thin walls. If you want a soft core, you could specify a maximum core hardness in addition to ECD. Specifying total case does not guarantee that a thin part is not carburized through to the core.

    I would never specify total case depth and effective case depth for the same part. It is too likely that one result would conflict with the other.

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    Ken Kirby
    Snap-on, Inc.
    Kenosha WI
    (262) 748-3836
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  • 4.  RE: Case depth call out for carburized components

    Posted 03-18-2021 09:51
    Over twenty years ago I was working as an intern in a large automotive manufacturing plant which did a lot of carburizing, nitriding, and case depth and this argument about optical metallography versus hardness for case depth measurement was alive and well between the metallurgists in the group I worked in.  I think the basic approach then was to use etching of samples taken from the manufacturing lines as a quality control measure since it was relatively fast and easy (and was done multiple times a day), but the process development/materials group preferred to use hardness traces (with interns doing a lot of hardness measurements :) ). I am not familiar with what is being done today, but what I will say is the automated hardness tools available today allow you to rapidly get much more data quickly and since hardness is easily quantifiable, after you develop a process it should be easy to implement.  When I moved to EPRI, the first tool I got our metallurgical lab was an automated hardness tester, it basically doubled the efficiency of our metallographer instantaneously because we eliminated having to do manual readings and in some cases eliminated the need to etch samples.  This double win more than paid for itself in a year and our lab throughput increased.

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    John Shingledecker
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252
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  • 5.  RE: Case depth call out for carburized components

    Posted 03-18-2021 11:34
    Why not to use Standards?
    There is ISO Standard 18203: 2016_Steel - Determination of the thickness of surface-hardened layers
    We in Italy have also the UNI Standard 5381, that reports methods to determine Total and Effective Hardened Depth
    There I have always found the proper solution.

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    Donato FIRRAO FASM
    Professor
    Politecnico di Torino
    Torino
    +393351494032
    Italy
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  • 6.  RE: Case depth call out for carburized components

    Posted 30 days ago
    I think it is very important to characterize a process with both the effective case depth and the total case depth i.e. by the whole profile (Hardness Vs depth). This is because the hardness profile can give an idea on the residual stresses profile that have a huge impact on bending fatigue of gears (as an example). The relationship between hardness and Residual stresse profiles could be determined when validating the process, but for quality control only hardness profile (cheaper and quicker) is required on test-parts or test-coupons.
    Industrial specifications such as AMS2759/7 call for hardness profiles to be able to specify effective and total case depth. Internal industrial specifications (Boeing, Pratt & Whitney, Safran, ...to name just aerospace companies) are very clear as for this requirement.

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    Nihad Ben Salah
    President
    NBS- M&P Consulting
    Canada
    https://www.nbsmpconsult.com/en/home
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