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Carburizing of 4140 steel

  • 1.  Carburizing of 4140 steel

    Posted 11-23-2020 22:13
    Hi metallurgist friends,
    For a specific design, we need to carburize a 4140 shaft and to do a liquid nitriding after. What are your thoughts on below:

    1- Is there a risk of getting carbides network on the surface due to saturation of carbon? Does shorter carburizing cycle time help with that? Any recommendations for carburizing time?

    2- Are there any risks in liquid nitriding of carburized 4140 steel? E.g flaky surface etc,..

    Neda Mandkarian
    McLAren Engineering

  • 2.  RE: Carburizing of 4140 steel

    Posted 11-24-2020 01:38


    I carburised 4140 for test bearings many years ago. We did not have any issues with carbide network formation. The carburising was at a temperature above 900 deg.C and the specimens were subsequently rehardened to give a distribution of fine spheroidal carbides. Rehardening was followed by tempering, of course.

    I do not know what you mean by "liquid nitriding".  


    Aidan Kerrigan         ​

    Aidan Kerrigan
    Senior Steel Technologist
    Skf Engineering & Research Centre BV

  • 3.  RE: Carburizing of 4140 steel

    Posted 11-24-2020 08:24
    Thanks for the feedback, maybe salt bath nitrocarburizing is a better term to use.

    Neda Mandkarian
    McLAren Engineering

  • 4.  RE: Carburizing of 4140 steel

    Posted 11-24-2020 09:10

    We have low pressure vacuum carburized 4140 at 950°C.  Because of the high carbon content in the base alloy, the amount of carbon supplied to the alloy (boost time) during the carburizing process was substantially less compared to conventional low carbon steels in order to satisfy the case hardness and case depth requirements without forming carbides or excess retained austenite.  Additionally, because the base alloy already has high carbon content, the rate of diffusion is also slower and required more diffusion time.  Essentially there is less driving force for the carbon diffusion due to the low differential of carbon from the case, core, and carburizing atmosphere.

    We have gas nitrided 4140 and 4150 and found the 4140 had higher surface hardness however shallower nitrided case depth compared to 4150.  I do not have experience if this trend would continue as the carbon continues to increase to the levels seen in carburized steels nor the risks associated with white layer composition as a function of carbon content.


    Trevor Jones
    Solar Manufacturing Incorporated
    Sellersville PA
    (215) 721-1502 x 1220

  • 5.  RE: Carburizing of 4140 steel

    Posted 11-24-2020 09:58
    I haven't seen carbide networks when carburizing 4140. You are not going to end up with any more carbon at the surface than you would when carburizing a lower carbon steel. You will just have more carbon in the core. Just don't go with a ridiculously high carbon potential. I generally would keep at 0.8% carbon potential, but a little higher is fine too.

    The only concern I would have about nitriding after carburizing would be retained austenite at the surface, but that is not something I have as much experience with.

    Ken Kirby
    Snap-on, Inc.
    Kenosha WI
    (262) 748-3836

  • 6.  RE: Carburizing of 4140 steel

    Posted 11-24-2020 14:07
    Hi Neda,
    I am assuming that this is just being carburized without hardening prior to nitriding.?

    I used liquid nitriding (Tufftride and Sursulf) for many years and did not have any problems with carbides providing the case is properly diffused.


    Martin Reeves
    fontec-global LLC
    Holland MI
    (616) 635-4283

  • 7.  RE: Carburizing of 4140 steel

    Posted 11-25-2020 03:29
    I believe one of the issues you have with carburising of 4140 is that you do not tend to produce good residual compressive stress in the case. This tends to make carburised 4140 more crack prone than low carbon carburising grades. We certainly recommend against it.

    It takes a bit of thinking or modelling to understand the effects of carbon level on volume expansion and temperature at which transformation occurs in the core relative to the case, and how this effects residual stress. In simple terms if there is no differential in carbon you will produce residual tensile stresses on the surface when quenching.

    Michael Leahy
    RCR Heat Treatment
    Welshpool, Wa
    61 8 9355 8130

  • 8.  RE: Carburizing of 4140 steel

    Posted 11-26-2020 16:24
    shallow case hardening (.010-.015") has been used to first increase wearability due to increased surface hardness and to give resistance to pitting fatigue for 4140 and other approximately 1.0% chromium steels. I do know that Volkswagen used this processing for many if not all of their gearbox gears.  My experience with carbuurizing 4140 or other steels having about 1 % Cr is the carbide forming tendency with the higher Cr.  For this reason I would use .90% Carbon as the target during carbirizing to assure full hardness while minimizing carbide formation.  I would still expect the surface to be in compression. 

    Jon Dossett FASM
    JLD Consulting
    Westfield IN
    (317) 737-4331

  • 9.  RE: Carburizing of 4140 steel

    Posted 11-25-2020 08:23
    Hello Neda,

    We carburize 4140 on a fairly regular basis. We do not typically have issues with network carbides though it is something to be aware of. It is important to keep the carbon potential at or under 1% to target a sub 1% carbon content in the case.  The time component is going to be dependent on other factors namely case depth required, section size, carbon potential, and carburizing temperature. Depending on how aggressive the quench is compared to the section size it may be necessary to cryo treat the parts after carburizing to mitigate retained austenite.
    There is an increased risk of cracking when carburizing 4140 compared to carburizing grade steels but by being expeditious with getting it out of the quench and into a tempering furnace will substantially mitigate the risk of cracking.
    I cannot speak to effect that the carburized surface would have on response to subsequent nitrocarburizing.

    Steven Keckler
    Process Metallurgist
    Waukesha WI
    (262) 549-1878

  • 10.  RE: Carburizing of 4140 steel

    Posted 11-27-2020 07:08
    Liquid carburizing is ferritic nitrocarburizing (570°C) in aerated salt baths of alcaline cyanates with some content of regenerating chemicals (e.g. Potassium Cyanide or Sulfide). In contact with carbides, a very high Carbon thin (10 microns) epsilon-nitride layer forms, which is both very hard (800 HV ca.) and tough. Thus, provided that the Carbon potential of the carburizing last stage does not go beyond 1%, the combination of shallow carburizing  and liquid nitriding would yield a very hard and tough surface which has also a very low friction coefficient due to the exagonal crystal structure of the epsilon carbide and its columnar growth in the c-axis crystal direction, so that wear resistance is implemented. Nitrogen diffusion into the underlying 1% Cr carburized layer increases its hardness due to Cr nitrides precipitation, thus enhancing resitance to Hertzian contact pressure.

    Donato FIRRAO FASM
    Politecnico di Torino