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Carbon affinity to Cr and Ni

  1. Jan 29, 2013 #1
    Hi,

    Carbon has more affinity to Chromium or Nickel? What is the reason for more affinity to one material over another?

    Any help would be highly appreciated.

    Thanks
     
  2. jcsd
  3. Jan 31, 2013 #2
    Hi, why do you need to compare the affinities of these metals to carbon? I believe that such a comparison is very difficult to make (apart from being possibly unnecessary), especially because both chromium and nickel form several carbides each.
     
  4. Jan 31, 2013 #3
    Hi,

    During dissimilar welding, for example carbon steel to austenitic stainless steel welding by using austenitic stainless steel (18Cr-8Ni grades) filler wire, the chances of carbon migration from carbon steel to stainless steel weld metal will be high if we use a filler metal contains more chromium.But,the carbon migration will be less if the filler wire is a Nickel base alloy like Inconel 112 or Inconel 82. What is the reason for this phenomena?I am clueless.... By the way I have heard that Chromium,vanadium and niobium are strong carbide formers, but Nickel...I have no idea...Any help would be highly appreciated to solve my puzzle.

    Thanks
     
    Last edited: Jan 31, 2013
  5. Jan 31, 2013 #4
    Hi, I have done a bit of search and it seems that unlike chromium carbides which have very robust metal-like structures, Ni carbides are more covalent and more thermodinamically unstable, and decompose in water and dilute acids. This might explain your welding problem.
     
  6. Jan 31, 2013 #5

    Astronuc

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    Staff: Mentor

    Carbon is readily soluble in liquid Fe, and Cr, V and Nb (and Ta, Ti) are strong carbide formers, and Ni is weak in comparison.

    Usually if one wants to weld austenitic steel to carbon steel, it's best to avoid fusion welding and go for diffusion bonding without the melting.
     
  7. Jan 31, 2013 #6
    Hi All,

    Thanks for the valuable inputs and of course astronuc's solubility theory noted.However, I am curious to know whether this phenomena has any connection to the mobility of carbon in B.C.C and F.C.C lattice? In my case both 18Cr-8Ni grade weld metal and Inconel 82 or 182 are believed to be austenitic (F.C.C lattice). On the other hand 18Ni-8Cr contains little bit of delta ferrite (B.C.C lattice) due to F-A mode solidification to avoid solidification cracking but in inconel 82 or 182 the solidification mode is fully austenitic due to it's high nickel percentage. So the delta ferrite in 18Cr-Ni weld metal allowing carbon to move so easily in the stainless steel weld matrix compared to fully austenitic weld metal matrix in inconel?By the way I have heard that carbon has more solubility in F.C.C lattice than B.C.C lattice (For example Fe-Fe3C diagram).So I am little bit confused, Any help would be highly appreciated.

    Thanks
     
    Last edited: Jan 31, 2013
  8. Feb 1, 2013 #7
    I do not think carbon has any appreciable rate of diffusion in the solid metal lattice since its atoms are too large to move around like for example hydrogen atoms do. The atomic size of the other alloy components also matters. Moreover, one can expect similar structure phases to have relative affinity to each other, but nickel carbide has an almost covalent hexagonal structure which is not only different from yours but is also thermodynamically unstable (see e.g. http://digital.library.unt.edu/ark:/67531/metadc71810/m1/1/).
     
  9. Feb 1, 2013 #8

    Astronuc

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    Staff: Mentor

  10. Feb 2, 2013 #9

    phion

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    Gold Member


    Hope this might help: Delta-Ferrite Formation
     
  11. Feb 2, 2013 #10

    “The crystalline structure of the solvent material is important. For example carbon atoms diffuse faster in BCC iron than in FCC iron under the same carbon concentration gradient, because the atomic packaging is not as dense in the BCC structure. The lower atomic packaging factor in the BCC structure (0.68) as compared to the FCC structure (0.74) allows easier diffusion in BCC lattice.”

    http://books.google.co.in/books?id=...diffusion is more in b.c.c than f.c.c&f=false

    “BCC more open than FCC for interstitial diffusion.ie,it is easier to move from one interstitial site to another in BCC.But it doesnot say anything about the size or number of interstitial sites in each.actually as you will see FCC has bigger(more) interstitial sites.”

    http://www.public.iastate.edu/~bastaw/Courses/MatE271/Week6.pdf
     
  12. Feb 3, 2013 #11
    Although there are no stable carbides in the Ni-C (graphite) system, a metastable carbide (Ni3C) can be produced. (67Ruh) found that splat quenching(Approx.106 Deg.C/s) a molten Ni-C alloy produced a highly super saturated FCC phase, along with Ni3C. Subsequent work by (81Ers) facilitated the construction of a metastable equilibrium diagram that incorporated Ni3C,along with a greatly extended solid solution of C fcc(Ni) (fig.2).Major inconsistencies at concentrations greater than 25 at %C between the metastable diagram presented by (81Ers) and (66Pon) have led to the omission of this portion of the diagram in fig.2”

    http://link.springer.com/article/10.1007/BF02881419?LI=true#page-1

    By the way, could you explain in which welding process splat quenching occuring??:eek:
     
    Last edited: Feb 3, 2013
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