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Carbon hardening agent in Iron and metal alloys

  1. Jan 26, 2007 #1
    Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and 1.7% by weight

    Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another

    Stainless steel has a minimum of 10% chromium content mixed in it and then you get Stainless steel

    So the chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, meaning the metal stays shiny.

    So why does carbon and/or other elements acts as a hardening agent.

    So my big question is, do you get something like a softening agent to make metal softer

    And how do people get it right to mix metals with different groups, common example brass it made from Zinc and Copper, but not sure if that will melt together, it’s like to mix Copper with Lead and wont work.

    But if you melt tin with lead it works nicely.

    So if it has the same periods it works or something?

    But lead also goes with gold, that’s weird because it’s also in the same period, can someone please teach me this stuff, I’ll really appreciate it
  2. jcsd
  3. Jan 26, 2007 #2


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    Carbon in steels forms carbides - particularly a carbide of Fe - cementite (Fe3C). Carbides are hard themselves, but dispersed in steel, they strengthen the alloy by dispersion strengthening, which as mentioned prevents the glide of dislocations and sliding/slipping of atoms in the lattice.

    Add less carbon.

    See The Effects of Alloying Elements on Iron-Carbon Alloys

    Iron and Its Interstitial Solid Solutions

    The Iron-Carbon Equilibrium Diagram

    Strengthening mechanisms in alloy steel

    More at - http://www.key-to-steel.com/default.aspx?ID=Articles

    Elements of the same group or similar valence do often have better solubility.

    Solid solution is another strengthening mechanism. Sn is added to Zr to strengthen the Zr. Zr-Sn forms a solid solution.

    Any alloy of Cu and Zn will melt, but one has to look at the phase diagram.

    Cu is used as alloying element with gold - which also provides some strengthening.
    Last edited: Jan 27, 2007
  4. Jan 27, 2007 #3
    You said I must add less carbon, but I want to do a experiment with steel or any other metal to make it as soft as lead, but carbon makes it hard so what will make it softer, will adding oxygen makes it softer or/and oxidize it all.

    And something I don’t understand that good, then does Iron (Fe) becomes cementite (Fe3). Does that happens then carbon is added, and that type of carbon is use? I’m more sure but I’ve seen on TV that they take piece of burned up toast and then make diamond glass with it.

    Also with Pyrite (FeS2 ) then does that S get another small 2 and what does that mean?
  5. Jan 27, 2007 #4


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    I am not aware of any alloying agent that softens steel to the same degree as lead. Lead, silver and gold are the softest metals. Iron and steel are relatively strong elements, and alloying is usually done to strengthen or improve corrosion resistance.

    The formation of cementite just happens, but its distribution can be affected by temperature and cooling rate. Carbon is soluble in iron, up to a point. One can have low-carbon, medium-carbon and high-carbon steels. Above 2% (by weight), the alloy is considered 'cast iron'.

    See this page for some applications of cast iron - http://www.msm.cam.ac.uk/phase-trans/2001/adi/cast.iron.html

    I am not sure what one is asking. Sulphides in steel are undesirable from the standpoint of strength and fatigue resistance. Consequently, sulfur is removed from iron in the initial processing before alloying. However, S is added to some steels for better machinability, but that also limits the applications. Mo can be added to the steel to bind the S in MoS2, which would act as a dispersion strengthening agent.

    MoS2 is a dry lubricant but that is between surfaces, not in the alloy.
    Last edited: Jan 27, 2007
  6. Jan 29, 2007 #5
    I’m talking about that second small 2 added after the S in Pyrite, do you know what that means, I would like to know?

    Pyrite (FeS2 ) - appearance, yellow/gold
    Pyrrhotite (FeS ) – appearance, brown like wet rust
  7. Jan 29, 2007 #6


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    Are you asking about this "2" : FeS2??

    If you really are (though it makes no sense to me that you would be - after all, you seem to understand what Cr2O3 is), then all I can say is that you really need to look into a high-school chemistry textbook - it will explain what a chemical formula is.

    If that's not what you are asking about, you need to explain more clearly what "2" you are refering to. There is only one "2" in the formula for pyrite.
    Last edited: Jan 29, 2007
  8. Jan 29, 2007 #7


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    I think the question is why FeS vs FeS2.

    That has to with thermodynamic equilibirium, and that will depend on the relative amounts of each element. It seems that FeS2 is the more common and perhaps more stable compound.

  9. Jan 30, 2007 #8
    Here’s a probably more common example to work with: Oxygen

    Oxygen – O1
    Peroxide - O2
    Ozone - O3
    Tetraoxygen - O4

    How does it works if you want to add another oxygen atom to the existing oxygen atom

    Like say, normal oxygen just O and then O2 how does that transformation process works
    I know if you want to make some kind off ozone O3 you mix the oxygen O with some high voltage sparks and then you get a very fresh smell, kinda like it have just rained
    Last edited: Jan 30, 2007
  10. Jan 30, 2007 #9


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    The most common form of elemental oxygen is the diatomic molecule O2. Ozone, O3, is not so common because interaction with UV is break it down and it is also a powerful oxidizer so will more readily react chemically with other elements.

    Peroxides tend to be chemically unstable (and powerful oxidizers).

    I am not familiar with tetraoxygen, but likely its existence requires higher pressure than normal atmospheric pressure and a pure oxygen enviroment.
    Last edited: Jan 30, 2007
  11. Jan 30, 2007 #10
    But then does O becomes O2 it's the same this FeS and FeS2, then does FeS becomes FeS2 / FeSS I’m not able to make that Small 2

    I don’t understand this transformation yet, then one atom bonds with another atom with the same chemical symbol
  12. Jan 30, 2007 #11
    This is somewhat a dumb question... I've been studying carburzing for a while now, and infact I've even carburized materials at 1700 degrees F. I know the slightest amount of carbon adds an incredable amount of strength, but what commercial practical applications is it used for really... I know all commercial steels contain carbon, ect... and this question makes me sound dumb, but what are more of the vital applications of carburizing...
  13. Jan 30, 2007 #12


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    Carburizing (and nitriding, carbonitriding and boronizing are thermochemical processes in which the outer portion of a metal can be hardened by the formation of phases which are harder. The process is generically known as "Case Hardening".

    The objective is to form a 'wear-resistant' (hard) coating over a strong and ductile/tough (i.e. resists mechanical failure/fracture) body. Gears and transmission parts are typically 'case hardened'.

    See these -

    Ref. 1 - http://www.chta.co.uk/downloads/data_2.pdf
    Ref. 2 - http://www.burlingtoneng.com/case_hardening.html

    Ref. 3 - http://en.wikipedia.org/wiki/Case_hardening - actually pretty reliable description.

    See also

    Gas Carburizing

    and more at http://www.key-to-steel.com/default.aspx?ID=Articles#p7
    Last edited: Jan 30, 2007
  14. Jan 30, 2007 #13


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    To make a subscript type [ sub]2[ /sub] but remove the space before s in the first bracket and / in the second bracket. For the superscript replace 'sub' with 'sup'. Its html code.

    OK, I think I understand the problem here.

    The chemical formula simply gives a 'stoichiometric' formula, but does not necessarily (nor usually) give any information about 'structure.

    FeS means one S for each Fe. FeS2 means 2 sulfur atoms for each Fe. Both would have some crystal structure.

    See the image on the upper right on this page - http://courses.washington.edu/ess212/carbonatesetc_files/slide0017.htm
  15. Jan 31, 2007 #14
    This is a simple example, To my understanding adding carbon to iron is like to adding some kinds of diamonds structures to iron in nm structures or something
    Last edited: Jan 31, 2007
  16. Jan 31, 2007 #15


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    No, that is not correct. Diamond has a particular crystalline or lattice structure in which one carbon atoms shares bonds with 4 neighboring carbon atoms, which forms somewhat of a tetrahedral structure. See - Diamond (A4) Crystal Structure

    There are 'diamond coatings', but there will be metal and carbide substrates associated with the interface between metal and coating.

    In steel, carbon forms 'carbides' with Fe (e.g. Fe3C) and other alloying elements: Cr, W, Mo, V, Ti, Nb, Ta, Zr.

    See this article - Influence of Alloying Elements on Steel Microstructure

    Here is FeS2 structure - http://cst-www.nrl.navy.mil/lattice/struk/FeS2.html or http://cst-www.nrl.navy.mil/lattice/struk/c2.html

    Sulfur and its compounds - http://cst-www.nrl.navy.mil/lattice/struk/sulfur.html

    Crystal LatticeStructures - http://cst-www.nrl.navy.mil/lattice/
    Last edited: Jan 31, 2007
  17. Feb 1, 2007 #16
    thank you so much for the information Astronuc, you always have so much information
  18. Feb 1, 2007 #17
    Then does you know then different types of metal can be alloyed/mixed with each other, like Pb and Cu wont go but Pb and tin (Sn) works very nice and I think Pb and Al also mix, bit I’m not able to alloy Cu with Zn do you maybe know way this don’t want to work?
    I’m working with a oxy-torch the max temp is ~1700C
  19. Feb 1, 2007 #18


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    Cu and Zn do form alloys - brass.

    Sn and Pb form solder alloys, although Pb-free Sn solders are preferred nowadays.

    I am not sure why would want to make Pb-Cu alloys, but there are such alloys. See - Lead and Lead Alloys

    It is generally difficult to alloy metals of different densities and chemical properties.
    Last edited: Feb 1, 2007
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