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Steel variations questions

  1. Nov 20, 2003 #1
    would like to know several things;

    1. what exactly is the stuff that i keep reading about called "cementite", and what does it do to steel
    on an atomic scale to make it harder and more brittle?

    2. how does heat-treating and quenching (ie. heating a steel blade, lets say, and then dunking it into an oil bath,
    or whatever it is they use) help to harden steel?

    3. do they use ceramics for blades nowadays, and if they do, what advantages/disadvantages
    do they have?

    4. i forgot, might be added later on...

    if you have an answer to any of the above, it would be splendiferous to hear it. thanks.
  2. jcsd
  3. Nov 22, 2003 #2
    Cementite is the stoichiometric compound, Fe3C. It is hard and brittle. Hardening of steels by cementite does not happen at an atomic level (too small!) but rather what is termed as a microstructural level. One route to steel hardening is done by getting a coherent distribution of cementite particles throughout the parent metal.

    Heating/quenching does just that. There is much more, in terms of heating duration, temperature, etc. But you need to know about phase diagrams to start to tackle the problem.

    Ceramic blades are used; they are non-magnetic, light, very hard and wear resistant. The downside is that they are brittle.
  4. Nov 24, 2003 #3
    thanks a lot.

    here's another;

    do dendrites (those tree like shapes formed when a metal solidifies) only occur in alloys, or in any metal, pure or not? can they be explained in terms that an A-level student could grasp (i mean, why does the metal not crystalise in the same way salts do? why do they form tree structures?)

    and is there an easy way to explain why impurities tend to collect between these dendrites? is it just that they don't fit into the crystal structure well?

    thanks for your help!!
  5. Nov 26, 2003 #4
    Dendritic growth does not only happen in metals. A similar case would be for snow, which forms when very pure water is cooled below freezing and falls through the atmosphere. Upon hitting another drop or dust, it rapidly crystallises, forming the fractal (if extended by a few more generations) snowflake structure. There are many similar analogies in nature, from the fractal geometry of tree branches, to snowflakes and dendritic growth in metals.

    You get dendrites when a molten metal undercools significantly and is not disturbed (e.g. stirring) during the process. In this context, undercooling is when the liquid metal's temperature is below its freezing point. You can also see the reverse of this in other pure liquids - if you get some distilled water and put it in a clean Pyrex beaker. Put the beaker in the microwave and leave the thing on high for a few minutes. If you left it long enough, and drop some salt in, it will suddenly 'boil' and (be careful!) foam over the top. You can get undercooling if the metal is very pure (no solidification throughout), cooled quickly or cooled in microgravity without touching the container's walls, for example.

    At the base of the dendrite, you will have an impurity which started the solidification process. With less impurities, you will end up with less dendrites. As the dendrite grows into the undercooled liquid, the solidification process releases the latent heat of fusion. The shape of the dendrite is such that it maximises its surface area for dissipating this heat to the undercooled liquid. The shape of the dendrite is also governed by how quick the solidification time is; a rapid solidification time will yield dendrites with tightly packed secondary branches (or arms). You get quick solidification times if you undercool the liquid significantly, and this gives you a tougher, stronger and more ductile metal.

    The dendrites formed in alloys and "pure" metals would differ. Assuming the absence of container walls (or a very clean, smooth wall surface), you would be able to undercool a pure metal much more than an alloy. Because of the purity, you would not be able to get many dendrites, but when you do, they will be big. For alloys, it will consist of many, smaller ones.

    By "not crystallising the same way salts do", I am assuming you are talking about a supersaturated aqueous salt solution being cooled, and crystals forming in it. The short answer is that the salt crystals do not adhere well to the container's walls. In addition, in dendritic growth, heat dissipation is the dominant factor. In any analogy with an aqueous supersaturated salt solution, there would also be issues of solubility. E.g. as a crystal forms is would release heat which increases the temperature of the water around it and increases its solubility...

    Between a crystalline solid that contains impurities and one that does not, generally, a pure solid is at a lower thermodynamic state than an impure one. This makes it more likely to form.
  6. Dec 6, 2003 #5
    Damn Tyro...i gotta make you my "homework buddy" for my Materials class...i learnt more about that stuff reading your post then reading my textbook for an hour :)
  7. Dec 6, 2003 #6
    Reguarding that question, in not sure if they use ceramic blades for large applications such as in turbines. But composite blades are popular with small applications such as your floor fan or propeller Anemometers. The largest factor affecting meterial sutability wiht blades ic creep resistance which is just the materials "sagging" or "lengthing" over time in basic terms. Composite materials have great creep resistance but poor temperature range so for applications were heat is involved such as the turbine in a turbojet engine for example then nickel is used. But i am not sure why they would use ceramics for blades since ceramics are usually brittle, even most engineering ceramics. However if you are refering to "Cermets" which are Metal-Ceramic Alloys then sure, use them if the costs are low...but other then that, im not really too in depth witht the subject since my knowledge only goes as far as my 2nd year textbook will take me...
  8. Dec 9, 2003 #7
    I too, think traditional metal blades are better than ceramic ones simply because they are tougher and won't break when you accidentally hit them too hard. There may be a few niche applications where ceramic blades are more popular, but I am not a knife-maniac so I can only speculate. Fooling around with anti-personnel mines/explosives armed with magnetic anti-tamper devices are one. Or possibly, because of their hardness + wear resistance, for a very sharp initial cutting edge. Corrosion resistance is another possibility...God knows what a corrosive mix human blood and propellant residue can be on a bayonet [b(]

    Most jet engines are designed to be able to suck in a bird and be operational enough to avoid catastrophic failure. Tests are actually done using frozen birds, loaded into large pressure-driven cannons, and fired into engines (and various other key sections of the aircraft, like the cockpit windows).
  9. Dec 10, 2003 #8
    Ask away dude. It is actually quite fun, in a nostalgic way, to get asked the stuff I too, used to pore over (usually a few days before the exams [b(]). Of course, you should know that my exposure to materials science only came through the modules I took in my engineering course, and was not actually a materials science degree.
  10. Jan 21, 2004 #9
    ceramic blades, etc

    There's been a lot of work done in the aircraft industry (even did some myself!) on ceramic-ceramic composites. These are ceramic fibres in a ceramic matrix - often the same material, eg SiC-SiC composite (and opportunities for sic jokes). In these, the fibre toughens rather than reinforces the matrix (in contrast to, say, glass-fibres in epoxy). The aim is to be able to combine the high-temperature properties of ceramic (and so eliminate the cooling channels that go through a metal turbine blade) with the toughness of a composite.

    Not sure if you could use it for a cutting blade as opposed to a turbine blade - you might not get a very sharp edge.


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