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I need help

  1. Dec 7, 2004 #1
    i need urgent help

    can anyone suggest some link that clearly states the physical property needed for a stainless steel knife?
    i desperately need it today.
    thz

    any help will be GREATLY appreciated
     
  2. jcsd
  3. Dec 7, 2004 #2

    Integral

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    I like them sharp.

    You will need to provide a lot more infromation before anyone can answer your question.
     
  4. Dec 7, 2004 #3

    dextercioby

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    Lol.
    Me too.

    I think for daily use,the chemical properties of stainless steel would be more relevant/important.For any object which enters contact with acids (weak,i'm not assuming anyone would put a knife in some hydrochloric acid or sulphuric one (these acids in aquous solution)),it should be relevant the chemical behavior.

    To make a bad joke,if u had asked about a stainless steel sewing needle,i would have assumed u had swallowed it and i would have told not to worry,since the (weak) solution of hydrochloric acid from your gastric acid would gradually "eat" it...With knives,it's pretty hard to swallow one and live after...

    Daniel.
     
  5. Dec 7, 2004 #4
    um.. what i mean is..
    what's phy property make it suitable for this application
    y is stainless steel used.
    it must have something special to do wif it.. i know corrosion resistance is one.

    well, i think hardness should count. and may be it got great wear resistance. in general, anything to do wif the internal structure, like dislocation, grain boundaries will do, coz i think my teacher is lookin for that kind of thing
     
  6. Dec 7, 2004 #5

    chroot

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    Generally, hardness and corrosion resistance are a trade-off. A very hard steel that keeps a sharp edge will be more vulnerable to corrosion than a knife made from a softer steel.

    - Warren
     
  7. Dec 7, 2004 #6

    Gokul43201

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    You know what gives stainless its corrosion resistance. The chromium in the steel usually diffuses towards grain boundaries and outer surfaces, providing a chromium rich passivated surface. This is among the most useful properties for SS in this application.

    Other than this, SS has better hardness and yield strength than PC Steels. The rapid work hardening of austenitic SS (304, 316, etc.) upon cold rolling makes things easier. And another big advantage of SS is the fabrication flexibility : the austenitic SS's can be folded, forged (cold and hot), deep drawn, bent and roll formed (and I may have easily missed a few more).

    Another small advantage may be the poor thermal conductivity (actually the large ratio of heat capacity to conductivity, which results in a long thermal time constant), but I don't think that was a design constraint, just an small added bonus.

    The drawbacks are : poor machinability (it's a lot easier to sharpen if made of something else) and relatively high cost, but those are minor.
     
  8. Dec 7, 2004 #7


    Depends on the stainless steel. Some of them are resistant to HCl. Also, there's one S.S. alloy that's used for making tanks and pipes for handling nitric acid. But an interesting little story ;)... Some years ago, I ran into this guy at a local shopping center. Actually, he kind of ran into me, and asked if I'd like to see something amazing. With hardly a pause, he then pulled out a double-edged razor blade, popped it into his mouth, and swallowed it. Then he ate another one. Then he asked for a donation...
     
  9. Dec 8, 2004 #8

    Astronuc

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  10. Dec 8, 2004 #9
    alrite, thanks for the xplanation, here's just a year 12 stuff which i m stuck in:

    ss steel is tough coz it undergoes tempering, i think it's becoz ususally tempered steel r produced from reheating quenched steel(which has a hard structure due to the rapid coolin time), and because in tempering, u usually heat it to a lower temperature like 200 or 300 something, so the grains inside the steel gains little kinetic energy and recrystalline themselves, but the kinetic energy they've gained r not enough for full recrystallisation, so some regions recrystallise well and form quite soft and large grains, while there is some hard and small grains inside,
    is that correct? my teacher only talked about annealing and quenching in phy lesson but didnt mention much about tempering, i juz make it up.

    and for the tough structure of tempered steel, i dun really understand y it is tough.
    coz it is a polycrystalline structure, if there's a impact force acting on the tempered steel, it wont' break rite? i dun understand how's its gonna work in a polycrystalline structure with soft and hard regions...my gues is the soft regions absorb the impact force, but there should be dislocation preventing it from bending, so its not gonna work, i cant think of any other reasons, can anyone help? this is not only related to tempered ss steel, but juz steel in general, i'm a year 12 student, so can you explain it in a simplified way? thz
     
  11. Dec 8, 2004 #10
    You talk about this stuff in high school? Nice :)

    I'm not a materials guy, so I can only speak about some general issues. Here's what I make of it, basing on what I learned in a strength of materials course.

    Well, as far as "absorbing the impact force" goes, the amount of so-called strain energy absorbed depends on stiffness (e.g. Young's modulus) of material. In fact, it's inversely proportional to Young's modulus. So the stiffer the material gets (E goes up), the less energy it will absorb for a given load, because it'll also deform less. And strain energy is another name for deformation energy.

    I'm talking about bulk phenomena, but it also applies to small chunks of material. Generally, the softer grains will deform a lot, while the hard ones will deform less, and the ratio of deformations will be the inverse of ratios of Young's moduli -- this is a very broad approximation of course. I.e. if hard grains have E=300GPa, and soft ones have E=100GPa, the hard ones will deform to about 30% of the soft ones.

    The reality is much more complex, though. E.g. since the soft grains may undergo local yielding and even some plastic flow, the hard grains will usually deform much less than expected by ratio of Young's moduli. In fact, the grains are likely to be anisotropic themselves, so they will have more than two independent elastic constants/moduli, i.e. they may have up to 3 Young's moduli, etc.

    Now, as far as "breaking" or so called failure goes, there are essentially two modes: one is due to fatigue, and one is called rupture.

    In general, elastic materials can be classified as brittle or ductile, depending on how much they elongate if you pull on them (tension test) until failure occurs. Imagine a knife, mounted by its handle and its tip to a machine which will try to "stretch" it. Such machines are huge beasts, as the loads needed are very significant (tens, hundreds, thousands tons!). As you increase the force acting along the length of the knife, the knife will also stretch. For a while the stretch will be proportional to the force, and this is the elastic region of the stress (force/area) / strain (change in length/length) curve. As a certain force is reached, two things will happen: either the material will behave plastically, or it will just abruptly break.

    Now, as far as knifes go, my experience tells me that the typical silverware-variety (not e.g. butcher's knives) are quite brittle and you can break them into two pieces by dropping them "properly" on a hard surface. So you have brittle failure.

    As far as ductile rupture goes, a nice example would be e.g. copper wire. You pull on it, and after a very short elastic regime it will just stretch plastically, and it can stretch quite a lot before it actually break.

    Fatigue is a different beast. In fatigue you have repeated load/unload cycles. Due to internal dislocations that each such load/unload cycle presents, there may be regions where dislocations have self-organized to give an area of material with lower stiffness and/or lower strength than adjacent areas. Such area will most likely become a micro-crack, whose tip will slowly move, elongating the crack, during each load/unload cycle. When the crack is long enough it doesn't need repeated cycles to keep growing, the last static load condition will just make the crack grow through full width of material.

    Now as far as dislocations go, you're right in that if there was negligible amount of soft material -- only large, hard grains, the material would be very hard and brittle. OTOH, if there's enough soft grains, they will absorb most of strain energy, and thus there won't be enough deformation demanded of hard grains to even start them to dislocate. This is a very rough sketch of what might be going on, and I might even be wrong. I hope there are some material specialists lurking here who would know more.
     
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