Why material becomes hard?

It is usually said and performed practically that material becomes hard after quenching. but my question is that why material becomes hard after sudden cooling? what happens to its inner structure that it becomes hard?? waiting..

Replies

  • sweet_honey
    sweet_honey
    Alternately heating and cooling the material (say steel), changes the arrangement of iron and carbon molecules. Also it changes the way that the material (steel) responds to the mechanical stresses.
    Quenching causes the iron and carbon molecules to get arranged in crystalline structure, that is very brittle. Hence, it becomes hard after quenching.
  • Ramani Aswath
    Ramani Aswath
    #-Link-Snipped-#
  • CE Designer
    CE Designer
    Actually it is the surface of the steel that is hardened and its called surface hardening. When the steel is heated above the critical temperature the carbon molecules tend to move to the surface. When the steel is quenched in oil or brine and it cools rapidly the carbon molecules get trapped in their positions. A martensitic structure is formed on the surface which is very hard.
    The carbon content of steel determines its ability to be surface hardened. The more carbon content the greater its ability to be hardened. Usually when steels are surface hardened they are annealed to reduce the brittleness created by hardening it.
  • Ankush Sharma
    Ankush Sharma
    CE Designer
    A martensitic structure is formed on the surface which is very hard.
    dear will u please tell me that what is martensitic structure?
  • CE Designer
    CE Designer
    I will try to give a detailed description here:

    Plain carbon steel is made up of an iron and carbon solution. Iron and carbon together form plain carbon steel. Think of the iron as water and the carbon as sugar.
    When pure iron is heated from room temperature to just below the melting temperature, it changes from α-Ferrite (stable form of iron) to Austenite and finally just as it melts it turns into δ-Ferrite.

    Now Austenite and Ferrite are just names given to the way the iron and carbon atoms arrange themselves at specific temperatures.
    It is important to note that Carbon exists as an impurity in the iron and at elevated temperatures diffuses or dissolves into the iron.

    Depending on the rate of cooling and the carbon content you can have different micro-structures forming such as pearlite and bainite.
    Martensite is another microstructure. Martensite is formed when austenite iron-carbon alloys are rapidly cooled (quenched) to a relatively low temperature (near ambient).
    Martensite is formed when the cooling is rapid enough to prevent carbon diffusion. Essentially the FCC austenite transforms into BCT martensite upon quenching. All carbon atoms remain as interstitial impurities in martensite.

    Under slow cooling diffusion would occur and you will get the α-Ferrite and cementite forming as usual but when quenched diffusion is prevented and the carbon atoms are frozen. This is why you need rapid cooling to form martensite.
    Also important to note: As temperature decreases diffusion rate and solubility also decrease.

    If you still don't understand don't worry. You will catch up in time. If you still require more detail into the topic I recommend Materials Science and Engineering from Callister. All this information is in chapters 9 and 10 of that book, well in the edition I have.
    #-Link-Snipped-#

You are reading an archived discussion.

Related Posts

as is 456 describes that the strain in tension reinforcement should be more than .87fy/es +.002 what is the logic????
Hello Friends I was wondering if there any website for reading reviews of users and their ratings on any kind of product or service available in market
hi to everyone out there I am a mechanical engineering student,who is very much in to technical stuff. i created a blog in which they are posts on mechanical engineering...
I'm bit surprised that no one's initiated a discussion on the problem that's haunting the airline industry - grounding of all the Boeing 787 Dreamliner airplanes. The problem seems to...
CEans, we now have the sidebar on all the pages of CE Community. I want your ideas on what would you like to see in the sidebar. Your account photo...