Alloy Theory and Properties
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Questions and Answers

What is the primary requirement for a substance to be classified as an alloy?

  • It must be composed of only two chemical elements
  • It must be obtained by mixing only two metals
  • It must be composed of at least two chemical elements, one of which must be a metal (correct)
  • It must be a non-crystalline substance
  • What is the main purpose of adding alloying elements to steels?

  • To reduce the corrosion resistance of the material
  • To improve the mechanical properties of the material at various temperatures (correct)
  • To reduce the strength of the material
  • To increase the cost of production
  • What is the typical range of carbon content in ferrous alloys classified as steels?

  • Exactly 2.14% Carbon
  • Less than 1.0% Carbon
  • More than 2.14% Carbon
  • Less than 2.14% Carbon (correct)
  • What is the primary difference between ferrous alloys and non-ferrous alloys?

    <p>The presence of iron in the alloy</p> Signup and view all the answers

    What is the effect of adding alloying elements to improve the toughness of a material?

    <p>It improves the toughness of the material at any minimum hardness or strength</p> Signup and view all the answers

    What is the purpose of alloying elements in improving the wear resistance of a material?

    <p>To increase the wear resistance of the material</p> Signup and view all the answers

    What is the primary reason for the limited hardenability of medium carbon steels?

    <p>Insufficient Ni, Cr and Mo alloying additions</p> Signup and view all the answers

    Which type of steel is characterized by a high strength, hardness, and limited ductility?

    <p>High carbon steel</p> Signup and view all the answers

    What is the primary role of carbon in the microstructure of steels?

    <p>Occupying interstitial sites of Fe</p> Signup and view all the answers

    What is the minimum percentage of Cr required to make a steel highly corrosion resistant?

    <p>12%</p> Signup and view all the answers

    Which type of stainless steel is strengthened and hardened by cold work because it is not heat treatable?

    <p>Ferritic</p> Signup and view all the answers

    What is the primary advantage of adding Cr, V, Mo, and W to high carbon steels?

    <p>Improving wear resistance</p> Signup and view all the answers

    Which type of steel is commonly used in applications such as cutlery, razor blades, and surgical knives?

    <p>Stainless steel</p> Signup and view all the answers

    What is the primary reason for the relatively low electrical and thermal conductivities of ferrous alloys?

    <p>Relatively high density</p> Signup and view all the answers

    Study Notes

    Alloy Theory

    • An alloy is composed of two or more chemical elements, at least one of which must be a metal.
    • Alloys are crystalline substances obtained by mixing with other metals and also metals with non-metals.
    • Examples of alloys include iron and carbon for steels, and brass, which is a copper-zinc alloy.

    Purpose of Alloying

    • Alloying elements are added to steels for many purposes, including:
      • Increasing hardenability
      • Improving strength at ordinary temperature
      • Improving mechanical properties at either high or low temperature
      • Increasing wear resistance
      • Increasing corrosion resistance
      • Improving toughness at any minimum hardness or strength
      • Improving magnetic properties

    Types of Metal Alloys

    • Ferrous Alloys
      • Alloys in which iron is the major or principal component
      • They are produced in large quantities than any other metallic materials
      • In ferrous materials, the main alloying element is carbon (C)
    • Non-ferrous Alloys

    Ferrous Alloys

    • Disadvantages of ferrous alloys include:
      • Susceptibility to corrosion (environmental degradation)
      • Relatively high density and comparatively low electrical and thermal conductivities

    Steel

    • Steels are alloys of iron and carbon plus other alloying elements
    • In steels, carbon is present in atomic form, and occupies interstitial sites of the Fe microstructure
    • Mechanical properties of steels are very sensitive to carbon content

    Classification of Steels

    • Low Carbon Steel
      • Carbon present in these alloys is limited, and is not enough to strengthen these materials by heat treatment, except by cold work
      • Their microstructure consists of ferrite and pearlite
      • These alloys are thus relatively soft, ductile combined with high toughness
      • Typical applications include: structural shapes, automobile body components, sheets used in buildings, bridges, and tin cans
    • Medium Carbon Steel
      • These are stronger than low carbon steels
      • However, these are less ductile than low carbon steels
      • As hardenability of these alloys is low, only thin sections can be heat treated
      • Ni, Cr, and Mo alloying additions improve their hardenability
      • Typical applications include: railway tracks & wheels, gears, other machine parts which may require good combination of strength and toughness
    • High Carbon Steel
      • These are the strongest and hardest of carbon steels, and of course their ductility is very limited
      • These are heat treatable, and mostly used in hardened and tempered conditions
      • They possess very high wear resistance
      • Used for tool application such as knives, razors, hacksaw blades, etc.
      • Addition of alloying element like Cr, V, Mo, W which forms hard carbides by reacting with carbon present, wear resistance of high carbon steels can be improved considerably
    • Stainless Steel
      • They are highly resistant to corrosion
      • Steels are made highly corrosion resistant by addition of special alloying elements, especially a minimum of 12% Cr along with Ni and Mo
      • Stainless steels are mainly three kinds: ferritic, austenitic, and martensitic steels
      • Typical applications include: cutlery, razor blades, surgical knives, etc.
    • Ferritic Stainless Steel
      • They are principally Fe-Cr-C alloys with 12-14% Cr
      • They are magnetic
      • They are strengthened and hardened by cold work because they are not heat treatable
    • Austenitic Stainless Steel
      • They contain 18% Cr and 8% Ni in addition to other minor alloying elements
      • They are the most corrosion resistant because of the high chromium content

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    Learn about alloys, their composition, properties, and purposes of alloying elements in engineering applications. Understand how alloys are formed and their uses.

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