Carbon Steels:
    •     Carbon steels contain trace amounts of alloying elements and account for 90% of total steel 
          production. 
    •     Carbon steels can be further categorized into three groups depending on their carbon content:
          1.   Low Carbon Steels/Mild Steels contain up to 0.3% carbon
          2.   Medium Carbon Steels contain 0.3 – 0.6% carbon
          3.   High Carbon Steels contain more than 0.6% carbon 
    Alloy Steels:
    •     Alloy steels contain alloying elements like: 
          Manganese, 
          Silicon, 
          Nickel, 
          Titanium, 
          Copper, 
          Chromium And Aluminum
          in varying proportions in order to manipulate the steel's properties, such as:: 
               Hardenability, 
               Corrosion Resistance, 
               Strength, 
               Formability, 
               Weldability Or
                Ductility. 
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    •    Applications for alloys steel include: 
         Pipelines, 
         Auto Parts, 
         Transformers, 
         Power generators and 
         Electric motors.
    Stainless  Steels:
    •    Stainless steels generally contain between 10-20% chromium as the main alloying element and 
         are valued for high corrosion resistance. 
    •    With over 11% chromium, steel is about 200 times more resistant to corrosion than mild steel. 
    •    These steels can be divided into three groups based on their crystalline structure:
         a.  Austenitic: 
         •   Austenitic steels are non-magnetic and non heat-treatable, and generally contain: 
              18% chromium
              8% nickel and
               less than 0.8% carbon
         •   Austenitic steels form the largest portion of the global stainless steel market and are often 
             used in:
              Food Processing Equipment,
               Kitchen Utensils and
               Piping.
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          b.   Ferritic: 
          •    Ferritic steels contains: 
           Trace amounts of Nickel, 
           12-17% Chromium, 
           less than 0.1% Carbon, along with other alloying elements:, 
               such as Molybdenum, 
               Aluminum or Titanium. 
          •    These magnetic steels cannot be hardened with heat treatment, but can be strengthened by cold 
               working.
          c.   Martensitic:
          •     Martensitic steels contain : 
           11-17% chromium
           less than 0.4% nickel and 
           up to 1.2% carbon. 
          •    These magnetic and heat-treatable steels are used in knives, cutting tools, as well as dental 
               and surgical equipment.
          d.  Tool Steels: 
          •    Tool steels contain: 
           Tungsten 
           Molybdenum 
           Cobalt and 
           Vanadium
           in varying quantities to increase heat resistance and durability, making them ideal for cutting and 
          drilling equipment.                                                                                       4
      The art and technology  needed  for  the  production  of  iron  was  well 
          developed in ancient India. 
      The Asoka pillars, such as the one situated in the Qutub Minar complex 
          is an example of high skills of its makers. 
      The quality of iron used is so good that after more than hundred years it 
          is still rust free. 
      However iron  was not used in buildings and bridges during the ancient 
          and medieval period because of the following reasons :
          1.   For Buildings and Bridges, superior quality alternative materials like 
               very  strong  bricks,  stones,  strong  and  durable  timber  and  high 
               strength mortars were available.
          2.   Fairly complicated forms of construction like the variable thickness 
               barrel  roofs,  circular  and  elliptical  arches,  spherical  and  elliptical 
               domes and other types of structures were used.
      A  barrel  shell  roof  made  from  bricks  and  mortar  only  at  Bara 
          Immambara, Lucknow is known to be the largest shell roof of its kind in 
          the world.
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    Types of Structures :-   From structural point of view
          Shell Structures
         Framed Structures
    Shell Structures :-  
     These are mostly made up of plates and sheets. 
     In such structures loads are mostly taken up by plates which serves as 
        covering material also. 
     Tanks, airplanes and shell roof coverings are some of the examples of shell 
        structures. 
     The main advantage is the saving in the cost of the construction material.
    Framed Structures:-  
     Framed  structures  consist  of  elongated  members  assembled  together, 
        such as roof truss, beams, etc. 
     In framed structures the load is mostly transmitted by the frames and the 
        later do not act as a covering material.
    However, framed structures are more commonly used, as shell structures pose 
    some difficulties. The knowledge of shell structures is not widely spread and 
    such structures are not economical for certain layouts and spans.
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