Engineering Chemistry S Datta

316 ENGINEERING CHEMISTRY
Welding. It is the operation of joining the two pieces of a metal either by pressure or by
fusion to one compact mass.
Alloy. An alloy is a mixture or compound or both of two or more metals. When one of
these metals is mercury, the alloy is called an amalgam. So, an alloy may be considered as solid
solution or an intermetallic compound or both. Steel is an alloy of iron and carbon, a non-metal.
An alloy is often more useful than a pure metal. Controlling the composition of an alloy makes
it possible to vary its properties. Generally, alloys are classified as:
• Non-ferrous alloys
• Ferrous alloys
Non-ferrous alloys do not contain iron as one of the constituents of the alloys, e.g., Brass
which is composed of Zn and Cu. Whereas ferrous alloys always contain iron as one of the
constituents of the alloys, e.g., Stainless steel contains Fe, Ni, Cr.
Importance of alloy preparation
• to increase the hardness and tensile strength
• to improve the casting property
• to increase the resistance to corrosion
• to decrease the thermal and electrical conductivities.
• to lower the melting and boiling points of individual metal.
General methods of preparation of alloys
• By fusion. Component metals are mixed together in the desired ratio and melted
followed by cooling.
• By compression. Finely divided metals are compressed in the desired ratio.
• By reduction of the mixture of oxides to get mixture of metals in the proper ratio.
Examples of important alloys are
• Mild steel. An alloy of iron with carbon (0.1-0.25%). It is relatively cheap, can be easily
rolled into sheets, and can be pressed into shape.
• Stainless steel (Chromium steel) (10-15% Cr, 90-85% Fe). It is mainly used in making
utensils as well as reactors in industries.
• Manganese steel contains Mn 12-14% and the rest is iron. It is used in making rolls.
• Molybdenum and Tungsten steel or High speed tool steel are mainly used for
high speed machines.
• Wrought iron (C = 0.1-0.25%), is mainly used in making household furnitures.
• Ferro-titanium is used as deoxidisers to avoid formation of ‘blow holes’ from molten
steel.
• Brass, an alloy of copper (Cu:Zn = 60:40), has a gold colour and does not corrode.
• Bronze, an alloy of copper with upto 12% tin (Sn), a strong, hard wearing and resistant
to corrosion.
• Solder. An alloy of Pb and Sn that melts at low temperature, can be used to join metals.
• Duralumin. An alloy of 95% Al with 4% Cu and smaller amounts of Mg, Fe, Si, it is
stronger, harder, more resistant to corrosion than pure Al, used mainly in making the
body of aeroplanes.