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Sunday, 7 February 2016

Brass

Brass:
          Brass is an alloy of copper and zinc. With range of composition from 5 to 45% of zinc, the brasses are among the most useful alloys. They possesses excellent mechanical properties, and are corrosion resistant and are readily machinable. Commercially, two types of brasses are most important:
1. Alpha brass                    It contains up to 36% zinc and remainder is copper.
2. Alpha-Beta brass          It contains 36% to 46% zn, remainder is copper.
Alpha and Beta are names given to different phases of brasses. Alpha-Beta brass contain both alpha and beta phases.
          The tensile strength and ductility of brass both increases with increasing Zn content up to 30% zinc. If zinc content increases beyond 30%, the tensile strength continues up to 45% Zn, but there is a marked drop in ductility of brasses. β-phase is much harder and stronger but less ductile than α-phase. α-phase has excellent cold-formability and is used where the parts are wrought to shape. The mechanical properties of α-brasses also change with the amount of cold-work done on them.  α-β brasses are fit for hot working.
α-brasses can be sub-divided into two groups --
(i) red-brasses containing up to 20% Zn, and, 
(ii) Yellow brasses containing over 20% Zn.
          Red brasses are more expensive and are primarily used where their color, greater corrosion resistance or workability are distinct advantages. They have good casting and machining properties and are also weldable. One well-known red-brass is "gilding-brass" or gilding metal with 5% Zn. It is used for decorative work. Yellow brasses are most ductile and are used for jobs requiring most severe cold forging operations. The cartridges are made from a 70% Cu, 30% Zn brass by a deep drawing process, hence this composition of yellow brass has come to be known as cartridge brass. Other famous compositions of brasses are:
  1. Admiralty brass containing 29% Zn, 1% Tin, remaining copper.
  2. Muntz metal contains 40-45% Zn, remainder is copper.
  3. Naval brass contains 39% Zn, 1% Tin, remainder is copper.

Admiralty brass, naval brass and muntz metal are all used for ships-fittings, condenser tubes, preheaters, heat exchangers etc.,

Modified brasses:

(i) Manganese brass:
          The alloy referred to under this head is generally spoken of as Manganese bronze, but it is in really a brass since the main constitutes are copper and zinc. The general composition of this metal is 60% copper, 38% zinc, 0.5 to 1.5% of tin, and sometimes a little aluminium.
Physical and mechanical properties:
  1. It is extremely tough.
  2. It has high resistance even in sea water.
  3. In addition to its action as deoxidiser, manganese hardens and strengthens the alloy. 
Uses:
struts, gears, valve stems, bushings, cams, wear rings for pressing dies for wood pulp industry, high strength machine parts, hooks, frames, shafts, marine racing propellers, worm gears, pressing dies for wood pulp. Marine rudders, marine castings, boat parts, clamps.

(ii) Iron brass: 
          One of the most widely used alloys of this type is known as delta metal, its composition is 60% copper, 35% zinc, 3% iron.
Physical and mechanical properties:
  1. It can be easily cast.
  2. It resists corrosion.
  3. It is hard and tough.
Uses:
It is used for mild steel if corrosion is to be resisted.

(iii) Tin brass:
          Tin is one of the most useful metals added to brass. It increases the hardness and tensile strength but amount employed should not exceed 2%, because with higher amounts the ductility begins to decreases. Its composition is 60% copper, 35% of zinc and remaining other metals. It is generally used in naval construction so some times it is called as novel brass.
Physical and mechanical properties:
  1. It has high strength and hardness.
  2. It tenacity is high.
  3. It has excellent corrosion and resistance property.




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