The ability of a metal to be formed, cast, welded, or machined is called its fabricating characteristic.
Machinability refers to the ease with which a metal may be machined, i.e., the forces acting on the cutting tool are low, chips are easily broken up, good finish produced, longer tool life between two sharpening. The machinability of metals is improved by uniform microstructure, small and undistorted grains, spheroidal structure in high carbon steels, lamellar structure in low-and medium-carbon steels.
It is also improved by hot working of medium-and high-carbon steels, cold working of low-carbon steels and heat treatments (annealing, normalising, tempering. The addition of small amounts of lead, manganese, sulphur and phosphorus and absence of abrasive inclusions (Al2O3) also improve the machinability of metals.
Ferrite being too soft does not produce good shearing action. Lead (0.15 to 0.35%>), or manganese sulphides help break up the ferrite structure. Leaded steels are known as free-machining steels and can be used at 50% higher cutting speeds than corresponding plain carbon steels. Small amounts of carbon improve the machinability of plain carbon steels upto around 180 BHN, but beyond this further carbon increases hardness and decreases machinability.
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Alloy AISIB 1112 is considered to be most machinable and forms basis for 100% machinability.
Steel castings have the same machinability as comparable wrought metals. However the skin of castings has oxide scale which is detrimental to machining and should be removed by pressure blasting before machining.
Malleable iron is considered one of the most readily machined ferrous metal because of its uniform structure and the nodular form of tempered carbon. It has a machinability rating of 120%.
Formability of a metal is a direct function of ductility which is dependent on the crystal structure. Metals having face-centred cubic crystals can be easily formed because of number of slip planes. The grain size, amount of working of metal, addition of alloying elements, annealing and normalising, also help improve formability of metal.
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Small group sizes are preferred for shallow drawing of copper and relatively large grains for heavy drawing on the thicker gauges. Hot working of metals reduces the size of the crystals and distorts the grains (and produces the ductility). The amount of distortion is more in cold working and thus poor ductility and poor formability.
Ductility can be restored by heating such metals to recrystallization temperature. Most alloying elements in a pure metal reduce its ductility and hence formability. The atoms of alloying element either replace the atoms of pure metal or find place in between the atoms of pure metal and thus reduce the number of slip planes. Low carbon steels have good forming qualities because there are less carbon and alloys to interfere with slip planes.
Weldability of a metal refers to the ease with which it can be welded and the quality of the weld obtained. It is affected by heating and cooling effects on the metal, oxidation of base metal, gas vaporisation and solubility. It is generally observed that the deposited weld metal picks up carbon or other alloys and impurities from the parent metal, making the weld hard and brittle, resulting in formation of cracks on cooling.
Such metals which oxidise rapidly interfere with the welding process. Normally oxides have higher welding point than base metal which affects flow ability. Oxides may get entrapped in the weld metal, resulting in porosity, reduced strength, and brittleness. Gases formed in welding of some metals may become entrapped causing porosity and deteriorating the desired property of base metal.
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Plain carbon steel is the most weldable of all metals. Medium- and high- carbon steels will harden if allowed to cool rapidly just after welding. Preheating and post heating help to remove the brittle micro- structures and thus improve quality of weld. Extra high carbon steels (C—1.0 to 1.7%) and tool steels are not welded but usually joined by brazing with a low- temperature silver alloy.
Castability of a metal is influenced by solidification rate, shrinkage, gas porosity, and hot strength.