(Coursehero Solution)
The following factors affect the mechanical properties of a material:
(a) Grain Size: This has an important effect on the mechanical properties of metals. In comparison to coarse grained metals, fine grained steels offer better resistance to cracking, produce fine surface finishes and can be shaped better. Fine grained steels have higher strength but lower ductility and therefore, fine grain is specified for structural applications . Coarse grained steels have higher hardenability and better forging properties.
(b) Temperature: It has been observed that the temperature affects the tenacity, elastic limit, Young’s modulus and other properties of steel and wrought iron as follows:
(i) With rise in temperature, yield stress, toughness and ultimate tensile strength decreases, whereas Young’s modulus decreases.
(ii) At higher temperatures (~ 450°C) embrittlement of steel and creep occurs. It increases with time and thereby reducing the toughness.
(iii) With rise in temperature stiffness and fracture stress of many metals decrease.
(iv) Several metals and large number of alloys exhibit the phenomenon of superconductivity (infinite conductivity and perfect diamagnetism) near absolute zero temperature.
(v) For unnotched mild steels, the elongation and reduction in cross-sectional area is satisfactory upto 130°C. It goes down considerably below that temperature. Elongation rises with further rise in temperature.
(vi) Below 100°C, non-ferrous metals show better properties than ferrous metals.
(vii) With the rise in temperature, there is also rise in thermal vibrations of atoms causing changes in structural properties.
(c) Exposure to Atmosphere :When exposed to atmosphere, most metals get oxidised. Due to oxidation of metal surface, a film is formed. The presence of chloride in marine atmosphere increases the conductivity of the liquid layer in contact with the metal surface. The resistivity of metals decreases due to the presence of moisture, sulphur dioxide, hydrogen sulphide and other corrosive environments. Corrosive action is pronounced and varies with the humidity of the air. Obviously, care should be taken while selecting the materials. When exposed to atmospheric conditions, metals like chromium, S.S., aluminium and nickel show exceptional resistance.
(d) Heat Treatment: By heat treatment, one can improve mechanical properties like tensile strength, hardness, toughness, and wear resistance. Heat treatment is generally carried out to -
(i) Improve machinability
(ii) Improve resistance to corrosion
(iii) Improve chemical, magnetic, electrical and thermal properties
(iv) Improve mechanical properties, e.g. ductility, hardens tensile strength, shock resistance etc.
(v) Relieve internal stresses induced during cold and hot working
(vi) Modification of the structure and grain size
(e) Carbon Content : Compared to the other ferrous and non-ferrous materials, carbon steels are widely used engineering materials. There are many types of carbon steels, each designed by the metallurgist, to have particular properties required by the engineer. Carbon steel is basically an alloy of iron and carbon containing manganese (1.0% max); silicon (0.5% max); sulphur (0.04% max) and phosphorus (0.05% max). The last two elements are introduced as impurities by the raw materials used during manufacture of steel and kept as minimum as possible. Silicon is residue from the steel making process.