Ductility – Definition , Factor affecting on Ductility Of Steel
Ductility Definition :
Ductility is a measure of a metal’s ability to withstand tensile stress—any force that pulls the two ends of a material away from each other.
Meaning of Ductility:
Ductility is an essential property of material for its formability. However, ductility is not something like absolute constant for a metal or alloy under all conditions. In fact, it gets modified by the process parameters that is why the same material may show different formability in different forming processes.
Ductility is measured by the strain suffered by the material before fracture. In a tensile test it may be measured by percent elongation in engineering terms, or by logarithmic strain at the fracture point. In compression test similar measures may be used. In torsion test it is measured by the strain suffered by outer layer of material of test bar before fracture.
The tensile tests show low ductility because of neck formation and consequently the negative hydrostatic pressure in the neck region promotes crack initiation and propagation. This problem is not there in compression and torsion tests which show higher ductility for the same material. Many researchers have preferred torsion test for measurement of ductility while the strength properties are related to those measured in tensile test.
Factors that Affect Ductility of Metals:
Ductility is affected by intrinsic factors like composition, grain size, cell structure etc., as well as by external factors like hydrostatic pressure, temperature, plastic deformation already suffered etc.
Some important observations about ductility are given below:
(i) Metals with FCC and BCC crystal structure show higher ductility at high temperatures compared to those with HCP crystal structure.
(ii) Grain size has significant influence on ductility. Many alloys show super-plastic behavior when grain size is very small of the order of few microns.
(iii) Steels with higher oxygen content show low ductility.
(iv) In some alloys impurities even in very small percentages have significant effect on ductility. Ductility of carbon steels containing sulfur impurity as small as 0.018%, drastically decreases ductility at around 1040°C. This can however be remedied if Mn content is high. In fact the ratio Mn/S is the factor which can alter ductility of carbon steels at 1040°C. With the value of this ratio at 2 the percent elongation is only 12-15% at 1040°C while with ratio of 14 it is 110 per cent.
(v) Temperature is a major factor that influences ductility and hence formability. In general it increases ductility, however, ductility may decrease at certain temperatures due to phase transformation and micro-structural changes brought about by increase in temperature. Figure shows the effect of temperature on ductility of stainless steel. It has low ductility at 1050°C and maximum at 1350°C. Therefore it has a very narrow hot working range.
(vi) Hydrostatic pressure increases ductility. This observation was first made by Bridgeman. In torsion tests the length of specimen decreases with increase in torsion. If the specimen is subjected to an axial compressive stress in torsion test it shows higher ductility than when there is no axial stress. If a tensile axial stress is applied the ductility decreases still further.
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