p2w QTSi 2 2 2 s 2 2 XIt is not feasible to entirely remove impurities in
making either iron or steel. The final product always contains small percentages of the metallic
impurities like silicon, manganese, sulphur, and phosphorus besides iron and carbon.
Occasionally small percentages of copper and arsenic are also present. In well made steel these
impurities generally range between 0.2 and 1.0 per cent and their resultant effect on the
constitution of steel is often small. Of the common impurities, Phosphorus cannot be eliminated
in the process of manufacture, whereas most of the silicon and manganese are introduced to
improve the metal.
is often added to molten metal to remove oxygen and diminish blow holes. In structural
steel it rarely exceeds 0.25 per cent. Silicon up to 1.75 per cent appears to increase both ultimate
strength and elastic limit without decreasing ductility.
is considered to promote enlargement of the grains and thus produce brittleness.
The ductility of low-carbon steel decreases slightly by the presence of 0.3–0.5 per cent
phosphorus. However, yield point, ultimate strength and hardness of steel are increased.
Resistance to shock is also reduced by 0.1 per cent phosphorus and the metal is rendered cold
short (i.e., brittle when cold). A decrease in toughness appears to be more pronounced in high-
carbon than in low-carbon steels. The maximum limits for phosphorus are: for inferior grades
of structural steel 0. 1, for best grades of structural steel 0.055, and 0.02 per cent for tool steels.
readily combines with iron to form iron sulphide (FeS) which, when present in iron or
steel, has a tendency to segregate and form brittle networks at the grain boundaries. On
account of its low melting point, iron sulphide causes lack of cohesion between adjacent grains
when heated above a red heat. Such brittleness at high temperature is termed as red shortness
which makes steel or iron hard to roll or forge. Manganese sulphide has a much higher melting
point than iron sulphidle and does not render ferrous metals red short. Therefore, inasmuch as
manganese has a very powerful affinity for sulphur, it is possible to relieve red shortness by
adding sufficient quantity of manganese to the molten metal to combine with sulphur.
Theoretically the ratio of manganese to sulphur should be 1.70 to 1.0 in order to form manganese
sulphide and completely satisfy sulphur. Less than 0.15 per cent sulphur content hardly
exercises any appreciable effect on the mechanical properties of steel. When sulphur is present
along with manganese it improves the machineability of steel.
w has strong affinity for oxygen and sulphur and acts as a cleanser of the molten metal
by withdrawing much of the undesirable impurities into the slag. Manganese increases the
tensile strength, hardenability and dilutes the effect of sulphur. When more manganese is present
than required for sulphur and oxygen the excess manganese forms carbide and acts as hardener.
g
increases resistance to corrosion when present in small percentage.
e has a tendency to raise the strength and brittleness.
xE2s
2 are mechanically suspended in the metal and are often called slag
inclusions causing brittleness.
r2
The object of heat treatment is to develop desired properties in steel. The properties of steel can
be controlled and changed as well by various heat treatments. A steel of given composition