12 Manufacturing Processes
elements like chromium, nickel, tungsten, molybdenum, and vanadium are also present and they make
an appreciable difference in the properties of steel.
Before we go further, readers must note that in steels, besides iron and carbon, four other elements
are always present. These are S, P, Mn and Si. Removing these elements from steel is not a practical
proposition. However, the effect of sulphur and phosphorus on the properties of steel is detrimental and
their percentage is generally not allowed to exceed 0.05%. Similarly, the usual percentage of manganese
and silicon in steel is kept below 0.8 and 0.3%, although their effect is not detrimental to the properties
of steel. In fact, manganese counters the bad effect of sulphur. The presence of these four elements to
the extent indicated does not put plain carbon steel into the category of alloy steel. However, if higher
percentages of Mn and Si are intentionally added to steel in order to alter its properties, then the resulting
steels come within the category of alloy steels.
Plain Carbon Steels
Since the properties of plain carbon steels are so dependent upon their carbon percentage, these steels
are further classified into following categories on the basis of carbon percentage only:
(i) Low carbon or dead mild steel having carbon below 0.15%,
(ii) Mild steel having carbon between 0.15–0.3%,
(iii) Medium carbon steel having carbon between 0.3–0.7%, and
(iv) High carbon steels having carbon content above 0.7% (the higher practical limit of C% is
1.3%).
As the carbon percentage increases, the strength and hardness of plain carbon steel increases
while ductility decreases. Reference is invited to Fig. 2.1 (see figure on next page), which shows the
effect of increasing carbon percentage on certain mechanical properties of carbon steels.
Applications and Uses of Plain Carbon Steel
Dead mild steel. It has got very good weldability and ductility. Hence, it is used in welded and solid
drawn tubes, thin sheets and wire rods, etc. It is also used for those parts which undergo shock loading
but must have good wear-resistance. To increase its wear-resistance, the parts have to undergo case
hardening process; which provides a hard surface, while the core remains soft and tough.
Mild steel. It is used very extensively for structural work. It retains very good weldability if
carbon percentage is limited to 0.25%. Forgings, stampings, sheets and plates, bars, rods and tubes are
made of mild steel.
Medium carbon steel. It has little weldability but is stronger and has better wearing property
than mild steel. It is used for railway axles, rotors and discs, wire ropes, steel spokes, marine shafts,
carbon shafts, general agricultural tools etc.
High carbon steels. It is used for hand tools like cold chisels, cold working dies, hammers,
boiler maker’s tools, wood working tools, hand taps and reamers, filers, razors, shear blades etc. High
carbon steels can be hardened by the process of quenching and being hard can be used for cutting tools
which are not used in hot condition. If they become hot (above 150°C), they begin to lose their hardness
and become blunt.