Steels

Steel possibly is the oldest material of construction that has weathered and history and not only kept its foremost place in industrial application excluding also enhanced it greatly. The improvement of its position in industrial world is mostly because of its capacity to be produced in various alloyed varieties and response to several heat treatments. The cutting properties of sharp edges of steel were recognized long back while man began to employ swords and knives made in steel. The extremely similar properties of this material have been exploited to create cutting tools that wear very little and that machine parts like gears, bearing, shafts, etc. The steel is here being employed in every conceivable engineering structure. Machine bodies, rail road and railways rolling stocks, bridges, ships and boilers are a few illustrations. Various forms of steel are now obtainable commercially and all are produced to serve several specific reason. The needs of steel very largely and more often than one occupy consideration of their tensile strength, collision hardness and strength. Table no.2 shows some requirements and applications of some steels.

                                                       Table no.2: Typical Carbon Steels and their Applications

The steel carbon content plays a significant role in deciding its properties. Whether no carbon is representing in iron, it crystallizes in form of ferrite that is bcc soft material and extremely ductile. Pure iron with no impurities is perhaps employed only in laboratories and may be as expensive as gold. Pure iron is tough but not very strong. Along with addition of carbon, rising amounts of cementite (Fe₃C) crystallize in the construction. Cementite being hard decreases ductility considerably. Table no.3 would illustrate how ductility or measured as % elongation of steel is reduced along with increasing amount of carbon. While carbon is as much as 6.67 percent in iron, the entire structure crystallizes like cementite and is not at all usable commercially since it neither has ductility nor is machine-able.

                                                                      Table no. 3: Ductility of Plain Carbon Steel

The carbon content in steel gives outs to categorize steel as per  to its application. Table no.4 explains the application of various iron-carbon alloys. In Figure of Cooling Curve of Pure Iron with Steady Rate of Heat Loss, demonstrates the effect of carbon percentage on tensile strength of steels mostly. The tensile strength of pure iron or 0% C is around 250 N/mm² that increases to about 850 N/mm²  at a carbon percentage of 0.8. This has before now been pointed out that very low carbons content the entire metal is invented of ferrite grains that are ductile and soft. As the percentage of carbon raises more and more material is made up of pearlite, a substance such appears to have colour of mother-of-pearl, thus the name. At 0.4 percent carbon, pearlite materializes to be almost half the region viewed throughout microscope. At such level of carbon the tensile strength is about 540 N/mm². At 0.8 percent carbon almost whole region consists of pearlite whilst strength of 850 N/mm² is reached.

                                                   Table no.4: Carbon Percentage in Plain Carbon Steels and Application