METALLOGRAPHY OF PLAIN CARBON STEELS Overview: Plain carbon steels are alloys of iron and carbon in which carbon varies from traces to about 2 % by weight. Carbon in the steel forms cementite (Fe3C), a hard brittle phase, it both strengthens it and renders it amenable to heat treatment. Commercial steels, in addition to carbon, contain elements such as manganese, silicon, sulphur and phosphorus. These elements are always present in all steels. Sulphur and phosphorus are highly detrimental and are treated as undesirable elements and these are usually kept below 0.04 %. The goal of this exercise is to study microstructure of various plain carbon steel classes and provide enough exposure to students so that they may learn to differentiate between these classes in future.
Procedure: The students shall be provided with prepared metallographic samples of steel for microstructural study. The three basic types of steel would be differentiated on the basis of the phase morphology. The phase present in the microstructure shall also be identified. There are three main classes of plain carbon steels according to the percentage of carbon: 1. Low Carbon Steels (0.05 – 0.25 % Carbon) 2. Medium Carbon Steels (0.25 – 0.65 % Carbon) 3. High Carbon Steels (0.65 – 1.7 % Carbon) 1. Low Carbon Steels: Steels with carbon varying from 0.05 to 0.25 percent are referred to as low carbon steels. The microstructure of low carbon steel consists of two phases, black and white. The black phase is pearlite and white phase is ferrite. The quantity of ferrite is more in low carbon steels than pearlite. 2. Medium Carbon Steels: Steels with carbon varying from 0.25 to 0.65 percent are referred to as medium carbon steels. The microstructure of medium carbon steels consist of two phases, black and white. The black phase is pearlite and white phase is ferrite. The quantity of pearlite is more than in low carbon steels. The quantity of pearlite is either nearly equal to ferrite or more than ferrite depending upon carbon contents of steel.
3. High Carbon Steels: Steels with carbon varying from 0.65 to 1.7 percent are referred to as high carbon steels. The microstructure of high carbon steels consists of two phases, black and white. The black phase is pearlite and white phase present along the grain boundaries is cementite. Cementite forms a network along the grain boundaries.
Eutectoid Steels: Eutectoid steels contain 0.8 % carbon. In the phase diagram of steel at 0.8 % carbon, eutectoid reaction occurs and results into a eutectoid mixture of ferrite and cementite known as pearlite. So the microstructure of eutectoid steel consists of pearlite only, which appears black under microscope.
Very low carbon steel
0.25% Carbon steel
0.5% Carbon steel