Common steels are general purpose carbon steels.. These steels are considered unalloyed carbon steels., despite the presence of a certain content of alloying elements such as manganese and silicon, which got into steel for technological reasons during its smelting.
Annealed state of steel and phase diagram
The metastable equilibrium state of ordinary carbon steels is maximally achieved in their so-called annealed state.. For steels in the annealed state, a strict relationship is observed between the microstructure of the steel and mechanical properties of steel (picture).
Figure 1A shows the iron-cementite phase diagram.. Figure 1B shows the microstructures of three types of steels - hypoeutectoid, eutectoid and hypereutectoid. Figure 1B shows the ferrite content, perlite and secondary cement in steel depending on the carbon content. Figure 1D shows the change in the mechanical properties of steels with a change in their carbon content.
Effect on strength of perlite content
If we compare figures 1B and 1D, then you can see, that the tensile strength curve behaves similarly to the pearlite content curve in steel. Tensile strength increases linearly from 300 MPa for pure ferritic steel up to 900 MPa for eutectoid steel with 100 % perlite. Tensile Strength of Hypoeutectoid Steels, containing ferrite and perlite, directly proportional to the content of pearlite in the steel.
On the other hand, the relatively high strength of pearlite comes from the high strength of iron carbide, which is one eighth of perlite. Besides, high strength is also ensured by the fact, that both lamellar phases - cementite and ferrite - were formed from the same initial austenite microstructure. Therefore, strong atomic bonds from austenite are largely preserved between the phases, formed during the decomposition of austenite.
Reducing the strength of hypereutectode steel
The tensile strength of hypereutectoid steel decreases linearly according to the content of pearlite in the steel. In this range of carbon content, the atomic bonding forces between pearlite grains decrease in proportion to the thickness of the secondary cementite network., which segregates along the austenite grain boundaries. At these grain boundaries, randomly oriented iron atoms form, together with carbon, secondary cementite.. Orientation of ferritic plates, seven-eighths pearlite grains differs from the orientation of adjacent pearlite grains. Therefore, the binding forces of iron, which is located in the secondary cementite along the grain boundaries, much weaker than iron bonds, which is inside the cementite particles. This is the reason for the decrease in the strength of hypereutectoid steels with an increase in the carbon content..
More carbon - higher hardness of steel
Figure 1D also shows the change in Brinell hardness of steel with an increase in carbon content.. Can be seen, that hardness hypoeutectoid steel also increases linearly with an increase in the steel content of pearlite. The Brinell hardness value is approximately one third of the megapascal strength value..
In the same time, the hardness of hypereutectoid steels continues to increase even after reaching 100 % pearlite content in steel. In this range of carbon content, the hardness increases almost linearly - although to a lesser extent., than hypoeutectoid steels. This increase in hardness occurs due to an increase in the thickness of the cementite particles in the pearlite of hypereutectoid steels..
More carbon – higher yield strength
Yield strength changes with increasing carbon similar to hardness. The reason for this is, that cementite plates of pearlite grains prevent slippage, which plastic deformation requires as well as the network of secondary cementite. Relative extension (A), which characterizes the ductility of steel and its ability to form, as well as the relative narrowing (Z) decrease with increasing carbon content.
Source: M. chalk, Physical Metallurgical for Engineers, ASM International, 2002