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LECTURE 12: PLAIN CARBON STEEL - Coggle Diagram
LECTURE 12: PLAIN CARBON STEEL
IRON CARBIDE PHASE DIAGRAM
Single Phase Regions
Ferrite - interstitial solid solutionof C in bcc Fe
Austentite - interstitial solid solution of C in fcc Fe
Squiggly ferrite - interstitial solid solution of C in bcc Fe (different C content due to sizes of interstitial sorted in crystal structure)
Cementite - intermediate compound of C and Fe (very specific composition)
Plain Carbon Steel
Contains only Fe and C with traces of other elements (P, Mn, Si)
Low carbon steel (<0.25wt% C) is weakest and easiest to machine, medium carbon steel (0.25wt% < C < 0.6wt%) has high strength , wear resistance and toughness and can be hardened and tampered but only in thin sections, high carbon steel (0.6wt% < C < 1.4wt%)is the hardest and strongest but the most difficult to machine since it is least ductile
EUTECTOID COMPOSITION CABON STEELS
Eutectoid composition is approx 0.76wt% C and when cooled to 772 degrees austentite goes straight to ferrite with second phase cementite
Below eutectoid temp. Structure contains grains with lamellae alternating alpha and Fe3C known as pearlite
Pearlite structure helps to provide strength, determined by the cooling process, faster cooling = thicker layers formed = less effective at preventing dislocation slip
Hypoeutectoid
Through thre cooling process austentite changes to alpha phase (know as proeutectoid ferrite) starting at the grain boundaries, when it is cooled further the remaining austentite has eutectoid composition (0.76wt% C) and forms pearlite structure
Hypereutectoid
As it cools, a cementite precipitate forms in the austentite starting at the grain boundaries (pro eutectoid cementite) leftover austentite increases in Fe (eutectoid composition) and becomes a pearlite structure
EXTRA NOTES
Watch lecture for slide 10 notes
Feel like we have learned about the f calculations before
MECHANICAL PROPERTIES
Hypoeutectoid alloys
Consist of regoins of pearlite providing strength and ferrite providing ductility and increasing C content results in increased pearlite and therefore strength
Hypereutectoid
Regoins of pearlite and cementite, pearlite provides strength and cementite provides hardness but is also brittle, increased C = increased hardness = small decrease in strength