Chapter 9 Part 2 Flashcards
Primary alpha and Primary Fe3C
Primary phases that precipitate on the old gamma iron grain boundaries during slow cooling of hypoeutectoid and hypereutectoid steels, respectively
Peralite
The eutectoid microconstituent - thin alternating layers if alpha and Fe3C
Slow cooling leads to coarse pearlite which is softer and more ductile
Moderate cooling leads to fine pearlite which is harder and less ductile
Bainite
Cooling more aggressively (transforming at a lower temperature than) fine pearlite leads to feathery plates (upper bainite) or spiky needles (lower bainite)
Martensite
Metastable Body Centered Tetragonal phase created when steel is cooled so quickly that Fe3C particles do not get the chance to precipitate
Extremely hard and brittle
Hardness of martensite increases with carbon content of the steel, but only up to about 0.6% or 0.7% carbon by weight
Tempered Martensite
Very small spheres of F33C in alpha created by reheating (tempering) steel which has been quenched into martensite.
Nearly as strong and hard as martensite, but much more ductile but much more ductile and tough.
As a general rule, the higher then tempering temperature, the softer and more ductile tempered martensite becomes
Spheroidite
Very large spheres of Fe3C in alpha, created by heating steel to just below the eutectoid temperature (727 C) and holding for a very, very long time
Extremely soft and machinable, nearly like ferrite.
This microstructure is used for hypereutectoid (high carbon) steels to make it possible to form and machine them. After machining the steel would then be re-heat treated to a stronger microstructure.
Hypereuctectoid steels are rarely/never heat treated unto spheroidite because they are always machinable
Slowest to Fastest Cooling
Coarse pearlite + primary phase
(Most ductile / softest)
Fine pearlite + primary phase
Upper bainite
Lower Bainite
Martensite
(Least ductile / hardest)
