Steel Phases Flashcards
Where is austenite in the steel phase diagram?
The highest solid phase is austenite (FCC)
Where is ferrite on the steel phase diagram?
The far left, mid temp with very low carbon conc
Where is cementite on the steel phase diagram?
To the right, with mid-range carbon conc at a range of temps
What is the difference between a Eutectic and Eutectoid?
Eutectic - liquid phase transforms into two solid phases
Eutectoid - solid phase transforms into two different (other) solid phase
The v shapes on graphs
What is a peritectic point
The peak on a phase diagram. Two phases, at least one liquid, form a single different phase during cooling
What is a peritectoid point?
More common than a peritectic. During cooling a mixture of two solid phases becomes one single solid phase creating a peak on the phase diagram
What primary different steel structures are there, and in what phase do they occur?
The ferrite/cementite region
Martensite, Pearlite, Bainite
Describe the microstructure and formation of austenite + ferrite phase
Hold above nose of C-curves, above A1 temp (the flat temp line at 723 degrees C), grains of ferrite form on the grain boundaries of existing austenite, leaving a mixture of ferrite and austenite grains ( fairly evenly sized)
Describe the formation and microstructure of pearlite
Slowly cooled above the nose of C-curves (about 530 C) but below A1 temp (723 C)
While in the austenite ferrite phase, ferrite forms on existing austenite grains while in cementite/ferrite region, then austenite transforms to cementite/ferrite mixture (pearlite - parallel strips of cementite and ferrite)
Leaves mixture of pearlite and ferrite grains (roughly evenly sized)
Describe the formation and microstructure of bainite
Quenched to beneath the nose of the C-curve, diffusion more inhibited, so austenite transforms directly to ferrite +iron carbide in finely dispersed grains
This structure of finely dispersed grains within the layout of the old large pre-existing austenite grains is called bainite (cementite needles/grains in a ferrite matrix)
Carbide needles - think of like graphite shards
Describe the formation and structure of martensite
Quenching carbon steel faster than the critical cooling rate (CCR) to bypass the C-curve prevents diffusion phase transformations - a supersaturated solid solution forms - and is still in FCC rather than BCC structure
To resolve tension caused by this, the material shears rapidly, changing its lattice and forming a needle of martensite along the austenite grain boundary where the shear occurred
More of these needles form where undercooling occurs
The result is a lattice of martensite needles between austenite grains
What happens when martensite is tempered?
Yield strength decreases, fracture toughness significantly increases
Ferrite precipitates
When might low hardenability be desirable?
For when you don’t want to form martensite on cooling, for instance in welding
