Steels- Introduction Flashcards
Ferrite (α) information
BCC
Solid solution of C in α-Fe
Max solubility of C is 0.02 wt% at 727°C
Solubility of C decreases with temperature (0.008 wt% at 0°C)
Curie temperature 768°C so above this is paramagnetic
Soft and ductile
Austenite (γ) information
FCC
Solid solution of C in γ-Fe
Maximum solubility of C is 2.1 wt% at 1147°C (a lot more than α)
Cementite (Fe3C) information
Intermetallic compound of fixed composition (6.67 wt% C)
Orthorhombic with 12 Fe atoms and 4 C per unit cell
Hard and brittle
Ferrite (δ) information
BCC (but different lattice parameter to α)
Solid solution of C in δ-Fe
Maximum solubility of C is 0.09 wt% at 1495°C
Eutectoid reaction
At 727°C
γ(0.8%C) -> α(0.02%C) + Fe3C(6.67%C)
How do steel designations work?
Use 4 or 5 digit number
First two digits refer to the major alloy additions (don’t have to remember which alloying elements correspond to number)
Last 2 or 3 digits give C concentration in wt%. Divide the number by 100 to get the weight percentage
E.g XX120 has 1.2wt% C
Mechanisms of transformation of austenite on cooling for different cooling rates
Highest T and slowest is nucleation and growth.
Next is massive
Then bainitic
Lowest T and fastest is martensitic
Lines A3 and A1 on the iron-cementite phase diagram
A sub 1 is eutectoid temperature line (normally horizontal) above the mixed ferrite and cementite zone.
A sub 3 is the boundary between austenite and α ferrite (normally like exponential decay)
Nucleation and growth mechanism
Operates at slow cooling rates below the A3. Diffusion controlled mechanism. Smooth polygonal ferrite grains. Low dislocation density. Incoherent boundaries
Massive (acicular ferrite)
Transformation occurs at T under 740°C. Increased dislocation density (5x10^9 cm-2). Occurs by nucleation and short range atomic transport across α/γ interface (hard to distinguish from nucleation and growth). Product differs from N and G in that boundaries are irregular due to ferrite having both coherent and incoherent boundaries with parent austenite. Doesn’t produce surface relief
Bainite
Transformation occurs at about 640°C. Produces surface relief. Hybrid transformation
Martensite
Generic term that does not define a crystal structure. Occurs by cooperative movement of atoms (no diffusion and constant composition). Produces surface relief and high density of lattice defects (high dislocation density)
Displacive transformations
Athermal, diffusionless, shear or martensitic. Atoms move by less than 1 interatomic spacing and by making and breaking interatomic bonds. Atoms move one after another in precise sequence (military transformation). Speed of transformation roughly equal to velocity of lattice vibrations through crystal and not function of T. Extent of transformation depends only on T. No composition change. Always specific crystallographic relationships between product and parent phase.
Diffusional transformations
Atoms move over 1-10^6 interatomic spacings. Thermally activated diffusion from site to site. Atoms move randomly from one lattice site to another (civilian transformation). Speed of transformation is function of T. Extent of transformation depends on T and time. Composition of phases can change. Sometime crystallographic relationships between product and parent phase
Two groups of alloying elements
Austenite stabilisers (increase austenite phase field)
Ferrite stabilisers (decrease austenite phase field)