Steels- Decomposition of Austenite

Question 1

The two groups for decomposition of austenite

Answer 1

Reactions near equilibrium
Reactions far from equilibrium

Question 2

Features of reactions near equilibrium

Answer 2

Occur at small ΔT below critical temperatures so small driving forces. Reaction products determined reasonably well from phase diagrams. Reactions involve nucleation and growth

Question 3

Features of reactions far from equilibrium

Answer 3

Occur at large ΔT below critical temperatures (and high cooling rate). Reaction products not related to phase diagram. Reactions involve shear

Question 4

What is the A1 temperature?

Answer 4

Normally flat line on phase diagram. Eutectic temperature. Boundary above pearlite (not a phase)

Question 5

What is the A3 temperature?

Answer 5

The boundary between α ferrite and austenite on the phase diagram

Question 6

What are Ac temperature lines?

Answer 6

Arrêt chauffant (heating). The lines on the phase diagram when heating. E.g Ac3 is the A3 temperature line for heating

Question 7

What are Ar temperature lines?

Answer 7

Arrêt refroidissant (cooling). The lines on the phase diagram when cooling. E.g Ar3 is the A3 temperature when cooling. The A3 temperature line is between Ar3 and Ac3. Ar always lower than Ac

Question 8

Difference in γ decomposition for fine-grained and large-grained

Answer 8

For fine-grained, nucleation is primarily along grain boundaries. Carbon rejected from α diffusés to centre and relaxes the supersaturation.
For large-grained, the carbon content within the grain is unchanged and α can nucleate within the grain

Question 9

What does larger ΔT mean for γ decomposition?

Answer 9

More nucleation within grains due to larger driving force. Finer α grain size. More plate and needle-like morphologies because more driving force for interface creation is available

Question 10

Proeutectoid reactions for hypoeutectoid steel

Answer 10

Start with a C% less than eutectoid composition and at temperature above A3. Have grains of austenite. As cool and meet A3 temperature for that composition, primary α nucleates (mainly on GBs). The α grows as cooling continues. When reach the A1 temperature, remaining austenite transforms into pearlite. End up with grains of primary α (proeutectoid ferrite) and nodules of pearlite

Question 11

Proeutectoid reactions for hypereutectoid steel

Answer 11

Start with C% greater than eutectoid composition at temperature above the Acm (separating γ from γ+Fe3C). As cool and meet Acm line Fe3C nucleates (mainly along GBs). As cool further Fe3C grows (along GBs). When meet A1 temperature remaining austenite transforms to pearlite. End up with Fe3C films at former γ GBs and nodules of pearlite.

Question 12

Grain boundary allotriomorphs

Answer 12

Crystals nucleates at GBs and grow preferentially along GBs. Occurs at high transformation temperatures (8-850C). Have curved α/γ grain boundaries. Boundaries become facetted at lower temperatures. As temperature decreases these crystals develop facets on at least one side but often both sides of boundaries. Nucleate having a Kurdjumov-Sachs (K-S) orientation relationship with one austenite grain. {111}subγ (close packed planes) parallel to {110}subα

Question 13

Widmanstatten sideplates

Answer 13

Look like spikes. Primary sideplates grow directly from GBs into matrix phase. Secondary sideplates develop from crystals of another morphology (but of the same phase). Occur at slightly lower temperatures to GBAs. Grow along well-defined matrix planes (crystallographic planes)

Question 14

Intragranular idiomorphs

Answer 14

Roughly equiaxed crystals which nucleates within γ grains. Possess either irregular curved boundaries or boundaries with well-defined crystallographic characteristics. Occur within large γ grains. “blocky ferrite”

Question 15

Intragranular Widmanstatten sideplates

Answer 15

Similar to sideplates but nucleates within large γ grains.