Steels- Thermodynamics of Pearlite Nucleation Flashcards
How does the Hultgren extrapolation work?
Extrapolate the Acm (positive gradient) down below the A1 (eutectoid) temperature into the hypoeutectoid region. Extrapolate the A3 (negative gradient curve) down below the A1 temperature as a straight line into the hypereutectoid region
New key temperatures that arise from Hultgren extrapolation
For a hypoeutectoid composition, T1* is where that composition meets the extrapolated Acm line. Vertically above this and below the A1 is an arbitrary T1 temperature. For a hypereutectoid composition, T2* is where this composition meets the extrapolated A3 line.
What happens when you quench hypoeutectoid steel from the austenitising temperature (Tγ) down to T1?
Austenite is supersaturated in solute C with respect to ferrite but not Fe3C (as are left of extrapolated Acm). Ferrite forms as a grain boundary allotriomorph with perhaps some Widmanstatten side-plate formation
What happens when you quench hypoeutectoid steel from Tγ down to T1*?
Austenite is supersaturated in solute C with respect to both ferrite and Fe3C. Only then can we see their concurrent precipitation. Pearlite should from immediately without the formation of ferrite GB allotriomorphs.
How does nucleation and growth of pearlite progress in eutectoid steel?
Nucleation can be initiated by either α or Fe3C. For example start with an initial Fe3C nucleus (small plate). This grows into a fully-grown plate and α nucleates on the sides. These α plates become fully-grown and new Fe3C plates are nucleated. Could get a new Fe3C nucleus of different orientation forming at the surface of the original colony. This follows same steps to form a new colony adjacent to the original.
How does carbon concentration vary away from cementite and ferrite as pearlite grows in plates?
With a single Fe3C nucleus (or when Fe3C is on out plates) the C concentration in the surrounding austenite is low (conc needed to nucleates ferrite) at the interface but increases quickly back up to eutectoid concentration. When outer plates are ferrite the surrounding austenite has a high C concentration (that needed for Fe3C nucleation) at the interface which decreases quickly back down to the eutectoid concentration
In which directions does carbon move around the interface between austenite and pearlite (ends of all plate)?
In the austenite, carbon moves towards the ends of the Fe3C plates from the austenite. In the interface, carbon also moves towards the ends of the Fe3C plates. In the ferrite plates, this is also true but the C comes from close to the interface
How does flow stress vary with percentage of pearlite?
Increases linearly with increasing pearlite %. Because more pearlite means more interfacial area per unit volume (two new interfaces per lamellae). Interfaces inhibit dislocation motion.
How does tensile strength of hypoeutectoid Mn steel vary with carbon concentration?
Graph made up of three things. Lowest rectangle is solid solution hardening by Mn. Above is rectangle for grain size (bigger and slight upwards sloe for upper boundary. Top is triangle for pearlite with steep upslope at top. Because more carbon means greater % pearlite in a hypoeutectoid steel
How does tensile strength of hypoeutectoid Mn steel vary with Mn content?
Again three components to graph. Lowest is triangle for Mn in solid solution. Above is sort of rectangle parallel to slope of upper Mn in SS triangle. Above is pearlite region with top line of increased gradient to upper boundary for grain size.
How does energy absorbed vs test temperature graph vary for different carbon concentrations?
At low C wt% (like 0.11), energy absorbed is close to 0 until about -40C where is goes straight up to like 200J and stays there. As increase C wt%, this transition gets less steep and settles at lower energies at higher temperatures (up to 0.8 wt%)
