Steels- Thermodynamic Stability and Critical Temperatures Flashcards
Solute content vs 1/T graph
Solute content in (at%)^2. All metal carbides and nitrites have straight line from top left to bottom right. Those with lower lines are more thermodynamically stable and survive better at higher temperatures. The nitride is more stable than the carbide for a given metal. As N content in the solubility product increases it becomes more stable and line shifts to bottom left more
Log(%Nb) vs log(%C) graph
From top left going down is line below which all is dissolved in the austenite phase. Above and right is γ+NbC with some NbC not dissolved. This is for one temperature. There is a line from origin up that is stoichiometric ratio of NbC as gradient
How does the NbC wt% available for precipitation vary depending on ratio between Nb and C?
Most available at stoichiometric ratio NbC. Either side of this is steep decline with ever decreasing steepness
Why is more Nb not necessarily better?
At a certain temperature there is a maximum %Nb above which undissolved NbC will form which doesn’t contribute to precipitation hardening. Higher T means this maximum is higher
What are the three austenite critical temperatures?
Grain coarsening temperature T(GC) (sub GC)
Recrystallisation-stop temperature T(RXN) (sub RXN)
Transformation temperature Ar(3) or B(s) (sub 3 or s)
What do the three critical temperatures depend on?
The alloying elements used
Define grain coarsening temperature
The temperature above which grain coarsening by secondary recrystallisation (abnormal coarsening) commences
What is T(GC) related to?
The temperature above which undissolved precipitates can no longer suppress grain growth
What temperature is it important to know where the T(GC) relative to it is?
Reheating temperature. Since reheating of slabs or billets represents the initial stage for a given deformation process
How does normal grain coarsening progress?
At constant T over time. Start with all small grains. Larger grains grow and combine with other grains so fewer larger grains at end all of similar size
How does abnormal grain coarsening progress.
At constant T over time. Start as all small grains. Some grains grow while others stay small. Over time have lots of larger grain of similar size but still some regions of grains with the original small size. Eventually almost all the large size.
Why do we want a consistent grain size?
A mixed grain size gives a mixed set of properties
Avg grain diameter vs T graph to show T(GC)
A curved line up will start at some temperature along the x axis. This temperature is the T(GC) for that alloying element. Some don’t have a T(GC) like Ti (or is much higher) as they still have the ability to suppress grain growth
What can supersaturation be viewed as?
Potential for precipitates
How does supersaturation of carbide or nitride vary with temperature?
Supersaturation increases with decreasing temperature. From 0 up to 100% via concave curve starting at some temperature (high to low T x axis). The nitride has its curve further left (higher T) than the corresponding carbide.
