Steels- More Effects Of Alloy Elements and Solubility Products Flashcards
What can alloy elements affect in relation to transformation kinetics?
Nucleation rate
Growth rate
Nucleation and growth rates
What does adding Mn do to austenite to ferrite transition temperature?
It lowers the equilibrium temperature of that transition
What does adding Mo do to austenite to ferrite transition temperature?
It raises the equilibrium temperature of that transition
What do most alloy elements do to growth rate?
They decrease the growth rate the more of each is added. Large effect for Mo steels, less for Ni and Mn steels
What does cobalt do to growth rate?
Growth rate increases the more Co is added
How does grain growth rate vary with temperature?
Higher temperature has the faster growth rate
What else can form during the γ to α transition at slow cooling rate?
Formation of α and carbides and/nitrides
Third main effect of alloy elements
Interaction between substitutional and interstitial solutes
What can alloy elements be divided into in terms of phases they enter?
Those which enter only the α (or γ) phase.
This which form stable carbides or nitrides and also enter the α (or γ) phase.
Those which enter only the carbide phase.
All dictated by thermodynamic stability
Free energy change equation for general reaction
MC=M+C (metals carbide into metal and carbon in solution)
ΔG=-RT lnk=-2.303RT logk
Where R is gas constant
T is temperature
The decimal comes from converting to log base 10
k is the equilibrium constant
Formula for equilibrium constant
k=h(M)h(C)/a(MC)
Basically activity of products over reactants
Formula for activities of products and reactants
h(C) (h sub C) is f(C)wt%C
a(MC) is γ(MC)X(MC)
f and γ are activity coefficients
X is concentration
Assumptions for equilibrium constant formulae
Dilute solutions (alloy additions small)
a(MC)=1 (realistic)
f(M) and F(C) =1 (means ideal solution behaviour which is less realistic)
How do assumptions simplify equilibrium constant formula?
k=wt%Mwt%C
Two equations for free energy change and how they lead to general form of solubility product equation
ΔG=-2.303RT log([M][C])
ΔG=ΔH-TΔS
Equate and rearrange
log([M][C])=A-B/T=logk
When does thermomechanical processing begin for most processes?
At the point of reheating
What is control of the final microstructure based on?
Understanding of the way in which steels respond to hot processing and how the response can be altered through alloying
Why do microalloying elements enable the goal of thermomechanical processing to be easily achieved?
Because these elements permit forces retarding recrystallisation and grain coarsening to be governed by controlled precipitation during processing
What is Sv (S sub v)?
The effective number of interfaces in an area per unit volume.
Units mm^-1
Ferrite grain diameter vs Sv
Exponential decrease curve shape
What does a greater Sv do?
Increases number of nucleation sites so finer grains produced which increase strength and toughness
Role of microalloying elements
Help maximise Sv through their ability to generate retarding forces.
They contribute to hardenability when they are in solid solution and the transformation temperature.
They cause precipitation hardening of the transformation product.
Formulae for undercooling and supersaturation
ΔT=Te-T
ΔC=Co-Ce
Where e means equilibrium and T is temperature at
C sub o is some composition
C sub e is the equilibrium composition at that temperature
