Steels- More Effects Of Alloy Elements and Solubility Products

Question 1

What can alloy elements affect in relation to transformation kinetics?

Answer 1

Nucleation rate
Growth rate
Nucleation and growth rates

Question 2

What does adding Mn do to austenite to ferrite transition temperature?

Answer 2

It lowers the equilibrium temperature of that transition

Question 3

What does adding Mo do to austenite to ferrite transition temperature?

Answer 3

It raises the equilibrium temperature of that transition

Question 4

What do most alloy elements do to growth rate?

Answer 4

They decrease the growth rate the more of each is added. Large effect for Mo steels, less for Ni and Mn steels

Question 5

What does cobalt do to growth rate?

Answer 5

Growth rate increases the more Co is added

Question 6

How does grain growth rate vary with temperature?

Answer 6

Higher temperature has the faster growth rate

Question 7

What else can form during the γ to α transition at slow cooling rate?

Answer 7

Formation of α and carbides and/nitrides

Question 8

Third main effect of alloy elements

Answer 8

Interaction between substitutional and interstitial solutes

Question 9

What can alloy elements be divided into in terms of phases they enter?

Answer 9

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

Question 10

Free energy change equation for general reaction
MC=M+C (metals carbide into metal and carbon in solution)

Answer 10

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

Question 11

Formula for equilibrium constant

Answer 11

k=h(M)h(C)/a(MC)
Basically activity of products over reactants

Question 12

Formula for activities of products and reactants

Answer 12

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

Question 13

Assumptions for equilibrium constant formulae

Answer 13

Dilute solutions (alloy additions small)
a(MC)=1 (realistic)
f(M) and F(C) =1 (means ideal solution behaviour which is less realistic)

Question 14

How do assumptions simplify equilibrium constant formula?

Answer 14

k=wt%Mwt%C

Question 15

Two equations for free energy change and how they lead to general form of solubility product equation

Answer 15

ΔG=-2.303RT log([M][C])
ΔG=ΔH-TΔS
Equate and rearrange
log([M][C])=A-B/T=logk

Question 16

When does thermomechanical processing begin for most processes?

Answer 16

At the point of reheating

Question 17

What is control of the final microstructure based on?

Answer 17

Understanding of the way in which steels respond to hot processing and how the response can be altered through alloying

Question 18

Why do microalloying elements enable the goal of thermomechanical processing to be easily achieved?

Answer 18

Because these elements permit forces retarding recrystallisation and grain coarsening to be governed by controlled precipitation during processing

Question 19

What is Sv (S sub v)?

Answer 19

The effective number of interfaces in an area per unit volume.
Units mm^-1

Question 20

Ferrite grain diameter vs Sv

Answer 20

Exponential decrease curve shape

Question 21

What does a greater Sv do?

Answer 21

Increases number of nucleation sites so finer grains produced which increase strength and toughness

Question 22

Role of microalloying elements

Answer 22

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.

Question 23

Formulae for undercooling and supersaturation

Answer 23

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